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This is a sub-page of Sonic the Hedgehog 3.

Considering that Sonic the Hedgehog 3 and Sonic & Knuckles were worked on together, it's no surprise there are loads of leftover bits of data from the latter while it was still in development. However, there's still quite a bit in here.

• 1Knuckles Leftovers
• 2Sonic & Knuckles Zones
• 3Level Differences
• 4Mushroom Valley Object Data

Knuckles Leftovers

Some data for Knuckles as a playable character seems to still exist in the game. Game Genie codes ASCA-AAHC ASCA-BA9Y or Patch Code FFEF4C:0003 will allow you to 'select' Knuckles as your character. You will play as Sonic, but have Knuckles' character select, continue icons, and 'level-clearing' text. The end signpost will also feature Knuckles, though the image uses Sonic's blue palette, and the markers on the score screens that appear after a Special Stage use Knuckles' palette. As you might expect, this code will save properly, allowing you to apply the code, create a save, then continue to use 'Knuckles' even without the Game Genie.

FFFA80:0101 makes a few bosses behave like Knuckles' versions: Fire Breath shoots missiles, Robotnik doesn't fly down when he creates the water spout in Hydrocity, the spiked cylinder is in the Tunnelbot battle, and the second boss in Launch Base Act 2 plays a broken version of Knuckles' bomb cutscene when you defeat it (the bomb that should fly out of the wreckage appears with glitched graphics). This suggests that Knuckles was not meant to fight Big Arms at this point in development and was meant to go straight to Mushroom Valley/Hill, or even that Big Arms wasn't originally intended to be in the game at all. After this cutscene plays, nothing else happens. The game crashes some time after the cutscene for unknown reasons.

FFB038:0200 will add in Knuckles' ability to destroy barriers, allowing you to access any Knuckles-exclusive areas. This doesn't work in Ice Cap and will result in bounds of the map glitches (because it shared Tails' bounds). The code also disables the Insta-Shield ability as Knuckles does not have it normally.

Bugs and Oddities

• The cutscene where Sonic arrives on Angel Island is glitched. The cutscene starts where Knuckles would normally start in the level. For some reason, the fire-bombing cutscene starts while the glitched intro is going on and Sonic can fall through the ground and die.
• The Insta-Shield graphics disappear at times.
• The cutscene at the beginning of Ice Cap Act 1 is the same as Tails'.
• The pit at the beginning of Ice Cap Act 1 doesn't work correctly: the part where the bottom of the stage should connect to the top instead acts like a bottomless pit, causing you to lose a life.
• In Launch Base Act 1, the cutscene where Knuckles throws a bomb works, but the building does not load. This is because when you play as Knuckles in Sonic 3 & Knuckles, the building is already destroyed and the game simply does not load it.
• The game softlocks after defeating the Launch Base Act 1 boss. Sonic can still be controlled, but nothing else will happen. This is because Knuckles was supposed to fight two of them.
• If you use the last code mentioned above and break the rocks leading to Knuckles' boss arena, the water will lower (as it does for Knuckles), but the screenlock will not lower sometimes and prevent you from continuing.
• Although there is no boss in the Knuckles area in Angel Island, there are three hidden end-of-Act monitors in place: one each of Thunder Shield, Aqua Shield, and 10 Rings. Obviously, since there is no mini-boss to defeat and no signpost to uncover them with, they are completely inaccessible. These monitors are present in Sonic 3 & Knuckles, with the same items.

Sonic & Knuckles Zones

A few remnants of the Sonic & Knuckles Zones are still left in the game. As mentioned earlier, Flying Battery would have been played between Carnival Night and Ice Cap, but when the game was split into two that zone was moved into Sonic & Knuckles to give it more levels. Most of the data that once belonged to these levels either isn't present in Sonic 3 anymore or isn't currently accessible. None of these Zones have collision data intact, requiring the player to use Debug Mode to land on whatever collision the game recognizes as solid ground. Still, there's some leftovers of Flying Battery in Sonic 3: a data select icon, its music, and even its bosses!

You can 'access' these zones using the following Game Genie codes:

Each level can be played through the level select. Lava Reef, Sky Sanctuary, and Death Egg are accessible using the first, second, and third 2P VS options respectively while The Doomsday Zone uses the Sound Test option. The glowing spheres and slot machine bonus stages can be played by selecting 'Bonus' on the level select. 'ID 0D' can be accessed by selecting Angel Island Zone Act 1 and 2. Lava Reef Act 3 and Hidden Palace can be played by selecting Hydrocity Zone. Finally, the Death Egg final boss and Hidden Palace (Special Stage Ring) can be accessed through Marble Garden Acts 1 and 2.

Note that you need to access Flying Battery Zone in a specific way. Currently, using only these codes will crash the game for this Zone, but it'll still play the correct music. Flying Battery can be fixed using codes W4ZT-LE0R 3CZT-LJ8W for Act 1 and W4ZT-LE1G 3CZT-LJ9L for Act 2. It will be corrupted with glitchy collision and Ice Cap tiles appearing all over the place. However, some graphic tiles will not be overwritten and will use whatever was last loaded there which can make this Zone even glitchier. Other Sonic & Knuckles Zones such as the Death Egg final boss, Hidden Palace, Lava Reef Act 3/boss, and Hidden Palace (Special Stage Ring) can be accessed without the game crashing, although they are very glitched. The bonus stages will work, but only on a real console for some odd reason. Most of the Sonic & Knuckles Zones originally crashed due to reading a pointer table that did include entries for most of the Sonic & Knuckles Zones. However, the data the pointer table pointed to only went up to Launch Base, meaning garbage data was being read, causing the game to lock-up. When the 'crash fix' codes are enabled, they overwrite PLC data for Angel Island which causes it to crash, but only as long as the codes are enabled.

It is possible to use Sonic 3C's savestates to access the remains of the Sonic 3 Zones and for the most part it works the same way as using the codes above.

Flying Battery Zone

This Zone, perhaps, retains the most data out of all the Sonic & Knuckles Zones.

• Uses Ice Cap's graphics and palette.
• There are some rings near the beginning of each Act.
• No objects appear here other than bosses.
• The debug item list contains the usual ring, super monitor, debug rings, a vertical spike, and various facing springs; it also contains the two Flying Battery Zone badniks.
• Both the Act 1 and Act 2 bosses appear in this Zone. For some reason the Act 2 sub-boss does not appear here, even though it exists fully functional within the game.
• You need debug mode in order to reach the bosses as there is no easy way to get to them.
• Act transitions are broken: after you beat Act 1 it doesn't take you to Act 2 and likewise, it doesn't take you anywhere when you beat Act 2.
• The title card correctly refers to this Zone as 'Flying Battery'.

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Mushroom Valley Zone

Retains a little less data than Flying Battery, but still has some interesting leftovers.

• The title card refers to this level as 'Zone' followed by an Act 1 or 2 depending on which Act you're playing.
• This zone uses Azure Lake graphics.
• This zone contains an object layout consisting of path swappers, monitors, springs, and some deleted/broken objects.
• There are some rings scattered about in this Zone.
• The object layout does not match any known layout of this zone.
• Both Acts contain objects.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs; it also contains diagonal springs, horizontal spikes, and moving versions of the springs and spikes.
• You need debug mode to explore, otherwise you will just fall to your death. (Unless you can make it onto one of the monitors near the start of each Act.)
• Each act uses a different object layout.( Suprisingly it almost matches the Sonic 3C 0408's layout but a little bit higher )
• Both acts use the correct music.
• Both acts seemingly use some kind of Azure Lake palette.
• This zone cannot be completed; if an Egg Prison capsule is spawned in and is opened, the game will load data from an invalid Zone and crash.

Level ID 08 (Sandopolis Zone)

Not much remains of this zone. It's mostly a wasteland of broken data.

• The title card refers to this level as 'Zone' followed by an Act 1 or 2 depending on whichever you're playing.
• There is a repeating pattern of rings in this zone.
• This zone uses Angel Island's intro graphics.
• Act 2 uses Angel Island Act 2's palette.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Whenever this zone is accessed using the Game Genie codes above, the game crashes with a purple screen or freezes on the title card. This is most likely because most of the data that once belonged to this zone is no longer present.
• Both acts use the correct music.
• Act 1 uses Angel Island Act 1's intro palette.
• This zone cannot be completed.

Lava Reef Zone

A little bit less data than Mushroom Valley, but still a good bit more than Sandopolis.

• The title card refers to this level as 'Zone' followed by an Act 1 or 2 depending on whichever you're playing.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Act 1 of this zone has some palette data; it contains the lava palette cycle and the palette cycle for the dark flashing rocks.
• There are some rings strewn about the level.
• The start position array (Where you start in the level) is intact; you start from the top of the level and fall downwards, just like in Sonic & Knuckles.
• Act 2 does not have any palette data.
• This zone uses Azure Lake graphics, much like Mushroom Valley.
• The background has some animation or scrolling, however it is just composed of glitched Azure Lake graphics.
• Both Acts use the correct music.
• Both Acts seemingly use some kind of Azure Lake palette.
• This zone cannot be completed.
• You can actually land on solid ground and continue quite a good bit compared to the other Sonic & Knuckles Zones.

Two palette cycles can be found at this zone. They both resemble the ones found in Sonic & Knuckles with little difference. They can be found at offset 002EFE and 002F7E.

Level ID 0A (Sky Sanctuary Zone)

There is some strange wacky palette that can get loaded here.

• The title card refers to this level as 'Zone' without any 'Act 1' or 'Act 2'. This is because this Zone didn't use Act numbers in Sonic & Knuckles and Sonic 3 & Knuckles.
• There is a repeating pattern of rings in this zone.
• This Zone uses Angel Island's intro graphics.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Act 2 uses Angel Island Act 2's palette.
• Act 2 uses Death Egg Act 1 music for some odd reason. This could mean there was originally not going to be a second Act of Sky Sanctuary and instead Death Egg Zone would have three Acts. The unused Sonic & Knuckles bonus stages also have music oddities.
• Act 1 uses Angel Island Act 1's intro palette.
• This zone cannot be completed; if an Egg Prison capsule is spawned in and is opened, the game start playing the miniboss music and crash with a nonsensical palette.

Level ID 0B (Death Egg Zone)

This is the only other S&K level besides Flying Battery and Mushroom Valley to include an object layout. Otherwise, it is similar to Sandopolis with a different garbled layout.

• The title card refers to this level as 'Zone' followed by an Act 1 or 2 depending on whichever you're playing.
• There is a repeating pattern of rings in this zone.
• This Zone uses Angel Island's intro graphics.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Act 2 uses Angel Island Act 2's palette.
• For some reason, both Acts use Death Egg Act 2 music. This might have something to do with Sky Sanctuary's music oddity. (mentioned above)
• Whenever this zone is accessed using the Game Genie codes up above, the game crashes with a purple screen or freezes on the title card. This is most likely because most of the data that once belonged to this zone is no longer present.
• Act 1 uses Angel Island Act 1's intro palette.
• This zone cannot be completed.

• An object with an ID of $2, sub-type $89 can be found at coordinates X $550 and Y $0. It appears to be a path swapper/collision change object. It is the only object that remains in Death Egg Act 2, other than the default ring object. It is not present in the Sonic 3C 0408 prototype.

Level ID 0C (The Doomsday Zone)

There is not much to say about this zone, just another Sandopolis basically.

• The title card refers to this level as 'Zone' without any 'Act 1' or 'Act 2'. This is because this Zone didn't use Act numbers in Sonic & Knuckles and Sonic 3 & Knuckles.
• There is a repeating pattern of rings in this zone.
• This Zone uses Angel Island Zone intro graphics.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Act 2 uses Angel Island Act 2's palette.
• Act 2 of this zone was never used in Sonic & Knuckles. (This act is not to be confused with the Doomsday Act 2 that is listed in the level selects of both Sonic & Knuckles and Sonic 3 & Knuckles.)
• Whenever this zone is accessed using the Game Genie codes up above, the game crashes with a purple screen. This is most likely because most of the data that once belonged to this zone is no longer present.
• Both Acts use the correct music.
• Act 1 uses Angel Island Act 1's intro palette.
• This zone cannot be completed.

Level ID 0D

This zone has some wacky palette problems like Sky Sanctuary.

• The title card refers to this level as 'Zone' followed by an Act 1 or 2 depending on whichever you're playing.
• The way to start this zone without crash is use a new sonic 3 alone rom and don't hack anything, patching codes or play with it, start the game, go to the competition mode, select grandprix, select sonic and patch the code (FFF600:2400) before select sonic, go to the sonic 2 options, unlock sonic 3's level select using sonic 2 level select cheat way,go to the level select, patch the Game Genie uncrash code for id 0D zone and you are done (also patch the FFFE10:0D00 code), (this was done by Miya Saki and there's a video about it)
• Another way to start this zone is to enter the zone and immiediately escape it, then start any other zone(e.g. Hydrocity). Eventually,other zones,including 0D, will use any zone you first entered after title card and start location due to RAM corruption. This corruption is caused by 0D title card for unknown reasons, so using another zone title card or removing it will cause 0D to load normally.
• Entering this level causes the game to shove your character through to the left boundary of the stage. This now sets your position to illegal coordinates and begins corrupting the object load manager and other RAM addresses. The camera scrolls to the left, then seemingly freezes and the HUD vanishes for about 5 seconds. During this time the game enters demo mode and you can hear the sound of your character dying. Then, the game continues and the HUD reappears, but sets the camera to another position within the zone and locks up completely, but not before hearing the sound of your character dying again. The Z80 sub-processor also fails and lets out a loud ringing sound. This cannot be escaped with debug mode. This can, however be escaped by using Start + A or pressing the Start at the correct time during this sequence. The reason for this is because during the RAM corruption the game enters demo mode (much like breaking too many S Monitors in Sonic 3 & Knuckles). Upon returning to the level select, you are greeted with a near endless selection of real zones with strange glitches, and many invalid levels.
• This zone uses Angel Island graphics.
• This zone contains randomly placed rings.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Act 2 uses Angel Island Act 2's palette.
• Act 2 of this zone was used for the ending in Sonic & Knuckles and Sonic 3 & Knuckles, Act 1 was never used.
• This zone is almost exactly the same as it was in the Sonic 3C 0408 prototype.
• This zone uses Special Stage music.
• Act 1 uses Angel Island Act 1's intro palette.
• This zone cannot be completed, nor can it even be started normally.

Level ID 14 (Glowing Spheres Bonus Stage)

• The color palette arrangement for this level is exactly the same as that used in Sonic 3C (0408 Prototype), the color palettes have no color changes like Sonic & Knuckles.

• The title card correctly refers to this zone as 'Bonus Stage'.
• Some of the level layout is intact, it is nearly identical to the Sonic 3C 0408 layout.
• There are a few rings, but they don't appear to go with the level layout at all.
• The HUD uses glitched graphics, and only has 'RINGS'.
• You can access this Zone using a real console or a Sonic 3C 0408 save state.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• Palette data is intact. (As seen above).
• This Zone curiously uses the Slot Machine Bonus Stage music.
• You can only place objects near the beginning; if you try to place objects past a certain point nothing will happen. (This is also true for some of the other Sonic & Knuckles levels).
• This bonus stage cannot be completed.

Level ID 15 (Slot Machine Bonus Stage)

• Unused color palettes can be found at level ID 15, which is used by the Slot Machine Bonus Stage in Sonic & Knuckles. This arrangement is quite different from what is used in Sonic 3C (0408 Prototype) and Sonic & Knuckles, but it is still very similar.

• The title card correctly refers to this zone as 'Bonus Stage'.
• This Zone curiously uses the Gumball Bonus Stage music, like in the Nov-3-1993 Prototype.
• There is one ring collectible and another non-collectible ring, but they don't appear to go with the level layout at all.
• Palette data is intact (as seen above).
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• You can only place objects near the beginning; if you try to place objects past a certain point, nothing will happen. (This is also true for some of the other Sonic & Knuckles levels.)
• The level layout looks simple, the map is intact and the background is as well. However, the chunks, blocks, and tiles data has been removed. This map is very different from the Sonic 3C 0408 version.
• The HUD uses glitched graphics, and only has 'RINGS' unlike the Nov-3-1993 Prototype .
• You can access this Zone using a real console or a Sonic 3C 0408 save state.
• This bonus stage cannot be completed.

Level ID 16 Act 1 (Lava Reef Zone Boss/Act 3)

Looks to be nothing more than broken data. It doesn't even seem to use Lava Reef's data.

• This act has no title card.
• This act uses Angel Island Act 1's intro palette.
• This act uses Special Stage music just like ID 0D does.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• This act cannot be completed.
• This act contains a constantly repeating ring, some are collectible, while others are not.
• The HUD is missing the 'TIME' and 'SCORE' and only has 'RINGS' listed. The lives number is also strangely missing.
• You can only place objects near the beginning; if you try to place objects past a certain point, nothing will happen. (This is also true for some of the other Sonic & Knuckles levels).

Level ID 16 Act 2 (Hidden Palace Zone)

This level has a strange trigger that loads Launch Base Death Egg graphics.

• This act has no title card.
• This act uses Angel Island Act 2's palette.
• This act contains a constantly repeating ring, most likely as a placeholder. This ring is strange, however, as it appears even in levels with an ID of 18 and after it , which is odd considering these are not 'true levels' but garbage data.
• There is some kind of level 'event trigger' that causes Launch Base Act 2 Death Egg graphics to load about halfway through the Act. When this happens, it gets rid of most of the glitched collision and all of the garbled Angel Island graphics. Interestingly, the same thing happens in Sonic 3 & Knuckles, although it is done in a slightly different way.
• The HUD is missing the 'TIME' and 'SCORE' and only has 'RINGS' listed. The lives number is also strangely missing.
• You can only place objects near the beginning; if you try to place objects past a certain point, nothing will happen. (This is also true for some of the other Sonic & Knuckles levels.)
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• This act uses Special Stage music just like ID 0D does.
• This act uses Angel Island graphics until the event trigger occurs.
• This act cannot be completed.

Level ID 17 Act 1 (Death Egg Zone Act 3/Boss)

Another seemingly empty level slot, it highly resembles Lava Reef Act 3.

• This act has no title card.
• This act cannot be completed.
• This act uses Angel Island intro graphics.
• This act uses Angel Island Act 1's intro palette.
• This act uses Special Stage music just like ID 0D does.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• You can only place objects near the beginning; if you try to place objects past a certain point, nothing will happen. (This is also true for some of the other Sonic & Knuckles levels.)
• The HUD is missing the 'TIME' and 'SCORE' and only has 'RINGS' listed. The lives number is also strangely missing.

Level ID 17 Act 2 (Hidden Palace Special Stage Ring)

This Zone resembles Hidden Palace without any Death Egg graphics.

• In Sonic 3 & Knuckles, this act was the Hidden Palace you would go to when you entered a giant Special Stage ring.
• This act has no title card.
• This act cannot be completed.
• This act uses Angel Island intro graphics.
• This act uses Angel Island Act 2's palette.
• This act uses Special Stage music.
• The debug item list contains the usual ring, super monitor, a vertical spike, debug rings, and various facing springs.
• You can only place objects near the beginning; if you try to place objects past a certain point, nothing will happen. (This is also true for some of the other Sonic & Knuckles levels.)
• The HUD is missing the 'TIME' and 'SCORE' and only has 'RINGS' listed. The lives number is also strangely missing.
• Any Zone after this point is just strange garbage data. No other Zones are known to exist.

Level Differences

Flying Battery Zone

In Act 1, the mid-boss capsule is in a different position. In the Sonic 3C 0408 prototype, the boss is in the same position as Sonic 3.

A ring layout identical to Sonic 3C 408's ring layouts in The Doomsday Zone, Death Egg Zone final boss, and the Glowing Spheres Bonus Stage exists in this zone, suggesting that either Flying Battery barely begun development before moving to Sonic & Knuckles, or it was removed from Sonic 3 similar to Sonic 2's Hidden Palace Zone's removal.

In Act 2, the location of the boss is very different.

The prison egg was also dragged a bit far in Sonic & Knuckles.

Mushroom Valley Object Data

Object data of a zone that is found after Launch Base's data can be found at these ranges:

Act 1: 0x0069A4A to 0x0069B4B

Act 2: 0x0069B4C to 0x0069C0B

The data mostly consists of path swappers, springs, and monitors, and a Madmole, which deletes itself once it's loaded, suggesting that this object's data is no longer present or accessible. Interestingly, this zone (ID 07) went through a good bit of layout changes from Sonic 3C 0408 to the final version of Sonic & Knuckles. It could be possible that this is an object layout from a very early version of Mushroom Hill Zone (referred to as Mushroom Valley in this game).

Object List

Act 1

Act 2

Unused Enemies

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Both of these Badniks are curiously listed among all the other Badniks in the US and European manuals for Sonic 3.

• Technosqueek: This Badnik later shows up in Sonic & Knuckles. For some reason, if you place a sideways one in Debug Mode, it crashes the game.
• Blaster: Apparently the same as in Sonic & Knuckles, but their projectiles can be destroyed with a spin attack, freeing an animal.

Unused Bosses

• Gapsule: The Flying Battery Act 1 boss. Nearly identical to the one in Sonic & Knuckles.
• Flying Battery Laser Boss: The first boss of Flying Battery Act 2. Nearly identical to the one in Sonic & Knuckles, except for missing sound effects. This boss doesn't appear in the unfinished Flying Battery like in Sonic 3 Nov-3-1993 Prototype ; you can place it there with the following Action Replay codes:

• Hang Mobile: The second boss of Flying Battery Act 2. Almost the same as the one in Sonic & Knuckles, it is only slightly different (in terms of coding) and is missing sound effects.
• Knuckles' Marble Garden Act 2 Boss: This boss actually still exists in the game, but is not surprisingly unused. It acts nearly identical to the one in Sonic 3 & Knuckles, except that it's invincible.

Starting Points

In Sonic 3, multiple starting points found at offset 05E018 are left intact, which may suggest that many levels have already been cut with enough content. In comparison with Sonic 3 & Knuckles, the starting points for Flying Battery Act 2, Lava Reef Act 1, and the ending cutscenes have never been changed.

The other levels that have not been mentioned could have had different level designs from what we know. Hidden Palace, Lava Reef Act 2 Boss, the Final Boss of Death Egg, and the Glowing Spheres and Slot Machine Bonus Stages share the same starting points, which suggests that these levels were too incomplete or did not yet have custom starting locations.( although the glowing spheres has finished layout on the nov-3-1993 prototype )

Sonic & Knuckles is a stand-alone expansion pack from Sonic the Hedgehog 3 that is a good game in its own right, but when combined with Sonic 3 becomes what could arguably be the best Sonic game ever.

• 6Unused Sprites
• 8Unused Zones
• 9Unused Areas
• 10Misplaced Objects

Level Select

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The level select can be accessed by starting the game and grabbing one of the lifts in Mushroom Hill Zone. Once you have grabbed a lift, press Left, Left, Left, Right, Right, Right, Up, Up, Up. If done correctly, you should hear a ring chime.

When paused, you can perform the following actions:

• A - Return to title screen.
• B - Play at half-speed.
• C - Advance by one frame.

Press Start to pause, then A to return to the title screen. At the title screen, press A + Start to access the level select. From there, you can press C to choose between Sonic (01) and Knuckles (03) only, and trying to select Sonic & Tails (00) or Tails (02) will give you Sonic alone. The Sonic 3 zones are also listed here, but choosing any of them will redirect you to Mushroom Hill. Entries in the sound test which will normally have songs only heard in Sonic 3 and Drowning are empty and unplayable.

Notes about the levels:

• Lava Reef Act 3 goes to the boss room. Knuckles cannot select this level.
• Lava Reef Act 4 goes to Hidden Palace Zone. Each character goes to their respective entrance in the level.
• Sky Sanctuary Act 1 is Sonic's level and Act 2 is Knuckles'. Sonic cannot go to Knuckles' level and vice-versa.
• Knuckles can enter Death Egg, but he cannot defeat the Act 1 boss (he cannot jump high enough to hit the boss in the first phase).
• If Sonic beats The Doomsday Zone, the bad ending will play if you don't have all the Chaos Emeralds.
• The Doomsday Act 2 leads to the final boss of Death Egg. If Knuckles defeats the boss, the game will reset to the SEGA screen.
• Knuckles cannot enter either level of The Doomsday.
• Bonus 1 goes to the Glowing Spheres game. Bonus 2 goes to the Slot Machine game. Finishing each stage will restart it.
• The only remnants of the Sonic 3 zones are the patches made to various zones. These still rely on the data from Sonic 3.

Debug Mode

Debug mode cannot be unlocked by normal means. It can only be unlocked using two means:

1. By using the Game Genie code RFTT-A6XW, then pressing A + Start on the title screen or when in the level select.
2. By utilizing a glitch in Sonic 3 & Knuckles whereby placing a long string of 'S' monitors in a row using debug mode, followed by spindashing into them, will cause the game to crash with a garbled screen. Pressing the Start button on this screen will load Sonic & Knuckles with level select and debug mode enabled. Using the Wii Virtual Console's 'Lock-on Menu', it is shown that the game still thinks that it's locked on to Sonic 3.

When in debug mode there, you can enter both normal and edit mode in-game. Normal mode is similar to normal gameplay but with added debug features, while edit mode is a mode where you can place objects anywhere in the level.

When in normal mode:

• Press A to reverse gravity (reaching the top of the level will kill you as if you fell off a bottomless cliff).
• Press B to enter edit mode.
• Press B + C to play through all the frames of the current characters' animations. (May work in edit mode, needs testing!)

When in edit mode:

• The coordinates of the current object are shown by the top line of hex digits in the HUD.
• The coordinates of the current viewport are shown by the bottom line of hex digits in the HUD.
• Press A to cycle forwards through the object list.
• Press A + C to cycle backwards through the object list.
• Press B to enter normal mode.
• Press C to place the current object shown.

You can use debug mode to enter any level a character would normally not be able to enter (see above), though in some cases this causes player sprites to become corrupted.

Entering edit mode during some parts of the game (such as the light tubes in Death Egg) will crash the game.

8th Special Stage

If Sound Test is enabled, and any of the Special Stage options is selected, Sonic or Knuckles is taken to the 1st Special Stage. The end of it however, spawns a Yellow Super Emerald. This is when the Special Stage is marked as clear, but the normal Chaos Emerald amount doesn't rise. The Super Emerald amount will rise, but it has pretty much no effect due to Super Emeralds being unobtainable. This bug will allow access to a hidden Special Stage and allow for the 8th Emerald to be collected once the other six have, though.

It is also possible to access this Special Stage by enabling debug mode above, then setting the Sound Test selection to 07 and pressing A + Start while selecting any of the Special Stage options.

Tails

As with Knuckles being unavailable in Sonic 3 but with a few leftovers, Tails is not available in this game. Unlike Knuckles in Sonic 3, however, he is in an actual playable form! Unfortunately, most of his sprites are missing and are replaced with corrupted graphics.

If the Pro Action Replay code FFFF09:02 is used in Sonic & Knuckles, Tails will be usable, but just about every sprite of his except the transformation sprite will be missing, basically rendering him invisible (unlike when a savestate is used; a different, glitchy sprite is used in place of Tails' original one). There are four exceptions:

1. Tails' sprites used at the end of Sky Sanctuary Zone to run around the collapsing pillar are actually stored in the S&K ROM, and therefore are visible. The same sprites are also used in the same context in Lava Reef Zone.
2. Tails' hanging sprites from Sky Sanctuary are also visible.
3. Tails hanging onto the mushroom paragliders in Mushroom Hill Zone.
4. In Tails' bad ending, you can see him normally when he and Sonic fly away from the island.

Hidden Palace's Hidden Area

There's a hidden area in Hidden Palace Zone that is only accessible by debug mode or having Tails in Sonic 3 & Knuckles.

Just before entering the inside of Hidden Palace, fly from the two steps left until a ledge is reached. Walk left and six power-up monitors will appear, mostly shield monitors (Ring, Bubble, Fireball). In some of the S&K prototypes, an 'S' monitor is present instead of the Speed Shoes.

If you run right, leap off the ledge, and keep running right, you will find two Ring monitors and a 1-up monitor.(video)

Unused Sprites

Hidden Message in Slot Machine Bonus

Japanese characters are used as placeholders for slots animations. Read from top to bottom and from left to right, they are the first few parts of the Iroha poem.

Unused Music

A short 'Game Clear' jingle heard by playing sound 32 in the Sound Test. Though Sonic 3 used this sound ID for its own jingle played after defeating the final boss, it isn't used in Sonic & Knuckles at all. A longer version of this jingle can be heard at the end of the credits medley, though.

Unused Zones

There are a few acts that go unused.

The True Doomsday Act 2

Contrary to what the level select would lead you to believe, the real Doomsday Act 2 is inaccessible by normal means (unless a glitch is used or using patch code FFFE10:0C01). The level is basically empty except for some chunks off-screen. Super Sonic dies shortly after transformation due to being given 0 rings at the start of the level.

Slot Machine Bonus Stage Act 2

Basically the same as the regular version.

Glowing Spheres Act 2

The only major difference is the different spawn point, which places you inside a wall.

ID 0D Act 1

Crashes on entry, presumably before the title card even loads. This zone uses Angel Island graphics (which are non-existent in S&K) as a placeholder, which is one of the reasons the Act crashes. The only thing this act is ever used for is giving Hidden Palace its own title card. In Sonic 3, this zone has its own unique ring layout, possibly suggesting it was something entirely different at one point in development. For reference, ID 0D Act 2 is the ending sequence for Sonic.

Gumball Machine Bonus stage

Sonic The Hedgehog Toys

This bonus stage was in Sonic 3 (alone) and in Sonic 3 & Knuckles. It goes absent in this game and simply crashes on the title card.

Unused Areas

Lava Reef Zone Act 1

There are three empty bonus rooms in Lava Reef Act 1. They appear to have been used to hold Special Stage rings at some point in development. Inside, there are tunnels that would have allowed you to enter and exit them, except that they have been blocked off by invisible solid objects. They appear to be remnants from an early version of the stage, like the one that was restored from the Sonic 3 Nov 3rd, 1993 prototype. The locations are as follows:

1. To the right of the first Spin Dash elevator at coordinates 09C005C0.
2. To the right of the Spin Dash elevator encountered after the driller badnik, at coordinates 13C00740.
3. Beneath the crushers on the top path shortly after Sonic's path splits into two, at coordinates 21400540.

Death Egg Zone Act 1

At the top of the first wide conveyor belt elevator sequence in the Act is a hidden room containing two ring monitors and an invincibility monitor. However, the corridor leading to the room is completely blocked off by spikes, making the room impossible to get into without debug mode.

Misplaced Objects

Flying Battery Zone Act 1

There are two yellow springs buried in the ground near the first giant purple tube.

Sonic The Hedgehog Slot Machine

Mysterious spikes are hidden in the floor of Flying Battery Zone 1.

Death Egg Zone Act 1

There are three rings placed in the floor under the first black antigravity ball. It is possible to collect these rings by returning to this point with a Lightning Shield.

Sonic 3 Level Select Code

The Sonic 3 level select code... code is still present in the ROM and can be reactivated via hacking. Interestingly, it was updated to work on the Sonic & Knuckles title, and to be inputted at all times in the Sonic 3 title (unlike Sonic 3 Alone, in which you can only input it after the Sega logo but before the title screen appears, which is made harder via the ridiculous number of lag frames).