One of the things that ''Franchise/SonicTheHedgehog'' is known for is its wealth of DummiedOut content, to the point where entire communities are dedicated to taking the games apart and forming theories about the games' development cycles.
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[[folder:''Sonic the Hedgehog'' (1991)]]
* While not nearly as much as later games, ''[[VideoGame/SonicTheHedgehog1 Sonic the Hedgehog]]'' had its fair share of removed content; the [[http://info.sonicretro.org/Splats Splats]] enemy never appeared without hacking, yet [[WolverinePublicity prominently appeared in promotional art and merchandise]]. There's also an unused checkered ball obstacle, whose graphics were reused for the first boss and showed up in some capacity for the ''Sonic 2'' prototypes.
* Going into debug mode in the special stages in the first game, and teleporting yourself outside the maze will soon enough reveal random graphics for powerups that were never used in the final game, including a 'W' powerup, '1 zone', '2 zone', and '3 zone' powerups, and a 1-up, which is mentioned in the manual.
* The [=iOS=] port cleans up and adds coding for most of the game's unused objects, but they still aren't used.
* There's an [[https://www.youtube.com/watch?v=fwXkhaqrt6c 8-bit rendition of Marble Zone's stage theme]] buried in the Platform/SegaMasterSystem [[ReformulatedGame version]]'s code, suggesting it shared levels with the Mega Drive version early in development.
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[[folder:''Sonic the Hedgehog 2'']]
Overall, ''[[VideoGame/SonicTheHedgehog2 Sonic 2]]'' is '''legendary''' for its removed content, largely thanks to the prototypes dumped on the internet over the years.

* There are ''many'' unused sprites and animations for Sonic. Some of them were referenced in the spritework for ''VideoGame/SonicMania'', and most of them are buried in the code for the 2013 mobile remaster.
* The game code has level slots intended for scrapped zones. One intended for [[TheLostWoods Wood Zone]] leads to an objectless, miscolored version of Emerald Hill Act 1, and the other, intended for the [[UrbanLegendOfZelda infamous]] Hidden Palace, retains that level's layout and palette, but not the collision--thus Sonic and Tails just fall to their deaths.
** The latter stage plays the unused [[https://tcrf.net/images/7/74/Sonic_2_HPZ.ogg track #10]] from the SoundTest. [[https://tcrf.net/images/5/5f/StH2_Unused_Song_Masa%27s_Demo_Version.ogg Masato Nakamura's demo]], unlike all the other stage music demos, ends properly and doesn't loop, suggesting it may have been intended for a cutscene of some sort.
* During the fight against the Silver Sonic in Death Egg Zone, Dr. Eggman lurks from a little window in the back. He has a fully implemented laughing animation for when Sonic gets hit, but since Death Egg [[DroughtLevelOfDoom has no rings to collect]], you die in one hit and all animation is frozen.
* [[http://info.sonicretro.org/images/3/3e/DesertlevelCorrectColor.JPG This promo screenshot]] showing a desert zone baffled fans for years. Level designer Brenda Ross confirmed its name as [[http://info.sonicretro.org/Brenda_Ross_interview_by_Deviance_(February_2001) Dust Hill Zone]] in an interview; she'd also planned a snow level that would've [[PaletteSwap reused most of its graphics]] with the cacti replaced by Christmas trees.
* The manual for the [[ReformulatedGame 8-bit version]] clearly mentions a SprintShoes power-up, but it only appears once in the Platform/SegaMasterSystem version, and is replaced with a ring box in the Platform/GameGear version.
** Tails may have been intended to be playable in this version; despite his highly-detailed spritesheet, he's non-playable.

!!Prototypes

* The '''Simon Wai prototype''' is so named after the man who released its ROM to the internet, after finding it on a Chinese ROM site. The cartridge containing it was stolen from a New York toy fair in 1992, then dumped into the internet and endlessly bootlegged by Chinese and South American pirates.
** There are three level slots for scrapped levels. Hidden Palace Zone and Wood Zone are semi-playable, but many areas can't be reached without debug mode. [[https://www.youtube.com/watch?v=LZAIyzRAyug Hidden Palace]] is about two-thirds complete, while [[https://www.youtube.com/watch?v=NWf_XmEu9cA Wood Zone]] has a garbled and very incomplete layout. The third one is a level entry for the aforementioned Dust Hill, which actually leads to Mystic Cave; because this build was sent to gaming press, Mystic Cave was often called by the former's name.
** The third one is for the [[NamesToRunAwayFromReallyFast woefully-named]] Genocide City Zone. Despite the name, though, Yuji Naka stated in the ''VideoGame/SonicJam'' strategy guide that it was meant to be a single-act industrial level to follow up Metropolis Zone. Level artist Tom Payne, [[http://info.sonicretro.org/Tom_Payne_interview_by_SageXPO_(July_30,_2009) when interviewed]], said that its layout was recycled into Metropolis Act 3, and the level itself didn't last very long in development. As for the name, [[DidNotDoTheBloodyResearch Sonic Team just didn't know English that well]], and as soon as they figured out the implications changed its name to Cyber City before it was finally scrapped.
** Other interesting fragments of sprites from non-Sonic games in the ROM memory suggested that the development team cannibalized spare/rejected ROM chips from other games to burn their prototypes into.
* The '''''Series/NickArcade''''' '''prototype''' appeared in an episode of that show and dates even earlier than the Simon Wai build, but was found later.
** The build still has a lot of assets left over from ''Sonic 1'', including stage music, Green Hill's tiles and some raw code left within.
** Sonic bounces off walls if he runs into them, and Tails loses rings for the player when hit.
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[[folder:''Sonic CD'']]
* ''[[VideoGame/SonicTheHedgehogCD Sonic CD]]''[='=]s ending video and the D.A. Garden screen hint at a scrapped [[ShiftingSandLand desert level]]. Since design and programming of this game's levels were given to individual people, it seems this level was scrapped because [[ScheduleSlip its designer couldn't finish it on time.]]
** [[UpdatedRerelease The 2011 remaster]] contains an EasterEgg with a [[https://www.sonicstadium.org/wp-content/uploads/2011/12/Sonic-CD-Desert-Dazzle.png mockup of the desert level]]; developer Christian Whitehead said on the Sonic Retro forums that Sonic Team didn't want the rerelease to deviate too much from the original version, so the level stayed in the cutting room floor. The desert level concept would eventually be revived in ''VideoGame/SonicMania''[='=]s Mirage Saloon.
* The 1996 PC version's level files skip from [=R1=] (level 1) to [=R3=]; there is no [=R2=]. [[http://info.sonicretro.org/File:SCDconcept-R2RainbowWaterfalls.jpg This extremely crude concept art]] intended for [=R2=] shows a structure much like that of ''Sonic 2''[='=]s Aquatic Ruin Zone, plus rainbow waterfalls and some mountains to the back. The ending video's second sequence shows a ruins area, with moss and marble tiles, featuring an antlion enemy trying to catch Sonic in its pincers.
** Whitehead also shared [[http://www.christianwhitehead.com/Stuff/R2Sprites.png some enemy sprites]] left over from [=R2=], including the antlion enemy from the ending video and an incomplete boss sprite for Eggman in an unicycle.
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[[folder:''Sonic the Hedgehog 3'']]
* ''Sonic 3'' seemed ''full'' of this, with unreachable areas, half-implemented bosses, etc. The ExpansionPack ''[[VideoGame/Sonic3AndKnuckles Sonic & Knuckles]]'', released later, contained [[OneGameForThePriceOfTwo the rest of the game]]. There are still a few mysteries in the DebugMode, like a revolving cluster of red spheres that can be manipulated with the second controller, likely used to test the Special Stages' graphics, and sprites for Sonic on a surfboard.
* There's a Special Stage with a gold Chaos Emerald that can only be accessed via Debug Mode; you'll lose unless you have no eyelids. This stage's layout was recycled for ''VideoGame/SonicMania'', with rings in place of some blue spheres.
* The full game also inverts this: when playing Launch Base Zone, ''Sonic 3''[='=]s Big Arms boss and ending sequence are omitted, and finishing the level goes straight to Mushroom Hill Zone instead.[[note]]Unless you're playing as Knuckles. Or the ''Sonic Origins'' version.[[/note]]
[[/folder]]

[[folder:''Sonic Adventure'']]
* ''VideoGame/SonicAdventure'' had some details changed in localization: in the Japanese version, the title screen is brighter and static, and Sonic's feet blur at top speed like the WheelOFeet from the Mega Drive games. There's also the infamous cowgirl billboard in Casinopolis that was replaced with a generic marquee in the International version.
* Super Sonic also had text to recap the story events, like the other six characters. This ''can'' be viewed legitimately if you grab Sonic's optional upgrade in the Last Story and exit the game, but the recap [[UnintentionallyUnwinnable freezes the game and corrupts your save file]].
* Pre-release screenshots showed a [[http://sost.emulationzone.org/sonicadventure/dragon/dragon2.jpg two-headed dragon boss]] in Sky Chase, pursuing the Tornado. Its model is still on the disc, and modders rebuilt to to a working condition; but it has no attacks programmed, so [[https://www.youtube.com/watch?v=Yx-5O8pH3Jw all it does is follow you.]]
* Big the Cat was supposed to have to unlock the pool area of the hotel in order to enter Emerald Coast but instead the door simply opens for him when it's time to go to that stage. To do this he would have had to complete a small puzzle where he inserts two blocks into some slots that are adjacent to the pool door. Through glitching, this can still be seen in the final game and the puzzle is fully functional (although the text for the puzzle is untranslated from Japanese).
* There are two unused cutscenes in the game: [[https://www.youtube.com/watch?v=tEwkfHkudpE one]] of Eggman berating E-102 Gamma for failing to complete Final Egg, and [[https://www.youtube.com/watch?v=11sQUywrnUw another]] of Amy below the Egg Carrier deck before fighting ZERO which was never dubbed into English.
* [[https://www.youtube.com/watch?v=KyXIlPBANzY This unused voice clip from Tikal]] suggests that Super Sonic would've been playable outside the Last Story, by pressing the action button in mid-air with 50 rings like the Mega Drive games. In the final game, pressing action in mid-air just cancels the spin jump.
* [[http://forums.sonicretro.org/index.php?showtopic=31432 A fully-scripted prototype]] was discovered in 2013. The build dates from two months before the game's Japanese release, but has content from ''much'' earlier in development.
** [[http://sost.emulationzone.org/sonicadventure/beta_windy/windy1.jpg The early version of Windy Valley shown in the game's Japanese unveiling]] retains most of its geometry and level design, but is missing proper textures and refers to pointers even older than this build. [[https://www.youtube.com/watch?v=rAEgLe6PfaU This mod]] for the PC version of ''Sonic Adventure DX'' restores the level using resources from the [=AutoDemo=].
** Knuckles has many unused animations for fighting combos; none were used in the final or the [=AutoDemo=].
** The snake room in Lost World has a completely different layout, and the doors would have been obstacles that could crush Sonic.
* There are [[https://tcrf.net/Sonic_Adventure_(Dreamcast)/Unused_Audio several cut voice clips]] left over in the game data, including entire dialogue exchanges, that hint at unused mechanics.
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[[folder:''Sonic Adventure 2'']]
* Several levels have distant, unused development leftovers, like alternate goal rings and invisible level components (like the platform in Sky Rail and the grindrail in Meteor Herd).
* The Knuckles and Amy chao had stats much like the Tails chao, but the former two had no unlocking triggers.[[note]]Even then, the Tails chao [[OldSaveBonus could only be unlocked through]] ''VideoGame/PhantasyStarOnline Episode I & II'', [[GuideDangIt and not in-game through]] ''[=SA2=]''.[[/note]] [[UrbanLegendOfZelda Urban legends]] aside, those three chao were available in the [[http://chao.hippotank.com/sa2b/character-chao 2001 World Hobby Fair]], to be transferred to a Platform/NintendoGameCube memory card.
* [[https://www.youtube.com/watch?v=q8Im0r86dKE Much like in]] the first ''Adventure'', several voice clips were either cut or replaced with alternate takes. Interestingly, many of those unused lines from both games were [[https://www.youtube.com/watch?v=q8Im0r86dKE reused]] for ''Anime/SonicX''.
* [[https://www.textures-resource.com/resources/sheets/1/640.png Among Rouge's textures is one of her butt,]] clearly unused to keep the rating down.
* The Chao Kindergarten would've had a [[https://tcrf.net/images/thumb/b/bc/Sa2_ChaoLibraryLevel.png/320px-Sa2_ChaoLibraryLevel.png library]] and a playground. Cut text implies the library would've created new stories for the Dreamcast's Chao Adventure VMU game, while the playground is in a [[http://info.sonicretro.org/images/9/9e/Sa2kin_cophot2.png photo]] removed from all subsequent ports. [[https://tcrf.net/images/thumb/7/76/SA2ChaoPlaygroundRender.png/640px-SA2ChaoPlaygroundRender.png Here's what it would have looked like.]]
* [[https://www.youtube.com/watch?v=XGt__j7JOfc In this video]], Metal Sonic is the only character whose victory pose doesn't glitch. The ''Battle'' [[UpdatedRerelease release]] treats the [=2P=] mode's characters as Sonic and Shadow with altered stats; Metal Sonic's stats, however, are identical to Shadow's, and the game code doesn't differentiate between them.
[[/folder]]

[[folder:''Sonic Heroes'']]
* Team Rose have the easiest and shortest levels in the game, but since they play through the same physical levels as the other characters, the entire level data for each stage still exists. Normally a wall or an impassible gap separate the end of Team Rose's levels from the the rest of the other teams' paths, but in Casino Park and Frog Forest, you can use a Team Blast and the subsequent speed boost to just barely clear the gaps, and continue as normal. Team Rose can also go into the ghost-filled well in Mystic Mansion, by hitting a particular switch and flying very carefully. Interestingly, the characters have dialogue for level elements they don't normally encounter (like the VIP pinball room and the giant mushroom).
** There are also [[https://www.youtube.com/watch?v=srBLlPB7N2Q several unused voice clips]] hinting at removed gameplay mechanics.
[[/folder]]

[[folder:''Sonic the Hedgehog'' (2006)]]
* Modders have found a half-implemented "Rainbow Gem" in the game's code, which was intended to transform Sonic into Super Sonic. However, if hacked into the game, all it does is play an animation. There are unused particle effects for Super Sonic, but otherwise there's no code suggesting at what he was supposed to play like.
* The manual lists a classic-style Shield as an available power up, but none ever appear in the game. Modders have found that it's fully functional in the game's code and can be placed by modifying a power up box, only missing some minor polish like a sound effect when collected.
* The file containing the shop's text lists a speed chip for Silver, as well as music from past Sonic games, as purchasable items.
* Another text file lists mission objectives such as collecting rings, finding lost Chao, or finding a new goal ring in a different location, making it seem like each level was going to have multiple objectives like in Sonic Adventure 2.
[[/folder]]

[[folder:''Sonic Generations'']]
* Big the Cat was slated to appear in the game, with voice actors listed in the end credits. He was likely the host friend for Speed Highway, cut because he had nothing to do in the stage--being a fisherman and power lifter in a stage with no water or physical obstacles.
* The game files contain an unused bio for Super Sonic.
* Alongside the other skill upgrades are descriptions for the Orange and Pink Wisps, suggesting they were planned to be available as unlockable skills. In the final game, they only appear on Planet Wisp.
* There are lots of leftovers from ''Sonic Unleashed'', the biggest one being the '''entire renderer for the game'''. This is notable because Sonic Unleashed renders the game world with better lighting and shadows than Sonic Generations, but is more resource-intensive. On the PC version, it can be re-enabled with a few hex edits to the game's EXE, allowing users with higher-end [=PCs=] to see the game with even more visual fidelity.
[[/folder]]

[[folder:''Sonic Mania'']]
* Green Hill's unused wrecking ball object has returned--while ''[[RunningGag still]]'' [[RunningGag unused]], it can be placed in DebugMode and behave logically as an obstacle.
* Several levels have details and features obscured by the camera, only accessible with DebugMode: Press Garden has the word "Hello" backwards somewhere below the main path, Hydrocity Act 1 has a texture panel hidden at the far top-left, close to the player's spawn, and Knuckles's portion of Lava Reef Act 2 goes well beyond the camera's locking point, well into the Hidden Palace mural.
* Hidden in the data files are some unused sprites and assets from the mobile remasters of ''Sonic 1'', ''2'', and ''CD'' used to test the engine. There are also leftovers from the preview builds, like the "coming soon" screen from the end of a public demo, original level layouts used for the teasers and previews, and even SEGA's original level map data for the legacy zones.
* There are some other unused elements and even bosses in the code, such as a scrapped and partially-functioning boss for Studiopolis.
[[/folder]]

[[folder:Other games]]
* An early version of Sunset Park Act 3's music is in ''VideoGame/SonicChaos''[='=]s SoundTest. While not used in-game, it returned for ''VideoGame/SonicTripleTrouble''.
* ''VideoGame/KnucklesChaotix'' has at least one character, plus several items and level features cut from the final. ''Chaotix'' began development as ''Sonic Crackers'', an internal engine test featuring Sonic and Tails; it can be inferred that since Mighty is ultimately a spriteswap of Sonic, "Wechnia" is Tails. He was probably removed because two flying characters in ''Chaotix'' would be redundant and [[GameBreaker throw off the game balance]].
** More traditional ''Sonic'' level elements like loop-de-loops can be found in prototypes, but aren't in the final, likely due to programming issues. The final also has many unused graphics, including sprites for a possible removed cutscene with Super Sonic and the Tornado.
* In ''[[VideoGame/SegaSuperstars Sonic & Sega All Stars Racing]]'', Final Fortress's track descriptions talk about laser gates and a cameo that don't exist.
* The ''Sonic Gems Collection'' compilation had a few games planned for inclusion, but removed - like the ''VideoGame/WonderBoy'' Mega Drive games and ''Sonic Eraser'', a highly obscure puzzle game released on the Japan-only Mega Modem peripheral.
** ''VideoGame/BonanzaBros'' and all three ''VideoGame/StreetsOfRage'' games were in the Japanese release, but were removed for the US version. However, there are translated versions of their descriptions still unused in the US version, suggesting they were planned for inclusion and later dropped.
** WordOfGod states that ''VideoGame/SegaSonicTheHedgehog'' was also considered at early stages, though eventually turned down due to trouble emulating the trackball controls.
** A significant amount of the game's art gallery is dedicated to ''VideoGame/KnucklesChaotix'', suggesting it might've been planned for inclusion at some point.
** An Xbox port is also mentioned several times throughout the code, but it was never released.
* ''VideoGame/SonicChronicles'' has the 5 Viral Chao. Said Chao were to be released in events, although only one was ever officially released. The rest of the Chao, excluding one, were included in the Japan release.
* From test environments floating far away from normal access, to lines involving Perci, one of the [=NPCs=], still activating in a level as if she was playable (even though she just stands around and gives you quests at best), ''VideoGame/SonicBoom: Rise of Lyric'' bears all the scars of its TroubledProduction.
* ''VideoGame/ShadowTheHedgehog'' was found to have [[http://www.sonicretro.org/2011/03/two-test-levels-found-in-shadow-the-hedgehog/ a pair of unused maps]] buried in the code of the game, which were only found six years after the game's release using an old Renderware Graphics World Viewer. The odd design and small size of the maps suggests they were used to test the physics and were not going to be made accessible.
* ''Sonic Mega Collection'' has data indicating that ''VideoGame/MonsterLair'' and ''VideoGame/{{Vectorman}}'' were originally going to be included, and some unrelated videos (including footage of a San Francisco tram, and an interesting 3D animation about a human boy, [=R2-D2=]-esque robot, and DraconicHumanoid who travel through a portal to a fantasy world) that take up significant amounts of space on the disk, even though lack of space was supposedly the reason ''VideoGame/SonicCD'' wasn't on the compilation.
[[/folder]]