The original game had its fair share of removed work, most notably the enemy "Splats the Rabbit", who never appeared without hacking, yet whose image got into the promotional artworks, and turned up in the comics and toy lines for it. There's also the checkered ball obstacle, which had its graphics reused for Robotnik's wrecking ball and was sort of brought back for use in the Sonic 2 prototypes.
Going into debug mode in the special stages in the first game, and teleporting yourself outside the maze can produce some interesting results. You will eventually run into random graphics of powerups that were never used in the final game, including a 'W' powerup, '1 zone', '2 zone', and '3 zone' powerups, and an extra life powerup, the latter of 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.
It seems that the Game Gear and Master Systems versions of the game were originally supposed to have the same levels as the Genesis/Megadrive versions, as deep within the Master Systems game code there exists an unused 8-bit rendition of the Marble Zone theme.
Quite a few different prototypes of Sonic 2 have been found. Two in particular are quite interesting: The first one discovered, the so-called Simon Wai prototype (named after the person who released the ROM to the Sonic community; he in turn found it on a Chinese ROM site) is a dump of a fairly late build that was stolen from a toy fair in New York in 1992. The second is the Nick Arcade prototype (sometimes called the early proto, since it is from a fairly early development stage), which seems to match a build used on an episode of Nick Arcade (starring Melissa Joan Hart!)
Hidden Palace Zone's music can be found in the Sound Test in Sonic the Hedgehog 2 and the level layout is also contained in the ROM. The sprites, however, aren't, which means that hacking to access the level produces a very garbled screen where Sonic and Tails fall and die. It also contains the palette for the further-deleted Wood Zone. However, Hidden Palace Zone, after over 20 years, was made available in the Android and iOS ports in December 2013. Here is a look. It uses the music that was used in the beta version, which is Mystic Cave Zone's 2 player music.
The beta also had the blank Genocide City Zone, a planned single act zone whose map became Metropolis Zone Act 3. The story goes that one of the (Japanese) designers wanted a word that sounded "dangerous" and overshot. Once they realized what it meant they quickly changed that name to Cyber City before time constraints led to them making it into Metropolis Act 3. They didn't have enough time to give Cyber City all its own graphics, but they didn't want to waste a perfectly good level map.
A promotional screenshot showing a "Desert Zone" baffled fans for years. In the "Simon Wai" prototype, there is a level select entry for "Dust Hill Zone," which in fact led to Mystic Cave Zone. This led many fans to assume the desert level had been named Dust Hill Zone and possibly worked on before being overwritten by Mystic Cave Zone. Much research has been done, but fans have yet to agree if it ever existed and what it was called, and the screenshot has been confirmed to be a mock-up. Supposedly, the zone would have also had a winter-themed level use most of its graphics later on, with the cacti replaced by Christmas trees.
Sonic 3 seemed to be full of this sort of thing - unreachable areas, half-implemented bosses, enemies which appeared in the manual but not in the game proper, etc. - until the Expansion PackSonic And Knuckles was released, and gamers were allowed to buy the second half of the game. There are still a few mysteries that appear in the Debug Mode, though, like a surfing Sonic, and a giant cluster of red spheres which can be affected by messing with the second controller and was probably used to test the Special Stage pseudo-3D graphics. Surfing Sonic was meant for the intro, but was pretty obviously scrapped because someone on the dev team decided that Sonic going super to infiltrate the island would be more interesting.
This also works backwards: When playing Launch Base Zone (the final level of Sonic 3) in Sonic 3 & Knuckles, the original end boss and ending sequence of Sonic 3 (including the original end credits music) are omitted in favor of carrying straight on to the first level of Sonic & Knuckles instead. It's still possible to fight this boss by playing Sonic 3 individually as Sonic or Tails, or by playing Sonic 3 & Knuckles as Knuckles.
There's a Gold Chaos Emerald Special Stage, which can only be accessed via Debug Mode and is extremely difficult. You could try it by using debug, but you'll lose unless you have no eyelids. Some people think it might have been intended to be the Master Emerald, although this is rather unlikely.
In Sky Rail Zone, one of the platforms near the end of the level has an invisible section on its right-hand side. It would appear that another path was originally present here, and rather than remove or even dummy out this path, Sega simply made it invisible, so that gamers were unlikely to stumble upon it and wouldn't be able to follow it very far if they did.
Several levels have distant components, such as alternate goal rings or tiny, almost invisible platforms, that the player can see in the extreme distance, but are impossible to reach without cheat devices.
And to add more to the dummied-out bin, there were the Knuckles and Amy chao. They were both exactly like the Tails chao in terms of immortality, that both look somewhat like their namesakes and so forth, except that unlike the Tails Chao, both the Knuckles and Amy chao had no trigger to unlock either of them (even so, the Tails Chao requiredPhantasy Star Online Ep I and IIto unlock it andcouldn't be done so in-game withinAdventure) and because of this they had many rumors on how to unlock them among the debates on whether or not either exists. In truth they still exist and are fully functional, but both of them can only be unlocked by using Action Replay codes. Most likely, they were meant to be available through some kind of event, similar to the Pokémon series' Mew and Celebi. If such an event was held was previously unknown, however, Wikipedia has claimed they were downloaded to player's memory cards in an event only held in Japan only.
Metal Sonic is the only character in this video whose victory screen after the fight doesn't glitch, since the game more or less treats the 2P alternate characters as as alternate skins of Sonic and Shadow with slightly altered stats in the Battle version (They literally were plain alt skins in the original game) Metal Sonic's victory scene is done correctly because as far as the game is concerned, he's Shadow. This can be observed in other stages and bosses, making this more an example of Dummied In.
Knuckles Chaotix has at least one dummied out character, which the hacking community has dubbed Wechnia, and a number of items and level elements that weren't implemented in the final version of the game. This is because Chaotix was meant to be "Sonic Stadium", but somewhere along the lines developers and executives decided that the gameplay was too divergent from the previous games to keep with the idea. If you compare the sprites from the unreleased Sonic Stadium (also known as Sonic Crackers) closely, it's clear that Mighty is a sprite-edited Sonic, and actually shares moves and gameplay properties (he is the fastest character in the game, and in most games made after this one Sonic can wall jump), and "Wechnia" is Tails (he had the right palette in a rather early beta of the game, and in an even earlier version, he and Sonic both have character portraits). Tails was probably removed from the game because having two flying characters would be redundant and overly Game Breaking. The dummied out concept of a hub world made its way back into several later games, such as Sonic Battle. In addition more traditional Sonic level aspects such as loop de loop platforms are found in prototype versions (albeit working improperly), but are absent in the final product (likely due to programming issues). Multiple other unused graphics are also found in the final game or it's betas (such as bubble areas, implying water areas were intended at some point, and graphics of what seem to be an unused cutscene, including Super Sonic and the Tornado).
An early version of the level music "Sunset Park 3" can be found inside the Sound Test of Sonic Chaos. It was not used in that game, but was brought back for Triple Trouble.
In the Game Gear version of Sonic 2, the Sprint Shoes are clearly marked inside the manual, and yet they never appear in the game. The powerup only makes a single appearance in the Master System version of the same game, and is replaced with a box of rings in the Game Gear release. Furthermore, it was probably once intended that Tails be playable in this version of Sonic 2, as careful searching reveals that Tails has extraordinarily detailed sprites for a non-playable appearance, even including his Idle Animation.
In Sonic the Hedgehog (2006), the "Super Sonic" gem is partially in the final game, and the hacking community have found an actual transformation sequence for Sonic during gameplay. However, coding to actually allow one to play as Super Sonic isn't present.
Super Sonic's story also had text that recapped what happened in the story, like what happens with the other six characters. In a subversion, this is possible to view if you grab Sonic's optional upgrade in the Last Story and then exit the game, but after playing said recap scene, the game freezes, forcing you to create a new save file if you want to complete the game.
One example from Sonic the Hedgehog CD that many people miss is the unnamed desert level. This level shows up in the ending video and on the interactive Sound Test via a "world map" option, but since level design and programming were given to individual people, and the person in charge of this level was late in finalizing the look of the level, it was dropped late in development. There was, according to some interviews, yet another level planned for the game, but it didn't even get past concept phase. Furthermore, the PC version has all the levels contained in separate folders and one is conspicuously missing, R2, so that it goes R1, R3, R4, etc. The level is glimpsed in the ending animation, however, featuring an enemy that looks a hell of a lot like the first boss from Sonic 2 for the Game Gear, or should we say a boss that looks like an enemy from Sonic CD. This legacy was actually carried on in the downloadable remake in 2011, with an early screen of the desert level imagined into programming form found inside the games' memory. The reimagined "Desert Dazzle", along with a new final boss stage called "Final Fever" (itself sharing its name with the boss music for the final level, Metallic Madness, in the final version of both the original and remake) were planned as extras for the remake, but were turned down due to time restraints and SEGA's preference that the game not be altered too much from the original rendition.
"R2" has been revealed as being modeled on Marble Zone, but extremely crude concept drawings of the zone have it resembling Sonic 2's Aquatic Ruin, but with rainbow waterfalls interspersed throughout the stonework. R2 was either dropped so early in development that it didn't warrant having a graphic on the DA Garden, or was meant to segue into the "Desert Zone," again given the graphic shown on the DA Garden. Both of these ideas were likely saved for the Sonic 3 zones Marble Garden and Sandopolis.
In the Famicom Sonic the Hedgehog bootleg port Somari, the Scrap Brain Zone is dummied out. The game goes straight from Star Light Zone to the Final Zone.
Sonic Generations was to feature Big the Cat in some capacity, to the point of having voiced lines, as confirmed by his voice actor. It can be speculated that he was to be the host friend for the "Speed Highway" stage (as Sonic Adventure is his debut game) but was probably cut because he likely couldn't actually do anything in the stage, being a fisherman/power lifter in a stage with no water or physical obstacles.
There is an unused Super Sonic bio that can be found in the game's files.
In Sonic Heroes, Team Rose have the easiest and shortest levels in the game. However, since (as in the original Sonic Adventure) 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 stage the other teams explore, but in two stages (Casino Park and Frog Forest) you can use a Team Blast and the subsequent speed boost to just barely clear the gaps, and continue the stages as normal. It's also possible for Team Rose to go into the ghost-filled well in Mystic Mansion, by hitting a particular switch and flying very carefully. What's interesting is that as you do so, the characters will continue to comment on things that they wouldn't even encounter playing the game normally (such as the VIP pinball room and the giant mushroom), indicating that there could be dummied out dialogue for the other stages as well.
There are also unused voice clips of all the characters apparently reuniting with their team, and complaining about something being heavy. Hints at removed game mechanics?
Sonic Gems Collection, a compilation title that contained Sonic the Fighters, Sonic the Hedgehog CD, and Sonic R, had a few games that were planned to be included but were removed (not counting the US version's removal of the Streets of Rage games). These games included the Monster World Genesis games and Sonic Eraser, the latter being a highly obscure puzzle game released on the Mega Modem peripheral in Japan. Word of God states that SegaSonic the Hedgehog was also considered at early stages, though eventually turned down due to the trackball controls making emulation difficult.
Sonic Chronicles The Dark Brotherhood has the 5 Viral Chao. Said Chao were to be released in events, although only one was ever officially available. The rest of the Chao, excluding one were included in the Japan release.