Sonic the Hedgehog 2 (Nick Arcade prototype)

The Nick Arcade prototype version of Sonic the Hedgehog 2 is the earliest known pre-release version of the game, predating the famous "Simon Wai" prototype version. This prototype receives its name from the game show Nick Arcade due to an earlier version of Sonic the Hedgehog 2 being showcased in two episodes where its build is similar, if not the same as this version used on the show.

Notably, this prototype shows that Sonic the Hedgehog 2 is based off of the original Sonic the Hedgehog as Green Hill Zone.

Differences

 * If Sonic runs into a wall with a high velocity, he will bounce back off of the wall. This feature was canned sometime after this build, but the sprites it uses are still present in the Simon Wai prototype.
 * Each Zone uses music from Sonic 1.
 * There is a large amount of raw code contained within the ROM.
 * Tails can lose rings for the player if he is hurt, just like in the later beta.
 * If Tails dies, the camera will follow him as he respawns until he goes off screen.
 * If Sonic & Tails are hit at the same time, lots of rings will sprawl out regardless of how many rings they actually have (provided the player has at least one ring).
 * The backgrounds of Hill Top and Chemical Plant don't move.
 * Invincibility doesn't show stars, but plays the music. The music and invulnerability lasts until the end of the act.
 * Sonic and Tails are in different symmetrical designs on the end signpost
 * Chemical Plant Zone uses a purple-pink accent in the background instead of olive green; buildings are also much lower
 * Seesaws similar to that in the final Hill Top Zone can be seen in Emerald Hill's object debug - An interesting connection because HTZ's uses Emerald Hill's art.

Green Hill Zone

 * This Zone from Sonic 1 is still available, with its level art converted to Sonic 2 ' s format and the necessary changes have been made to make loops work.
 * Collision detection is very glitchy, as the collision system has been changed to the system for Sonic 2, but Green Hill Zones collision pointers are still set up for Sonic 1s' collision. Oddly, the old Sonic 1 collision index still exists in the rom, and can be restored via cheat codes (doing this will fix Green Hill Zone, but cause similar collision issues in every other zone in game).
 * Tails Life Monitors are present just as one is in Hidden Palace Zone, as all the monitors in Green Hill Zone haven't been changed to use Sonic 2s' monitor system. As a result, nearly every monitor in Green Hill Zone has been "switched" with a different monitor.
 * You can see here that the Sonic 1 springs still are here in the game and fully functional; the springs in the Sonic 2 levels are different and match the final game.
 * Many badniks and obstacles use glitchy sprites, including rings.
 * The Crabmeat badnik uses a lost sprite from Sonic 1.
 * If one passes into Marble Zone (Chemical Plant Zone), the rings will still use the glitchy sprites they had in Green Hill Zone.
 * The spikes used are the same ones from Sonic 2. They use the correct graphics (aside from the green outlines) when seen in Debug Mode but use glitchy graphics when placed or seen in normal gameplay.
 * The boss for the zone also uses glitchy sprites, as well as using the same lost sprites as the Crabmeats in place of Robotniks' face.
 * Tails can stand on the spikes without getting hurt.

Marble Zone

 * This is Chemical Plant Zone, and it is heavily under development, with major differences in the level's design from either final or the later prototypes, and is very unfinished.
 * Chemical Plant Acts 2 and 3 have the same layout as Act 1, but have the ring and object layouts of Green Hill Zone 1.
 * It uses a pink palette instead of a green palette used in the later betas and final versions.
 * Some of the buildings in the background are shorter, and match that of the footage of the Nick Arcade build.
 * There are extra objects in the zone for Marble Zone in Sonic 1 accessible via debug, but they use garbled tiles.
 * There is a falling block for Chemical Plant also accessible via debug
 * The transportation tubes were apparently used in a different way, but they aren't functional in this build.
 * There is a prototype boss for this zone found in the ROM through hacking.
 * The unused RAW data contained with the ROM is loaded when you down the zone.It does a illegal instrucion error.
 * It loads if Tails falls to the area too.

Spring Yard Zone

 * Actually Hidden Palace Zone, the zone layout is as it is in the later Simon Wai prototype.
 * Act 2 has some leftover object placement from Green Hill Zone Act 2.
 * There is a third act which has water that constantly rises and falls. This zone loads Green Hill Zone Act 1's ring locations.
 * If under water in Acts 2 and 3 until the countdown starts and you jump out the Labyrinth Zone music will play.
 * The water level can be controlled using and  on the 2P controller.
 * If you move the water up so a badnik is submerged, his orange spot turns bright yellow. This is because Sonic's underwater palette is really from Labyrinth Zone in Sonic 1.
 * In Act 3 you get placed in the blocks, what makes the camera go crazy (after some button mashing you will eventually get placed into the level)
 * In Act 3, shortly after you exit the tube, the water will suddenly appear.
 * All of the debug mode objects besides rings and monitors have broken previews.
 * There is a strange bouncing object using bat's graphics found in the debug list.
 * It has two objects that uses the animals' graphics. One only flips, other flips AND shoots fire.
 * The other one looks like the unused Tycerobot.

Labyrinth Zone

 * An empty zone, it loads Marble Zone's ring layout, Chemical Plant Zone's artwork, and has Labyrinth Zone's rippling effect. Third act is Scrap Brain Zone Act 3.
 * Labyrinth Zone's layouts are very broken, and seems to load broken data from Chemical Plant alongside leftover garbage data. Using debug mode several people have found various strips of "land" but that data is often still just garbage.
 * Several people have debated that this could supposedly be an early version of Genocide City Zone (because of the city-like rippling lines that overlap your character) but somehow got removed from the Simon Wai Prototype. However, it's more likely just garbage data leftover from Sonic 1, as this zone doesn't have any of it's own data, beisdes that which is leftover from Sonic 1.
 * If Marble Zone (CPZ) is loaded first, going back to the level select and going to Labyrinth Zone will cause it to load Chemical Plants background and some of its layout. (some layout tiles are mysteriously missing though)
 * Scrap Brain Zone Act 3 does the same thing, although it loads its own palettes. Oddly, you can easily land on solid ground in SBZ3 if you do this trick, and you can actually go forward by quite a bit (getting to the top of the level and beating it is impossible, though).

Star Light Zone

 * Actually Emerald Hill Zone (which the later prototype confirms kept the name Green Hill Zone for a time) and is as the same as it was in the Nick Arcade video. The layout is very different in Act 1, and slightly different in Act 2. Star Light Zone's music plays in this zone.
 * This is the only zone where Tails is disabled, suggesting this particular version was tailored for the Nick Arcade show in order for the contestants not to have their rings lost by Tails, as well as hide the fact that he can follow the player, as this feature hadn't been directly featured previously. Tails can play in this zone via 2 player mode, though.
 * The boss behaves exactly the same as in the later prototype, but it does have explosion effects. These are the same as the ones from Sonic 1.
 * The 4th buzzer's shots (and every other buzzer past it in the level) rarely hurt the player.

Scrap Brain Zone

 * Actually Hill Top Zone, this zone is also under heavy revision, as it finishes with a pipe leading to instant death because the screen-limit hadn't changed yet.
 * The playfield limits are still the same as they were in Sonic 1, but the level does not end at these limits. As a result, Act 1 cannot be played through.
 * The secondary collision array for Act 2 is missing, so as a result, loops don't work.
 * Second act crashes at about the midpoint. This is because, as Sonic 2 is built off of Sonic 1, the level event handler calls a subroutine from Sonic 1 that no longer exists (the transition between Acts 2 and 3 of Scrap Brain Zone). Final Zone crashes at the second tree for the same reason.
 * The lava doesn't hurt you in this prototype.
 * Green orbs are used as the weight on teeter-totters instead of a Sol without fireballs.
 * The lifts are not fully functional yet and partly broken
 * It is possible to move Tails past the point in Act 2 where it crashes and drop down, transporting you to Act 3.

Final Zone

 * Hill Top Zone (matching behavior in Sonic 1, where Final Zone is tucked into a corner of SBZ2), crashes to pink if you continue onwards because the game is trying to access the Final Zone boss data which is not there, dead end backwards. If you use debug and continue past the second tree, when you see the pink screen, rapidly press, at the top of the screen you might see a garbled "illegal instruction" error.

Special Stage

 * Same as Sonic 1
 * Unplayable (crashes to lila)
 * Playing music in the sound test and then selecting this has a 46.8% chance of carrying an infinite note

Trivia

 * In the episode of 'Nick Arcade'' featuring the prototype (see the video below), Melissa Joan Hart is seen playing a brief section of this prototype.