Sonic the Hedgehog 2 had quite the rocky development. Several scrapped ideas and changes were made, including the creation and later removal of Hidden Palace Zone. For a long time, the only places these could be seen were small images and clips spread throughout various video game ephemera in the 1990s.
That would all change in 1998, when a man named Simon Wai discovered a prototype build available for download on a Chinese GeoCities website.
What makes Sonic 2 interesting compared to most other games in the series is the relatively vast amount of info available about its development. A number of prototypes were discovered and dumped during the 2000s and 2010s, giving a unique look into the game's history.
Nick Arcade Prototype A very early build still using many Sonic 1 assets. Named for being very similar, if not identical, to the build used on the game show Nick Arcade in 1992.
Simon Wai Prototype Considerably more fleshed out, but still clearly an early alpha. Named after the person who found it in 1998, Simon Wai.
August 21st, 1992 A very unstable alpha build. While many features still don't exist, this build is overall more complete than the previous prototype. Occasionally incorrectly known as the "Sonic 2 Alpha Build".
September 14th, 1992 A late alpha (or "pre-beta" as Sega called it) build featuring elements seen in some prerelease videos and pictures. Also known as the Gamesmaster prototype.
CENSOR Prototype The earliest known beta, circa September 16, 1992. Named after the scene group that leaked it 12 days before the final game's release.
Beta 4 to Beta 8 Dated September 18-24, 1992. Internal testing builds that span the last week of development, discovered among Sega's Quality Assurance BBS archives.
Knuckles in Sonic 2
To do: Prototypes beyond 0524 need to be properly documented, not just a link to Sonic Retro.
Include proper downloads for the prototypes.
Comparisons between the prototypes and final build.
Any bugs you come across, document them.
0524 Build Dated May 27, 1994. Has a number of unfinished elements.
0606 Build Dated June 5, 1994. A bit more fleshed out than the previous build, such as including the "Knuckles" banner on the title screen.
0608 Build Dated June 8, 1994. Slight improvements, including more fixes.
0612 Build Dated June 12, 1994. Very close to being finalized.
Almost all prototype versions borrow the header from Sonic the Hedgehog, with the only tweak being a "2" added for the titles. This was only changed to the final version's header in Beta 8.
Similarly, the Nick Arcade, Simon Wai, and August 21st versions use Sonic 1's serial code, while all of the post-August 21st builds have Sonic 2's serial code.
If Tails jumps and hits an enemy in just a few frames after Sonic dies, the score will continually increase until the fade-out. This happens because, while all Badniks are frozen and can't be destroyed just when Sonic dies, Tails can keep moving, get stuck in an enemy, and continuously gain points - a bug that (as seen in the video at right) awards 1,452,800 points and nearly 30 extra lives.
This bug is mostly present in the CENSOR build and later, but can be done in just about any prototype. It was fixed in the final by having Tails immediately go into his flying animation when Sonic dies, but can be reproduced through another bug.
When going up slopes with Tails' AI, if Tails does not successfully make it, its AI will walk back to a stop and then attempt to spindash up the slope. If Sonic dies during this time, Tails will continue performing this action until the Spin Dash is released, which then puts him into the flying animation described previously. If you take Player 2's controller to control Tails after Sonic dies and before Tails' scripted action finishes, you will have full control of Tails until the game fades to black, allowing this bug to occur when Tails jumps into an enemy.