From Sonic Retro

This Sonic the Hedgehog CD prototype has many differences over the final game, such as this cluttered title screen.

Occasionally an incomplete version of a Sonic game finds it's way to the general public. As these versions are incomplete, they often play very differently to the final version. Levels and/or environments may be broken or non-existent, numerous bugs and/or glitches may be present, and some features may even be absent from the final game entirely. On even rarer occasions a game that was never finished may emerge.

This category covers beta/prototype versions and release candidates of Sonic games, as well as unreleased Sonic titles.