Sonic the Hedgehog CD (Prototype 510)

CD Sonic the Hedgehog, or more commonly known as Sonic the Hedgehog CD Prototype 510 is a very early prototype of Sonic the Hedgehog CD, the build is very early with many bugs present.

The entire game is present on the disc, but only three levels are normally played before the game shows a "Comin' Soon" screen, suggesting this was meant to be a demo version. It could also be because the rest of the stages are incomplete.

The build was discovered by a group of fans called Sega Extreme in Summer 2000, the original disc has since been sold and is now lost.