Sonic Generations (lost game demo; 2010)
Sonic Generations is a game made by Sega to commemorate the 20th Aniversary of Sonic the Hedgehog. It has gone through several versions and codenames during development, such as an early demo version named Sonic Boom and the early codename of Sonic Anniversary.
Whilst in development, there were several demos, such as the aforementioned Sonic Boom demo, only ever appearing at 2011's E3 Expo. More demos include the first level from the finished game being playable on PSP, Xbox 360 (Only for Gold Members) and PlayStation 3. However, a demo version of the game that never saw a home release but only available in select stores such as Gamestop and Walmart.
The game consisted of only Green Hill Zone in a Sonic 4 style. It was the same level for the classic Genesis ported into a Sonic 4 format but was only available in stores for a limited time. It is unknown if it was a disc that was put into the consoles or if it was a special version that was loaded onto the consoles. Alas, it shall remain a mystery.