The Sonic Generations Demo is a playable version of Sonic Generations released to gold members on Xbox Live Arcade and PlayStation Network users on the 23rd June 2011, twenty years after the release of the original Sonic the Hedgehog. Silver Xbox Live members were given the game a week later on the 30th.
The demo acts as a one-level trial version of the game, in which the player can control "Classic Sonic" through the first Act of Green Hill. It only remained active for twenty days, however it was found that adjusting the system clock could extend the play time indefinitely on both PlayStation 3 and Xbox 360. Due to Xbox Live setting the system clock automatically when online, to play the demo on the 360 past the date the player had to disconnect from Xbox Live. The build was said to be older than the version shown at E3 2011 (dating to May 2011) and was listed internally as "Demo #1", suggesting further demos were on their way.
Questions were raised about the game's 650MB+ file size, despite only claiming to contain one level. Hacking of both the PSN and XBLA versions has shown that in fact the demo contained masses of unused assets, including a supposedly fully playable Act of Green Hill featuring Modern Sonic, which would be later on used in the second demo. It also unveiled clues for the rest of the game, which at the time had not been officially announced by Sega.
Most importantly, the demo confirmed the level list (including Casino Night as downloadable content), bosses, in-game items, Badniks, missions, cameos, Super Sonic and the existence of a Sega Mega Drive emulator, which would later be used for emulating the original Sonic the Hedgehog in the final game.