Sonic Generations, as an unlockable bonus on the Xbox 360 and PlayStation 3 versions.
What Could Have Been: The original stage setup for Sonic to run through was Green Hill, Labyrinth, Marble, Star Light, Spring Yard, and finally Scrap Brain. The reason everything between Green Hill and Scrap Brain was changed wasn't because of its pacing, but rather due how Labyrinth would be too hard for a second level. This intended level order can be seen in the REV 00 level select menu.
Scrap Brain Zone was originally going to be called Clock Work Zone.
Stealth, the same person who assisted Whitehead on the iOS port, made a demonstration port of the first 3 games to present to SEGA for the DS, running well ahead in terms of frame rate before Classic Colection for the DS. SEGA never responded.