Hilarious in Hindsight: The earliest known prototype of Sonicthe Hedgehog 2 features an unused "wall crash" mechanic, which causes Sonic to get stunned and bounce back off a wall if he runs into it at a high speed. Fast forward 19 years, and Generations has an unused wall crash.
This mechanic was used in the Adventure games, as well as the PS2 versions of Sonic Unleashed.
I Knew It/Interface Spoiler: People looked at the Xbox 360 Avatar rewards for beating the game and speculated that Classic Eggman has a link to the Time Eater, since said Avatar parts are all patterned after Classic Eggman. They were right.
Meaningful Release Date: The first demo was made available on June 23, 2011, exactly 20 years after the first Sonic the Hedgehog was released. This is fitting, as it's a game about Sonic's history.
The second demo was released on October 19, 2011. When Sonic Generations was one of the Daily Deals for the Steam 2012 Summer Sale, it was discounted to $10.19.
And it was later revealed that Japan was getting an exclusive special 20th Anniversary set as well. 'Tis a sad year to be an American Sonic Fan...
Talking to Himself: Quite literally with Classic and Modern Tails, who are both voiced by Kate Higgins. Averted in the Japanese version, where Classic Tails is voiced by Takuto Yoshinaga, while Ryo Hirohashi voices Modern Tails.
There was also a rumour of DLC stages being released. Apart from the Casino Night pinball minigame this was false (Though Sega did mention that they did consider making a DLC stage based on Wacky Workbench)
What Could Have Been: According to Word of God, they had hoped to make all acts playable by Classic and Modern Sonic alike. Indeed, it is possible to hack or glitch the game so that you can play Modern Sonic in Classic levels, viewed from a 3D perspective — the Classic levels look very unfinished when viewed in this way, however. The game reportedly crashes in various ways when attempting these hacks: e.g. when Classic Sonic triggers a rainbow loop, or when Modern Sonic doesn't stop after the Classic Finish Panel and falls through the level geometry... You can, at least, unlock the Modern Sonic homing attack, à-la-Sonic 4, for Classic Sonic, after collecting enough red rings in the Modern Era Act 1 levels...
Big's voice actor Kyle Hebert recorded lines of dialogue for Big's part of the game, but Big was later scrapped from the game's plot and levels. It's currently unknown where in the game he would've been, but Hebert's name remains in the credits, and Big's statue can be seen in the Statue Room.
Sega mentioned that they planned to make a stage based on Wacky Workbench, initially dropped to keep the "3 stages per era" set-up and later dropped fully when they decided against full DLC stages.
That's only going by the PS360/PC version of the game (Heroes is considered to be part of the Modern era in the 3DS version) and purely for pragmatic reasons (so that there'd be an even number of stages per era), though.
To clarify, while the PS 360 version of the game has Seaside Hill to represent Sonic Heroes, the 3DS version uses a boss instead, the Egg Emperor, that serves as a boss for the Modern Era.