In the remake of Sonic 3 & Knuckles included with Sonic Origins, there is a hidden credits cheat code in the title screen.

Ever seen this weird sprite quirk with the lava and platform in Marble Zone in Sonic 1? Here's why that happens.

There's dummied out code that exists in Sonic 1 for when Sonic runs into a wall, where it will attempt to stop Sonic in the direction he's moving, and then move him upwards. It works best when he's on the floor, where he will climb up the wall. Otherwise, it's pretty broken.

There's some bugs with the peelout in Sonic CD. There's certainly shenanigans to be had. (This does not apply to the remake, as that implemented it differently)