Create levels (S1)- Enable Debug Mode & Level Select. Go to Marble Zone Act 3 & go straight to the boss. Now turn back to Sonic (make sure you are above the floor & lava) then turn back and you should be able to go under here and create a
level using certain blocks...

Running glitches (S2,S&K,S3&K)- If Sonic is running fast on Sonic 2 then Tails may walk in the air after you. And in any game with Knuckles except Sonic 2, at the end of the second part of a stage jump on top of the yellow part and fall off so you are facing it, but not actually on it. Knuckles should run on the spot instead of his celebration pose.