There are some particular glitches which either "shift" the beginning of the act to a later part (well, that could be considered resetting time to zero) or make the game think that you have completed an act while, in reality, you haven't.
Examples:
The former -- Play Ice Cap 1 as Knux. Reach the boss without hitting any checkpoints. Die from the boss. The act will restart directly from the boss fight and you can easily get a small time.
The latter -- Launch Base 1:
1) Take an elevator. You need one particular, but I don't remember which, I saw it in a video anyway.
2) Either take damage or turn super while inside the elevator. You'll maintain control.
3) Spindash a few times to pause the camera. The elevator will unload and you'll be stuck in a wall.
4) Zip to the right. If you took the correct elevator you'll be able to reach Knuckles's boss area. Now, he has TWO guys to fight, right? Sonic, however, normally has only one.
5) Hit both of the bosses 7 times.
6) Kill one of them.
7) Wait for the game to give you "act 1 completed" message".
8) As soon as you regain control, kill the other boss.
9) The game will credit you for completing "act 2" under 2-3 seconds.
Are such actions allowed? I certainly hope not, cause if they are, such bugs will utterly destroy any competition on the levels vulnerable to them.