theres too many, one that makes me laugh is in GTA1, try this:
1. Steal a motorbike and then get off it
2. hold down the punch button (make sure you're not holding a weapon else boom!)
3. Press the button to get back on the motor bike while holding the punch button
4. You will seem to slide over the top of it, now steal a car
5. Now crash into the motorbike so it gets knocked over
6. Check out what happens