A very frequent occurrence when you are learning to drive a manual car. It happens when you take the clutch out too early and too quickly, that is, when you drop the clutch. If you haven't got enough revs then the car will stall, if you have too many, then the bunny hopping begins!

The car starts to go, and then stops suddenly, starts up again, the abruptly stops. Up and down, up and down, forward then stop, forward then stop. In extreme cases the car will begin to literally bounce forward along the road.

This amusing spectacle continues until the driver either stalls the car, or brings the clutch in slowly enough to make a smooth take off. For the driver it is hell, for an instructor it's routine and for everyone watching it is so funny you will piss yourself laughing.

Edit: Thanks to andersa for pointing out that the correct way to avoid this happening is "floor the clutch immediately and then try the start over again."