I accidentally threw my old F350 dumptruck into reverse doing ~30mph a few years back. It popped out of gear and instead of throwing it back into fourth I put it in reverse. It locked the rear wheels up and sent me slidding sideways with a 1500 lb. set of pre-cast stairs in the back. Lets just say I was more careful after that.

I really need to be careful though as the car I drive now is a six speed and all my trucks are only four or five speeds.