I've been fortunate enough to have only wrecked my mini-motorcycle once.
It had rained that morning, and the roads had dried up for the most part, so I figured I'd hop on the bike and cruise around the neighborhood streets a bit. Everything was working great! No problems with anything on the bike, and I was pretty darn excited that I'd finally worked all the kinks out of it, and had a reliable neighborhood cruiser! So I'm riding along, and a couple seconds before approaching an intersection I decide I want to turn there. I hit the brake to slow down a bit before leaning into the turn, and the front tire(a skinny 26" bicycle tire) loses traction. Bike fell over on its side, with my arm and leg underneath it, and slid about 15 - 20 feet, crushing my ankle and tearing up my arm. Went to the doctor, where I found out I'd fractured my ankle.
Learned a couple of lessons. 1. Don't ride a motorcycle with a skinny bicycle tire on it. 2. Don't take turns on wet pavement going 30 MPH.
Picture of the arm(also posted in my other thread)