When I was younger, my older brother was pushing me in circles through my house in one of those old red and yellow "tyke" cars. Sure enough, he hit the corner of the wall by the front door and my head nailed the corner- whole lots of blood went everwhere as it had ruptured some bloodvessels in my nose!

Another good one was playing hockey in my roller blades, I stood in the middle (on the peak) and my blades went opposite directions and I fell flat on my ass. Didn't get hurt but it was quite entertaining!