I've told this story before, but it's really the only time I got hurt that I really remember.
I was in 2nd grade, during recess. Somebody dared me to try and jump to the 3rd or 4th bar on the monkey bars. I was able to grab onto the 3rd or 4th bar, but I had jumped with such velocity that I swung all the way up, lost my grip and landed flatly on my back on the playground floor. Had trouble standing up straight for about a week after.