Well if we're going back to childhood, when I was about 11 or 12 we were in an 8' allyway at the back of my mate's house with his Grifter, and a plank of wood and some breeze blocks. Everything went wrong, I went into 'slip gear' (wtf was the point in that anyway?) unexpectedly, or stupidly, my foot slipped off the pedal (is that why it was called slip gear?) I went up the plank all wrong, it too slipped and the front wheel went into the breeze blocks, I went over the handle bars and hurt my face.
Another time I was on my Striker in the street and tried to wheelie up the kerb, but failed, and I cut my nutsack on what I suppose was the stem. That hurt more.