Oh man, I've had so many stupid things, though most were accidental.
1) Thinking that that gap between a building and a lightpole was big enough for me to cycle through. It wasn't. Hit the pavement head first, teeth went trough lip and a corner chipped off, whole forehead bloody mess.
2) While leaning forward, knocking my head against a lock of an open cabinetdoor. Wound was pretty deep and as I was pretty young, seeing that much blood coming out of my head = panic. Still got a scar.
3) This one was just really stupid: At work, Imagine a metal box, but open only from the topside and the front side. It was stuck in larget metal box with the same design, and I needed the smaller one to go down (it was stuck a little above the bottom of the larger one). So I slammed the bottom hard so that the force would make it slip down. Unfortunatly I just put my other hand under the small metal box. Goddamned that hurt.
4) Dropping a heavy bag on a chair. Slam was so hard that somehow the chair bounced up, landing on my little toe.
Out of all, I would say nr 3 is the dumbest. The other 2 were just stupid accidents.