You Tube is full of them. It's really quite astounding what some people are willing to risk for just a little bit of a joy fling. Very serious injuries too.
The worst "stupid" injury I've ever seen anybody do in person was when this kid in gym class shouted out "hey, watch this!" and proceeded to allow himself to slide down from the top of a long rope rope as if it were a fireman's pole. Well, the blood curling scream that he made caught everyone's attention, followed up by his bloody hands after having shredded the skin off during that ridiculously dumb stunt.