We've all done something obviously stupid in retrospect, whether or not it resulted in injuries. Most of us know better than to even admit it, although it can make a good story to tell at family reunions. Readers submitted stories to Cracked about injuries they've suffered from doing something extremely dumb.
Most of them resulted from not thinking at all while you're doing something too fast to give consideration to safety. But the top rated story is thoroughly stupid. Read them all at Cracked.