Not the worst, but probably one of the stupidest:
When I was 16, I worked at a 72 lane house in Milwaukee. We had two sides of 36 lanes, and I was on the dead side with one other houseperson, cleaning up after earlier leagues. I was putting away the balls from the returns, and I had 3 6lb balls in my arms. Well, I had just taught myself how to juggle earlier iin the week, and I thought i was pretty hot stuff. So, with a "Hey, check this out!" I proceeded to attempt juggling the balls in my hands. 1st ball up, then the 2nd ball went up, and then up went the 3rd ball....and met the 1st ball on it's way back down. One went straight away from me, and the other one, BLAM, right into my forehead. The next thing I remember is hysterical laughter and having to get off the floor.
Needless to say, I didn't make it onto Letterman.