Most absent minded would have been the ubiquitous run-in with a parked car while looking backwards when I was about 15. Bent the fork and rim but the car was fine. I've been damaged goods ever since.

Dumbest thing was adjusting a threadless headset/stem -- actually, trying to get rid of a creak --while riding the bike. After loosening the headset tension screw and the upper stem bolt, but before I loosened the lower stem bolt I realized what I was about to do... I went back home. Put the bike away and proceeded to make margarita's since I realized I had no business being on a bike that particular day.

Dumbest thing to bystanders was bump-practice on the football field with a cycling club... you know, where you intentionally overlap wheels, bump tires, crash, throw elbows, learn how to "move" other cyclists and otherwise learn the fine points about cycling as a contact sport.