Have your ever done something that seemed perfectly brilliant at the time only to realize it was a really dumb idea after the fact?

I am thinking along the lines of those prolific youtube videos that always seem to involve college-aged dudes, beer, skateboards, and a house roof. If so, what was it? Did you learn anything from it? And, knowing what you now know, if you were able to redo that moment in your life, would you still have done it anyway?