Bit random, I just remember the time I found a Hot Wheel (toy car) in the toilet...instead of taking it out I thought "It's no bigger or heavier than a poop surely" so flushed it. Obviously it got stuck.
Can't think of any more off the top of my head. Come on I can't be the only one. What silly or "stupid" thing have you done that afterwards you thought What was I thinking?!