I'm sure everyone has left the self timer on there camera and forgot about it until they next used the camera. Generally resulting in a frustrating few moments of waiting for it to fire while you scream in your head that the must capture moment is about to disappear.
Guilty of the lens cap one too. I took a fantastic 15 minute exposure of the inside of a lens cap once
I actually had a shoot the morning after a night I was experimenting with long exposure. I had the camera on a 2 second delay followed by a 30 second shutter.
The next day's session went like this:
Me: Shall we get started?
Me after helping position the family: Alright, perfect, *click*
--nothing happens-- A second later, I took the camera away from my face, shortly after *click* uh oh, 30 seconds pass, then processing. Yea, it was great. If great means awkward, than yes, it was definitely great.