Have you ever done an upper decker?

If you don't know what it is, you probably haven't.

I'll educate you anyway:
An Upper Decker is when you take a shit in the tank of the toilet.
You don't flush, and typically put the used toilet paper inside drawers, magazines, or the tank itself.
Thus, the next person to use the toilet will get a nasty surprise when they flush.

Typically performed as an act of revenge against someone that you're upset with for whatever reason.

http://www.urbandictionary.com/define.php?term=upper%20decker