I once sort off cleaned someones windows with urine.
I am not proud of this, but it made me feel better at the time - a couple of years back running up to xmas a customer rang me to say I had cleaned her windows while she was out, but se wanted the insides of the conservatory doing and she was mad that I had done them while she was at work, which I do every month!
Anyway she went on and on, so I said allthough I am busy I will come and do it, it was a fair way out of my way too. When I got there she started again, not wanting to drop the customer as it is in the middle of a big run, I went to my van to get my stuff and had a crafty pee in my bucket prior to cleaning the windows!!
Revenge is fun!