in a stall at my school a couple of years ago, in one stall a girl had written "I have a penis is a pickle jar" so my friend meghan goes and under neath it writes "holy shit, then where's the pickle?!" classic.
I remember something smart actually written on a bathroom wall. There was a piece of sheet rock missing on the wall, and you could see the bricks that lay behind it. So someone wrote "Here lies fotunado" [/poe reference]
The lock was broken in the place i was going to take shit in and beside it wrote "Microsoft Security". Then another person wrote "Apple sucks, Microsoft is better". Then I assume the same guy who wrote the first thing wrote "The only reason microsoft better is because their pc are cheap" and war between apple and MS kept going on and filled the door. I think a third guy came in and said "Linux owns all"