A couple of classics,
"Those who write on bathroom walls, roll their shit in little balls: Those who think they have great wit, eat those little balls of shit."
I remember that from when I was 12.
A friend of mine who lurks here alot used to work at a petsmart. Whilst taking care of business one day, he noted that someone had written "i love erica" on the stall door. He popped an "Am" in front of "erica" to make the grafitti patriotic. I always thought that was clever.