I find fried rice poured into the sink on a regular basis at my school. I have no idea if it's the same person who really hates fried rice or if it's multiple people dumping out their lunches. Occasionally it's pasta, but it's almost always fried rice.
There was also the time I found a drawing of a popular girl having sex with the principal taped to the inside of a stall. I just left it there, and later one of her friends found it, starting a massive shitstorm. To this day we have no idea who did it.