Topic title says it all. What's the most dickish thing you have done in the past?
One memory comes to mind. I remember in my freshman year 5~6 years ago, my university's residence staff put me in the cheapest, worst dorm house that they had available (apparently my residence application was lost in the mail). It was an all guys dorm that reeked of piss and stale beer; the dorm was basically a giant frat house.
I was on the fourth floor, and each floor shared
one bathroom. A combination of laziness, naivete, and home sickness caused me to place all of my toiletries in my floor's bathroom. Later that night, I went to the bathroom and all of my belongings were gone (even my soap bar and toothbrush, which was disgusting). I was pissed, but replacing those items wouldn't cost much at all so I let it slide.
Later that week, I was doing laundry on the first floor and went to the bathroom to take a leak. I found all of my things including a half-empty shampoo bottle with my local grocery's price tag on it. Here is a pic of the subject:

Bored and having nothing better to do -- I opened the shampoo bottle, pissed in it, and left it behind. The shampoo bottle was finished by the end of the week when I checked up on it.
I felt like badass when I did it back then, but thinking back now I kind of feel like an idiot. I just feel sorry for the guy who washed his hair with my piss for a week.
So that's that. Anyone else got any good stories they'd like to share?