I made a tape outline of a body on one of our kitchens at work, then put tattered police tape all over.
I police-taped up my dev managers office.
I sealed someones cube with plastic tarp, put up biohazard warnings w/ 'Do not enter by order of the City of Santa Monica' and, of course, police taped the hell out of it.
Lesse... I switched the stickers on the water machine so the hot was cold and the cold was hot. I sent out RFC 3514 notices to everyone on my team, and finally announced that my product (Norton Internet Security) was going to be made part of the SystemWorks suite. That last one is probably only really funny if you work at this company, though.
Oh, and of course I posted AH2 screenshots in general.