[Stayfree]

Several years ago, I printed a bunch of bumper stickers on photographic paper cut into bumper sticker sizes after printing. Using these phrases for people who came too close for comfort in parking lots:

I HATE COPS!
I AM A FLAMING ASSHOLE
I AM A PORNSTAR!

[USNBandit]

A friend of mine had a prank war going with another friend. He was driving around town and was getting all sorts dirty looks. At his next stop he found a magnetic bumper sticker on the back of his car that said "I (heart) gay porn."

Eventually a truce was called after a mail in request for the Book of Mormon. That was really hard to turn off.