I saw one once that said: "Unless you're a hemorrhoid, get off my ass!" You know those stick figures that you put on the back window of the car and you get people for every member of the family? Well my sister thought it would be funny to get an old lady figurine and surround her with a bunch of cats, lol.
That reminds me of the bumper sticker I saw that said: I'm just one bad relationship away from being the crazy cat lady
I saw one the other day that said "this sticker is covering a rusty hole"...and it was flapping in the wind because it came unstuck from one side of the hole
I was stoned with my friend Bill driving and he hit on the brakes really quickly as traffic stopped right behind this old beat up van that had a bumper sticker that said 'Bill says no to drugs but yes to burritos'
This one isn't funny but I certainly liked it.. it said "Sure you can trust the government... just ask an Indian."