Not a bumper sticker, but...I saw this vanity plate back in the 80s in DC when I was a bicycle courier. It read: TIH2WA   

I puzzled on it for a few moments. When I rode, I always had a rearview mirror on my helmet. When the car passed, the joke became instantly clear.

A local newspaper columnist wrote about it sometime later, raising a BIG ole hue and cry about the travelling obscenity that kids must be protected from (as opposed to the obscenity of WAR). People protested, and the message reached the DMV (Department of Motor Vehicles), and they revoked the plate. Some people just have NO sense of humor! But wherever people laugh and share bumper stickers and vanity plates, it lives on.