The best two I have seen so far:
"Hey! YOU! Outta the gene pool!"
and the other one was a cartoon like wood screw that was intended to slap over the hearts on bumper stickers that say things like "I (red heart picture) my Terrier" so they appear to say things like
"I (screw picture) my Terrier"
Priceless. The same guys came up with a sticker that said "learn how to park JackasS"