Not really a bumper sticker, but on my way to work this morning, I saw a frail old woman on a mobility scooter being given the "who do you think you are? Nigel Mansell?" treatment by a flak-jacketed copper.
In my head, he was saying "You pwned me on AS Nat last night, BUT WHO'S LAUGHING NOW, BIYATCH!!!"
Would work as a poster...