Any far left or far right stickers. But especially Obama stickers, I find that almost every time I notice one of those I'm behind some jackass who's driving is horrible.
I have a small old nole logo sticker on my bumper.
There's a lady that works in my building that has a "I Listen to Classical Music and I Vote" bumper sticker. I keep hoping to see her getting into her car one day so I can ask what major issues classical music listeners are facing.
This is probably spot on, but I wouldn't rule out the reverse; that the guy caught his wife and she convinced him it was his fault she cheated, and now he's overcompensating to prevent her from sleeping with anyone else (which isn't working either).