A car with no working brake lights (including the ones in the windshield too). When this car comes to a stop, it may suprise you, especially if the driver is a hard braker.
Sadly, sometimes hard braking is intentional if the driver knows his brake lights are out. If you rear end a car, you are always at fault, no matter what.
I kept an eye on the suprise car infront of me, I think the shady foreigner inside was trying to get a cheap screwjob.