Last night when I was commuting home on my bicycle a car overtook me in a very stupid, unneccessary maneuver (since I was going pretty much the speed of traffic anyway) and when I reeled the car in a block or so later I decided to to give him an ironic "thumbs up" sign to show him what an idiot he is. Unfortunately at exactly the same time he had to break hard because of the car in front of him and I bumped into the car's back pretty hard.
I'm fine and my bike's too. But my recommendation is: don't do that. Someone cuts you off - let it go. Won't get you home any faster to "teach" the offender a lesson or whatever.