When another driver can't wait for you to get comfortably past the vehicle you are passing, and just as you activate the turn signal to change lanes, swerves around and between the two of you.
It's especially annoying when it's a middle-aged white guy wearing a button down and tie in an SUV doing the swerving and not some t-shirted punk in a souped up sports car.
The latter I almost expect to do something like that, the former I expect to have a little bit more decorum. But I guess appearances can be deceiving.
What's bugging you?