The guy in the passing lane on the interstate who comes up quick then drives just fast enough so you can't move over to make your pass.
Yeah, that and other variations on the same theme -- camping out in the passing lane and causing various problems.
Here in Rhode Island and Mass,, we need to invest in about 1,000 "keep right except to pass" signs and post them in the median . . . but it probably won't help, those people are completely oblivious to everything that is going on around them.