The one that gets my dandruff up is the 18 wheeler passing another 18 wheeler, doing 1/2 mph faster or less. This usually happens on a hill, and thereby backing up the "4 wheelers" for some distance, until that truck finally passes, or usually, the truck he's trying to pass pulls ahead, and then he falls back into the line where it should have been in the first place.
Like others have said here also, you'd think getting passed on the right, on a 2 lane interstate, would give you a clue that you are driving too slow?? I have found of late, usually the ones doing that have one ear with a cellphone attached. Don't know why, but when they get on the phone, they seem to back off on the throttle, for some reason. Glad to see the laws changing with no mobile cellphone use, unless handsfree, although, I think it is going to hard to enforce for the majority of the time.