I've been pulled over by various unmarked cars.
Example 1, unmarked car crosses 3 lanes with no turn signal cutting people off, I speed up to see if it really is a cop. Cop looks over, then pulls behind me hitting the lights. I pull over he walks up and asks if I knew why he pulled me over. To cut things short I tell him I just wanted to see if it was really a cop that broke several moving violations for no discernible reason. I asked for his badge number and info, then takes my credentials back to his car. He comes back and says slow down, have a nice day.
Example 2, I see an unmarked car (which turns out to be an airport authority cop) doing well over the speed limit. I of course match the speed for about 2 miles, then the cop suddenly slows down very very quickly gets behind me and hits the lights. I pull over and the cop asks me the obligatory dumb ass question, do you know why I pulled you over. I say no, to which the cop says, you have an illegal exhaust (driving an Evo VIII). I laugh and say, no sorry you are mistaken this exhaust is factory, would you care to climb under and look. I then ask the cop what authority gives him the ability to speed well over the limit then violently slow down and accuse me of having an illegal exhaust. He didn't even bother running my info. Simply said have a nice day.
I could go on all day with this shit. I don't mind cops, but I really hate the some of the fucktards they give badges to. I'm a bit of a dick when it comes to being pulled over for vague reasons, but I was also law enforcement for a while.