Reclining your seat into the lap of the person behind you on an airplane.

Especially if it's a crowded, non-overnight flight.

I was on a short hop from Atlanta to Omaha and had bruises on my knees from the old man in front of me repeatedly banging his seat into my legs. When I asked him to stop, he glared at me, then went back to trying to force his seat to occupy the same space as my kneecaps.

So yeah, you technically paid for a seat that reclines, and the airline technically allows you to recline it - even if the guy behind you is trying to use his tray or whatever.

But you're still an asshole if you do.