When you have someone in a P-51 FINALLY gets far enough away from their base so you can have a good fight without the ack chewing you up, make 2 turns, get on their 6, then they dive out, run for their ack, THEN when you still manage to take their tail off they Alt-F4 so you don't get the kill?
I'm pretty sure I know who it was but I didn't film it so I have no proof. Pretty lame game play to me though.