The you must be cheating because you beat me type of gamers.
Happens to me all the time, I could be the lowest rank in the game, I end up winning and the next thing I know I get a message from a person, at the highest rank saying. "You so hacked, there is no way you could beat me, I'm a higher rank than you, cheater."
God I hate those guys.
Also the people who refuse to help you in an objective game, because they would rather raised their KD ratio.
Also people who stereotype you.
For example I was playing online, and some guys found out that I was Irish, so they started asking me questions not sensible questions. Like what part of Ireland are you from? I'll answer those. The kind stereotypical questions, like.
Are you drunk yet?
Does your dad beat up your mum?
Is it true, that everyone in your country are alcoholics?
This why I rarely use a mic.