I had a fun match during which a little kid kept screaming racial slurs into my ear for an entire match in MW2, so me an a group of 4 of my friends started trolling him, trapping him in corners with riot shields, pushing him off of ledges, and things like that.
After a while the kid started crying, and called his mom, and started telling her about how we were bullying him. She got on the mic and started telling us off. We patiently allowed her to finish, and then told her some of the things her little brat had been screaming at us not 10 minutes prior. She didn't believe us, but a friend of mine had been recording all of our matches, and put up one of the matches on youtube (the one which had the little kid screaming at us), so we sent her the URL as proof.
Five minutes later the mom came back, took the mic away from her son (who had been quiet for the first time in half an hour), and apologized to us for her son's behavior, and with violent anger in her voice, told us that she would make sure that he never used that kind of language again.
That was a pretty good day.