I have had 3 come to mind in my 2 years+ on Xbox Live,
Number 1) Guitar Hero 3, Pro Face Off, opponent was highlighting the "Expert" level, and being an honest and fair gamer I was went expert, unfortunately s/he then locked in medium once I locked Expert. Played through song in which I had then defeated them by a few thouseand points, then getting messages saying I was unfair and must be hacking.
Number 2) Ace Combat 6, 1 V 1 online, decided to make it a fair fight and went with F18/E with SAAM (Semi Active Air-Air Missiles) and my opponent decided to go F22/A with QAAM (Quick Maneuvering Air-Air Missiles) which are practically impossible to dodge in an F22 let alone an F18. Got my ass handed to me, and then get a message about how noob I was and that I must be a girl because I was not getting angry that I lost. Sometimes people confuse me.
Number 3) Red Dead Redemption, some punk was going out of his way to kill me in free roam, so after the 5th death, I went and hunted him down, at which point I get a message saying "leave me alone, I didn't chase you down", and that was after death number 2. Guess it's not so fun when your opponent decides enough is enough and grabs a rifle eh punk?
So yeah, I think I've had a good run on Live for the time being.