Several times. The one that has stuck with me is the time I was accused of hacking on Shogo. The game was ten years old at this point, and the people still hanging out on the dying servers had been there for a long time; I had been playing for a good six years myself. Well, I checked the server (there were never more than three servers active at a time) and saw that there were two people on it. I logged on and started playing.
The two guys on the server were absolute noobs -- they must have found a copy of the game lying around somewhere and decided to give it a spin. Shogo was a completely skill based game online, and the Lithtech engine that it ran on had some quirks that made it not match up to any other game, except for other Lithtech games on modded servers. Well, I completely raped the poor guys -- we're talking K/D ratios on the order of 25-0. It was made worse by the fact that they apparently thought camping was a good idea. On Shogo, the health totals were high enough that very few weapons one shotted a character, and the ones that did could be easily dodged; campers were sitting ducks. Anyway, one of them started accusing me of cheating.
I denied it -- because I definitely wasn't cheating -- but he wouldn't shut up. Well, there was a glitch in the community anti cheat; namely, if one zoomed in and out with the assault rifle or the sniper rifle really quickly, the anti-cheat would frag you, and give a warning about altered field of view settings. I did this a few times to mess with the kid. He promptly rage quitted, and I never saw him on the servers again. The guy who he was on with apologized in a later match.
The sad part is that I was only a mediocre player compared to the rest of the community.