3 times in the last 2 years!
The first one I had no one but myself to blame. GM was a friend and coworker, his wife, and a couple of their friends rounded out the table. I had made a mess of my personal life and was letting it have a major impact on the game. Showing up late, missing sessions at the last minute, that sort of thing. The GM pulled me aside the last time and said that the other players had asked that I be dropped. I didn't blame them and had been considering quiting since I couldn't concentrate anyway. I was also moving out of town in a few months. End of that story.
Second time, the game was not a great fit anyway. GM wanted good characters in a heroic game, which is just the sort of game I like. He even had a Yahoo group set up for the campaign (important detail for later). Unfortunately, for him this seemed to mean exaulted good. In one case, a gnoll surrendered during the fight, then attacked one of the characters left to guard him while we finished the fight. When we came running, he dropped his weapon. The GM felt we still had to accept it's surrender and not kill it. This resulted in a bit of arguing and some e-mail exchanges, but things seemed to settle down. A couple weeks later, my SO's character got killed and the GM wanted to delay her return to the game, perhaps run her through a few private sessions to improve her gaming. He also wanted us to sit apart at the table, as he felt our kissing was distracting from the game. She replied that she would just as soon not play anymore. I tried to send an e-mail saying that I would be leaving too, but it was rejected by the yahoo group, indicating I had been dumped from the group. This was minutes after my SO sent the e-mail saying she was quiting. Technically, I was dumped.
Third time around, the game wasn't too bad, some personality conflicts. We'd been playin a little less then a year when the GM asked my SO to run an NPC for him instead of her regular character. After several sessions, she became impatient to return to her character and sent an e-mail asking how much longer it would be until she got to play her character again. The GM e-mailed us saying we were out of the game because it was too stressful to run for so many people (the table had grown from 4 to 6, with us in the original 4). He also said he had talked to all the other players and they were okay with it. He capped it off by telling us he considered us just as good a friends as the others. We were less then convinced of his sincerity and and told him as much, explaining that we didn't consider his actions that of a friend, particularly since it was all said in an e-mail. I can handle be dropped, but be honest about it and don't B.S. me.