Two stories come to mind...
The first (and more recent) story was in a one shot at my annual mini-con "End of the World." I was playing a professional gunslinger in a post-apocalyptic western game. EOW uses some home-brewed rules that are ridiculously lethal, and character death is common, but someone I had managed to survive several gunfights that day, including a few I honestly shouldn't have. We get through about nine or so hours of gameplay and finally finish the scenario when one of the players says, "so that's it? We're done?"
When the GM said the story was basically finished, the player said "I shoot Rob's character in the back."
Apparently in the pre-gen PC backstory I had killed this kid's father, but hadn't connected the two together. The other player had waited the entire game to enact his revenge, after my usefulness had ended.
The second story goes all the way back to college, and a Cyberpunk 2020 game that I was in. I was playing a Fixer (tending towards my usual rogue- or rogue-like PC tendencies back then). Our group was your basic troupe of dystopic ne-er do wells: two Solos, a Fixer (me), a Rockerboy, a Techie, and a Netrunner. There might have been someone else, but I can't remember.
One night the GM calls me and asks me to meet him at a dark corner of the college in person for a little one-on-one roleplaying. Our group had been contracted to do some snatch-and-grab raid on a big corporate building--typical Cyberpunk plot line. The GM, in the guise of a trusted NPC, tells me that he's heard that one of the team members has agreed to betray us at an opportune time. I immediately thought of John.
Why John? Because he's the kind of player who would do that. At least I thought so. I didn't hurt that I didn't always like him very much and he was really weird. (He ate an entire bowl of raw chickpeas and black olives every day with his dinner at the cafeteria. He must have been as regular as a clock.) He would be the guy to totally piss on everybody else in the game for the sake of plot. I seethed. I would keep an eye on that guy.
So the night of the session when we would do the run began. The two solos (one of which was John), the Netrunner, and I would enter the building. The Rockerboy and the Tech would stay in the AV4 outside (since they had limited usage in this scenario) and would come get us if there was a problem. I stuck to John's metaphorical side like glue, waiting for him to slip up. While we were deep within the bowels of the corporate offices, the alarm went off. I immediately pulled out my plastic squirt gun (we all were carrying toy guns to the gaming session for effect--it was the early 90's and you could do that sort of thing back then), pointed it at John's head and said, "This bastard set us up! I'm going to blow your head off!"
John looked at me like I was insane and said slowly, "what...the hell...is wrong with you?"
"Don't you lie to me you sonuva bitch! I got word that we were sold out and you did it! Rockerboy, we need an extraction now!"
The Rockerboy's player said, "I'm not going to do that. The Techie and I were paid to make sure you failed. We're flying away right now."
All four of us were killed trying to get out of the building. John looked at me askance for months to come.