For several years I ran and played games in an area that caused many of the same people to wander in and out of the games. One of these we'll call Ben, Ben was actually a really good player, his charaters were interesting. His dialogue usually at least on topic, sometimes even witty. They always fit in with the group while still being slightly aloof from us, mostly because due to health problems Ben couldn't always make every session. But after a decade of gaming, those of us who played ith him the longest, noticed a pattern. In evey single game that went on long enough Ben's character would betray the party. Never openly, always with back room deals, demonic pacts, anonymous informants, ect. Once we worked this out it became an in joke about Ben's 'sudden yet inevitable betrayal' and a kind of side game where Ben's characters would set us up in increasingly convoluted plots, and we'd be trying to guess what was the GM's plot and what was Ben's. It became such a staple, some of Ben's more memorable characters made guest appearances in games he wasn't in to run a side betrayal plot.
Some of the more memorable were: In Shadowrun Assasinated minor NPC members of of our Metahuman rights 'politclub' to prevent our renovation of part of the Redmond Barrens.
D&D Murdered a sacrificial victim before a ceremony we were trying to stop could be completed setting off world changing cataclysms.
Earthdawn, sold us into slavery to six major theran slave merchants, at the same time. Then warning us when their soldiers were about to reach us so we had to fight them off while they fought each other
D&D had us arrested for piracy, so he could steal a better ship during our trial, then hung out in swimming distance of the gallows to see if we would escape.
Weird West: Convinced the party to rob the bank, a payroll train, and a cattle auction on the same day ambushed us all with a posse in a canyon, shot us full of holes, then stole the loot from the posse on the way back to town. Oh yeah, we were the Sherrif and deputies. We were also undead already so while we weren't happy we weren't really dead either. Ben shared the loot out and apologised, right before the buffalo soldiers he tipped off arrived, payed him the reward for our capture and hung us from a tree. He came back with his greatly expanded loot to cut us down and cut us in again. And that's when the Apache's showed up…..