I think all the times I've bailed have been before the game got started. The one that sticks out the most was the prospective DM who said there would be no demi-humans in his AD&D game. Not my first preference, but I figured I'd give it a shot. He was putting a lot of thought into this, and what is the worst that will happen? The game will get boring, and collapse, but I wasn't very busy at the time, and I might learn something.
Then later in this 1-hour conversation, he mentions how
no one can play demi-humans correctly. He is adamant about this, and it is counter to my literary sensibilities, informed by "all stories about aliens are read by humans." I think this is kind of cynical as a reason to ban them, but shrug and keep talking. Some of his other stuff sounds reasonably interesting.
Then from something I say, he realizes that I've got a wrong impression and moves to correct it: "Oh, there will be demi-humans in the game. Just no players can play them." Hmm. So he plays NPC elves and dwarves? Oh yes, he does. So I clarify: "So a group of players, none of whom you have yet played a second with, because this is an all new group, are incapable of doing a decent enough job or portraying an elf, when that is their one job in the game, but you can do it well playing a host of NPCs?" He says, "Yes," matter of factly, and goes right on telling me about the campaign.
I let the conversation wind down, wished him luck, and told him I didn't think I'd add very much to his game.
In a separate incident, and the only time I ever expected to walk, it actually worked out. We had a mini-con one weekend in college. This guy known for being a jerk DM planned a 2nd ed. AD&D game. He couldn't get enough people to play, and was hovering around everyone else's table. Well, the game I was running ended much sooner than we were expecting. So about midnight, the rest of the group broke up. My roommate and I had nothing better to do, so we figured we'd play in this guys' game until it self-destructed. (I can enjoy certain foolishness in the wee hours of the morning that I wouldn't tolerate otherwise.) We secretly agreed to watch each others' back, and see how far we could get.
With us, he had enough to get started. He started dicking with all the players immediately, and transparently trying to turn them against each other so that he could trash the characters. We all just kind of looked at each other and went with it. So for the next five hours, six players pretended to let this guy jerk us around, but we'd never actually pull the trigger and stab each other in the back. We kept successfully moving through the (rather tough) adventure, and it got to be kind of fun. Kind of like adventuring with one hand tied behind your back. We were roleplaying friction with each other just to yank the DM's chain.
He didn't know that my roommate and I knew each other. So we could push things such that he just
knew that we hated each other, and would be backstabbing any moment. We were in a zone, reading each others' minds. Then the rest of the players caught on and joined in. A couple of the players gave out as the sun came up, and we had to stop. I was a bit sorry, because I wanted to see how far we could go. But it was one of those magic moments that you can't replicate later.
I found out later the DM had asked some of the other guys about the game, because he had dimly sensed something special happening, but didn't know what it was. They honestly tried to explain it to him, without hurting his feelings, but he wasn't able to understand it. He was still young. So I hope he got it later.
