Munchkin

First, I use a simple system proposed in Illuminati: Any promise that has to be resolved this turn is sacrosanct. Any promise made that involves doing something later in the game is not (you can't control that the right events will occur later). But that's in a private game where people know that.

Now, if you make a promise about the combat you're in right now and don't keep it, I suspect you'll find yourself not getting invited the next time the group gets together to play Munchkin. Of course, if you promise a helper all of the Treasure after you take first pick of two cards, then suddenly make it so there is only 2 Treasure gained from the combat, that's one thing, but if you then say, "Heh, just kidding, I think I'll keep all of these Treasures for myself," you're only going to make trouble for yourself in the future. Personally, I'd never play with you again, and if it were my game, I'd send you away from the table or pack up the game if I couldn't make you leave. I know that some people feel that just because the tag line for the game includes "Stab Your Buddy" means you don't have to play fairly, but that's not the case. I'm confident that the purpose of the game is not to be a total turd to your friends.

Is it an official position that people should play fairly? You're asking me if it's okay to be a dirty liar? What kind of person do you think I am?

Yes, you should play fairly.

You'll all have to forgive me, but I just find this notion that fair play is unnecessary when playing Munchkin preposterous and I get offended when people suggest such. I know of groups who can handle playing this way, but that's something the group agreed to. In general, whenever you play a game, you should do your best to play fairly. That means honoring your agreements, that means not cheating, that means not lying.