So, in the limited number of games we've played, my group (I still can't bring myself to call them my "cult") tried some stunts that seemed a little shadey for me. I couldn't find anything specifically against their actions in the rules, but…
Well, two examples spring to mind:
1) I don't remember the specific Mythos card that was in play, but it was a weather card that kept the investigators from sealing a gate location. We were one gate shy of a Gate Seal Victory, and the doom track was at 2 to go before Yig (dumb ol' Yig! Three games and he's been the big bad twice! Poo!) showed up. My friend was playing Kate Winthrop, so she sailed through the Abyss and came out, camping on the explored gate, waiting to close it until (hopefully) the weather changed, and we could seal it off.
I suppose there's nothing wrong with that per se, it's just… I dunno, can you meta-game a board game? We won by that gate seal, BTW, after 5 (five!) passed turns. (We then ran through a mock fight with Yig and beat him up, too. Dumb ol' Yig.)
2) Throwing a fight against a monster… do you have to try to win a fight? Another of our investigators were faced with a Night Gaunt, and was actually wanting to do some other-worldy knees-bent running about advancing, so he thought he'd just throw the fight and take a free ride to R'Lyeh. Some of us said that, as Ashcan Peter has never seen a Night Gaunt before, he wouldn't know that that would be the end result of his failed [non-existant] combat check. Our friend laughed and said "It's printed right there on the card!"
I wept, and died a little.
My years of table top RPGs makes me want to play the investigators as "characters" and I cringe a little every time my friends use their "I'm a literate, breathing person" knowledge v. their "character" knowledge to pull something in-game. I'm secure enough to let others enjoy the game as they would like to enjoy the game, but I also don't want to break the rules of the game, such as they are.