My reaction is mixed.
Honestly, I don’t care if players encounter a humanoid monster and say “This sounds like a troll. Let’s set it on fire!” GMs don’t have to make trolls vulnerable to fire… but on the other hand, shouldn’t change that detail simply to keep the players in the dark. It’s easier to say “Anything you’ve read in fantasy lit, fairy tales, or mythology or seen in a horror movie might exist, or might not. Use that as a guide to what rumors your character might have heard.”
Players shouldn’t meta-game to manipulate the GM or other players, however. And GMs shouldn’t use knowledge about gear the players brought or plans for raiding the dungeon to “win”. There’s also the case of players who interject out-of-place references into play, for example talking about disco music and machine guns when the other players want a more immersive feel. All of these are interpersonal issues, though, rather than rules issues or even rules mastery issues. Those players and GMs are being jerks, and the other players need to consider whether they really want to play at the same table.
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