I'd like CharlesRyan, Cadfan, and others who believe in the entertain-first camp to comment specifically, but of course anyone can contribute...
In this 3.5 scenario, my PCs were exploring a partly sunken ship and encountered a giant living mass of seaweed. I described the creature as vaguely anthropomorphic including a gaping maw where a person's mouth would be.
The halfling wizard player glomped onto the idea of the gaping maw and on his initiative said, "I throw myself into the gaping maw!"
(I knew he was going to do the "activate a Swan Boat inside a monster" and instantly kill it cheese...)
Now, I knew the creature didn't really have a mouth. It was a detail to create an image of horror. Since it's a plant, it simply absorbs, not chews.
Now, I fully understand how Chekhov's Gun worked here. The player heard a detail, thought it significant, then acted on that knowledge.
Here's what I'd do in this situation.
Principally, I'd address the character's perception of the situation. You're right, the maw might be a Chekhov's Gun, so I want to make sure the scene he perceives matches what I think I'm describing. Have I imagined the "maw" as a sort of dimple that suggests a horrific face? Or have I imagined something that actually looks like an opening?
If the former, I'd probably just clarify: "OK, you're considering that plan, looking for an opportunity to get closer, but the more you look the more you realize it isn't really a mouth."
If the latter, I'd ask myself why I pictured it that way, and does it really matter? If not, I'd amend my vision*, and clarify like above. If it does matter, I'd give the player a shot at a Spot check to realize the plan won't work. (Frankly, since I'm not out to instantly kill the character, I'd probably set the DC pretty low.) Even if there's a strong reason why the maw looked like a real mouth, I'd still call for some sort of check (perhaps with a higher DC) to realize the plan wouldn't work**--"It's a plant creature, and it probably absorbs its food, rather than chews. Based on your experience, you suspect that it might not actually swallow you if you jumped in."
If those checks were failed or the player pressed on anyway, I'd play things out. If there was another opportunity along the way to use a check or something to give him a more accurate view of his chances, I'd take it. But otherwise, let him play. Yeah, the character might go down--but he did just throw himself at a dangerous monster, so that was a risk.
In short, whenever I suspect a player is going to do something stupid
because he doesn't perceive the situation the way I think I'm describing it, I look to sync up his perception with mine.
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* When I suggested I prefer a less rigid play style than Varis's, this is exactly the sort of flexibility I had in mind. Not to redraw the map or change monster stats, but to examine the details I've set up in my mind and see if they're actually serving the game. If a superficial detail--like the shape of a nonfunctioning maw--is going to create an effect I wasn't going for, I have no problem changing it.
** One "strong reason" why the maw might look like a mouth is because I might want to teach the player a lesson for relying on insufferably cheesy tactics. If that's the case, the DCs to notice otherwise might be a bit higher . . . .