Not just that, but it becomes unclear where the line is for what the player does get to arbitrate about what their character knows and what they don't.
I can't decide what my character knows about trolls - do I get to decide what my character knows about elves? Should I roll dice to see if my character has ever actually heard the name of the continent they live on?
In all cases, situationally dependent.
If your background is that of a well-educated and-or somewhat worldly type (or other factors combine to indicate such), all is good; and hey - you might have even heard the troll story too. If your background is that of an illiterate farmer who has never gone more than 20 miles from home then hells yeah you're rolling.
I'm not going to be surprised if I get told that is being ridiculous, but I genuinely don't know where anyone besides me draws that line unless they tell me.
In fairness, the line is probably going to be in a different place not only at each table, but for each given situation within that table as noted just above.
There's a flip side too: often character knowledge exceeds player knowledge, or the players have forgotten things their characters would remember (in many but not all cases because it's been a week or two for the players but 15 minutes for the characters); and in these cases the players are brought up to speed. One example from my current game: the players constantly forget the name of the game world even though the characters have all from one source or another known it for ages. So they ask, I remind them, and we carry on.
Elfcrusher said:
Yes, of course. I don't think anybody here is arguing that you should give away key 'secrets' if somebody else at the table is not in on it, and would have fun solving it. That's called "being a jerk", not "metagaming".
I almost get the sense that some here are arguing for just this...the ones who in their session-0 state metagaming is fair game and if the player knows it, the character knows it.
At the same time, if the fun of an adventure depends on secrets, that's a design flaw. Why design something that's susceptible to an honest mistake? "Oops...sorry; I thought everybody knew that. Guess there's no point finishing."
By "secrets" you mean, I assume, things that once were somewhat secret but are now well known among gamers e.g. troll-and-fire. Because every adventure in one way or another depends on secrets - be it secret information, or secret rooms, or secret effects, or secret items, etc. - and by no means are all of them design flaws.
But yes, regrettably, these days designing an adventure around the secret that trolls don't do well against fire has become kinda pointless. And that is a bug, not a feature.
Imaculata said:
I have a player in my group, who plays a priest. And occasionally he'll make presumptions regarding monsters, deities, or other game situations, based on his experience as a player. But he only does this in regards to things that he assumes his character as a priest would know.
Which means the player is self-limiting the character's knowledge...I can (with some reservation) kinda get behind that; and good on the player.
It's when they don't self-limit like that - going back to the trap example again but let's change it up a bit (water is too easily beaten in 5e

): make it a trap where the away scout falls 30' into a pit of acid; the players know this but the characters do not and can not. Party happen to have a scroll of Protection From Acid on board - one-shot, gives everyone within 10' acid immunity for half an hour. While searching for the scout they reach a pristine hallway and don't (or can't) for whatever reason notice the trap. Do eyebrows go up if only now does the party use the scroll, even though there may still be half the adventure left or more?
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As for the underwater adventure question: while it's quite true that the 5e version of Water Breathing allows for ongoing underwater adventures* (which is just fine; kinda cool in fact) I still maintain it's a bit broken the rest of the time as by in effect being permanent it outright neutralizes what might otherwise be a legitimate obstacle and sometimes hazard. It also kinda ruins maritime ship-board campaigns or adventures as perhaps the biggest ongoing hazard there is somehow going overboard. Can't swim? Who cares, you can't drown either.
* - just occurred to me now - can S-component spells be cast underwater?
Lan-"Akrayna. The world's name is Akrayna."-efan