Maybe Hussar is referring to my posts, where I implied that if a player is pushing the argument that the DM is wrong because there is a manticore in a forest and they won't let it go, he or she is a bad player. And I stand by that, because not only is it the DM's world, but nothing about having a manticore in a forest triggers the unreasonable flags.
I also agree that the player that objects to manticores in a forest and won't let it go is a bad player, but my reasoning is slightly different than yours. It's not merely the player found that unreasonable that bothers me. It's their perspective on unreasonable information makes it almost impossible to run an interesting game.
The problem I have with the scenario it shows a player who clearly prefers to metagame than to game. This is a player who has a highly developed ability to deal with problems at the metagame level, but no ability to play a character. As such, I would prefer ask that player after the session was over to leave and find a different DM to torment, because that player is a waste, having developed such bad habits as a gamer, that it would be almost impossible and perfectly torturous as a DM to try to teach them how to play. I've got no use for players that have never developed even basic skills despite being highly experienced.
The following things really trouble me:
a) First, the player relied on player knowledge rather than PC knowledge and didn't even attempt to discover what her player might know but took it for granted that anything the player knew was applicable. The player knows that the MM says that manticores are found in deserts, but the player doesn't seem bothered about acting on this knowledge. The first question that needs to be addressed is, "What does my character know about manticores? Is it reasonable that my character would find this unusual?" Instead, the player shows zero nuance of this nature, happily metagaming away without the slightest embarrassment.
b) As such, the player's first recourse was to verify and question the metagame out of game rather than questioning the game in game. There are all sort of things that a skilled player might consider upon seeing a manticore in a forest.
a) Something big must be happening in the desert to get manticores to flee from their normal habitat!
b) Since this is a forest and manticores are only found in deserts, this must be an unusual subspecies. Be on the look out for unusual powers. Don't take anything for granted.
c) Since this is a forest and manticores are usually found in deserts, something must have brought it here.
d) Since this is a forest and manticores are usually found in deserts, this might be a shapechanged creature.
e) Since this is a forest and manticores are usually found in deserts, this might be an illusion.
f) In this game world, manticores aren't in fact desert creatures.
g) The DM forgot that in 2e, manticores are exclusively desert creatures, but so what? Why should I care?
If it turned out that the DM just made an oversight and did not wish to recover by making up an explanation on the spot, then it's up to the DM to admit, "Err... I apologize, but I made an oversight and put the manticore in the wrong terrain. I didn't intend that to be a big clue of such singular importance that it would be worth following up on. Please just assume that while odd, it's not that important." Even then, it would be incredibly rude to demand a retcon rather than a handwave. I find it pretty incomprehensible that someone's enjoyment of a game depends to any large extent on manticores only showing up in the desert. That isn't even a common fantasy trope, to say nothing of trope violation is often interesting in and of itself. Learning that things are different than I expected them to be and the most common stereotypes don't apply is often fun. I shudder to think what sort of player doesn't like being surprised or having any of their expectations overturned.
It's certainly not up to the player to tell the DM what ought to be happening in his world. The existence of a manticore of great size and ferocity in a mountain pass was established in my game world by 1987. I don't intend to kick it out my own imaginings and ideas simply because some one writes something limiting in a book. But if I'm a player in a DM's game were manticores only occur in the desert with near religious rigor, then I'm ok with that as well. It's his world.
c) The player evidenced zero trust in the DM. To even question this out of game is to seek out of game clues for the explained behavior. For all the player knows, the DM is well aware that manticores don't occur in forests and has made this a major part of the in game scenario and is expecting the players to follow up on this discrepancy in game. By demanding out of game that the DM give an out of game explanation to the player, rather than an in game explanation to the PC (assuming they qualify for one), the player has essentially overturned the game. They've demanded the DM admit there is a reason and therefore that the appearance of the manticore is in fact important to the plot and so should be treated as such (or conversely that it isn't and so doesn't need to be treated with importance). But the DM is under zero obligation to reveal his secrets out of game just because one player is confused. Since the DM is the secret keeper, it is not only incumbent upon them to keep the secrets so that all the players can enjoy discovering them in due time, but also their privilege to enjoy the moment when the secret is legitimately revealed and the honor they deserve when they create good and interesting secrets. The player demanding an out of game explanation might as well have responded by taking their hand and flinging the pieces of a board game off the table and across the floor, and have been equally rude and aggressive. Whatever carefully constructed plans (if any) the DM might have are in danger of being dashed irrevocably. To stick to their guns in this situation and demand removal of the manticore (as if even in the real world animals aren't occasionally found far from their normal habitats) is so ridiculous I'd be hard pressed to deal with the problem privately and not respond to the rudeness by suggesting on the spot that if the canonical ecology of manticores is so important to them that perhaps they ought to find a DM that shares their love of canon.
It's not like Hussar was having rocs being encountered underwater or something.
And if he was, so what? All the above still applies. Who brought the rocs underwater? How are the breathing? What does my character know about rocs? Do some rocs have the lifestyle of penguins, or cormorants, or pelicans? Do I have the same picture of the environment that the DM has? Perhaps I ought to ask him to recap the situation again. All of that ought to be considered before you tentatively and tactfully ask, "You do remember we are underwater, right?"
