Which is all fine Celebrim, if I had, in fact, placed that Manticore intentionally with all those questions you asked earlier answered. But, as I said earlier, I didn't even know that manticores lived in deserts. I placed the manticore there (or the roc underwater) and I was 100% WRONG...Why does that fact keep getting ignored?
First, I'm not even sure that is possible for a DM to be wrong about this. Like the treasure type, the number appearing, the % in lair, the alignment, and even the HD of the monster, all of those things in the entry represent only the most common examples and are no more than guidelines for DMs intended to help them craft their campaign but which are not to be expected by either the designers or the players to be completely binding. It is after all only 'favored terrain'. If most manticores live in the desert, it doesn't imply they all do.
Secondly, you are missing the point. The biggest problem with the player behavior is that they actually never considered whether or not they had any way to evaluate whether or not the DM was wrong, but considered it their prerogative to question it anyway. In this case, it happened that you hadn't considered the favored terrain before placing the monster, but they had no way of knowing that. The biggest problem was that a player with actual skill as a player would probably have never been able to discover you'd made a mistake, but would instead have been left with a minor mystery - "Why is there a manticore in the forest?" You as the GM would have only been able to pick up on the player confusion through character activity, and only been able to infer that the player thought that manticore's were unusual in the forest by the sort of propositions that the player was making on behalf of the character.
Had I had some grand plan about why manticores were appearing outside of their favoured habitat, then fine and dandy. But, the player was 100% right. For your story above to be the same as my example, Alderaan would still be there.
What my example is intended to show is the player failing to engage the setting, the character, or actually to even play. This is a player that literally knows everything about RPGs but is repeatedly demonstrating that the don't know how to play an RPG. They have a lot of experience manipulating RPG metagames, although even then, not artfully or respectfully but crudely and rudely, but never really show any desire to actually play the RPG. For this purpose, it really doesn't matter if the GM has it all thought out, or he's just making this up as he goes by the seat of his pants. While I greatly prefer the sort of game where the GM uses forethought and preparation to construct a game, that is a preference and its not objectively bad to run a game with a different paradigm - not the least of which because all game styles require a certain amount of improvisation.
If a player is fishing to determine whether the DM is improving or using prepared ideas, that itself is poor play on the part of the player. (And it's poor play on the part of the GM if you can as a player actually reliably detect the difference. Good improv feels and seems a solid as prepared text, because otherwise it's very hard to avoid the game being disrupted by metagaming. This is particularly important when mysteries are to be solved. For example, you don't want the gardener to be revealed as having no information of importance and no relation to a crime because it's clear that the gardener is an unnamed PC you forgot to make notes on.) In other words, good players aren't trying to trip the GM up in the metagame because they want to experience the joy of the game itself and have learned to value and enjoy the game itself. The 'Luke' player in my example, just can't. He can't let go of the metagame. It's probably for that hypothetical player, the whole of the game. And as a DM, with a player that only enjoys the metagame, you are deprived of one the greatest joys of GMing - watching your players play. Not only that, but in all my experience, a person that defaults to metagame play also defaults to manipulating you as a person in order to solve problems, rather than defaulting to manipulating the shared imaginary space. And I have to say, dealing with a person that is all the time trying to bully me, brow beat me, rules lawyer me, cheat the dice, read me, get me to explain IC things OOC, wheedle me, conjole me, and so forth is just plain tiring.
There, is that clear enough for everyone? I placed the manticore there completely out of ignorance of the favoured terrain of a manticore. Not only that, I couldn't give a rat's petoot what the Monster Manual said because I simply do not care. I never bothered to read the Monster Manual. Heck, when people started to bitch about 4e putting Blue Dragons on the coast, I actually had to go back and read the older Monster Manuals because I had no idea that blue dragons live in deserts. Didn't know and certainly didn't ever care.
But, the player DID care. He DID know. HE WAS RIGHT!!!
No, he was wrong. If you the DM don't care what the Monster Manual says about manticore habitat, it's simply not operable. Manticores live wherever you want them to live. That's the actual rules. Favored terrain isn't a binding contract. It isn't steam rolling a player to say he found a manticore in a forest. It would be steam rolling a player to tell him he hated manticores and therefore had to fight the manticore, or anything else to do with his player. But an attempt to tell a DM that Manticores aren't found in forests is steam rolling a DM, and a DM resisting that is not rolling over his players but just avoiding being rolled over.
Again, the problem here isn't players speaking up. I welcome players speaking up. The problem is in this case, the player has no way to evaluate that this is an oversight and even if they suspect it might be an oversight, a skilled player's first instincts are going to be to determine if it makes sense for some unknown reason. A monster located outside its favored terrain isn't 'wrong' and certainly not '100% wrong'.
