"But your character wouldn’t know about that!"

After people play in several games (or play in many games over a course of years), they come to know all sorts of information that a first level character probably wouldn’t know. I used to play with a guy who would loudly say, “But your character wouldn’t know about that!” whenever another player would dare to utter something like, “Green slime? Get out the oil and torches!” or, “A potion? I hope it’s a potion of flying!” or something similar. And this was even when he was not DMing. It was as if he expected us to play ‘stupid.’ Often he would do stupid things that other players did not want him to do and then claim, “I was just playing my character.” I’d describe his ‘malady’ as a form of reverse rules lawyering. I found it very tiresome.

That said, I find it fun (and refreshing) to play with people who don’t know the Monster Manual inside and out. I think as a fellow player, the “gee whiz I wonder what will happen next” idealism of new players just introduces more fun and a less jaded energy to the group.

If I were to DM, I would not mind that player characters acted on player knowledge… to expect a seasoned player to sit there and let a rust monster eat his character’s +5 sword just because the character never encountered a rust monster before (but the player HAS) seems the height of folly to me. It’s a game, not a pure simulation. Just like someone playing their 10,000th game of chess is going to have an advantage over a new player who is still asking, “How does the horse one move again?,” so, too, the player who has been playing D&D for years can be expected to have a few advantageous nuggets of wisdom that may help his character in a pinch… then again, players that assume that everything is going to be the same in my campaign as in the one run by their chum in highschool might be dissapointed (I think it’s fair game to introduce variant monsters like a variety of green slime that is vulnerable to cold instead of fire or traps that strike the area that most seasoned players might expect to be safe). If you need a justification, just allow that the new character sat on his grandpappy’s knee every night while that retired adventurer told him about rot grubs, green slime, harpies and gelatinous cubes.

Now, I also think it’s perfectly fine to introduce house rules and rules variants to your home campaign. If these rules would possibly directly impact the player’s decision making process, it’s only fair that you would try to let them know ahead of the time when they are in the middle of a situation and trying to decide what to do. Failing that, allowing a player to ‘take back’ one action (especially if it seems obvious that the player would have chosen differently if he knew about a house rule), seems only fair. If the player isn’t a dick, they can probably be trusted not to abuse your patience by invoking the, “But I didn’t know” clause too often.