"We're playing a Living Module and the XP budget only allows so much, so that dragon cannot be real. I ignore it and attack the other guy"
"Although my character has no knowledge of aboleths, I do, so I stay at the back and let the other players charge it"
"I read about Tomb of Horrors online, so I'll let the others climb into that mouth and not die myself"
"The Gm knows that the young kid playing the druid will get super-upset when their animal companion gets hurt, so we'll use it to scout ahead"
In all cases, these are often unvoiced thoughts.
Exactly. So, you'd advocate to have a thought police in the game? It is often unavoidable to have a certain amount of player knowledge. But how does it affect how you have your character act?
Let's look at the examples:
1) Now, I haven't played any Living Modules, but I'd never rule out a dragon encounter, in which I'm not supposed to actually fight the dragon. And if I, the player, suspect the dragon may be just an illusion, I'd try to call on my character's knowledge, e.g. "Wow, there's a dragon living in this area and no one has yet reported anything about it? Surely it would have been noticed before."
2) That's, imho, simply playing it carefully: "There's a scary-looking, huge monstrosity hiding in that dark pool? Charging _anything_ unknown strikes me as a bad idea, and this seems triply so."
3) Another case of simple suvival instincts: "Climbing into a mouth of unfathomable darkness? Err, have we really exhausted all other options? How about trying to delve it first in some way?"
4) At first, I didn't even get what this example is about. It took me a while to figure out, you're assuming the GM is taking favorites. Sorry, this is simply not an issue in my games. No one and nothing is ever safe from harm. And even if the GM did take care not to harm the animal companion, how is that supposed to make it safe for the rest of the party to let the companion do the scouting? It could just as easily lead an angry mob of monsters to the party while fleeing from the resulting fight and hiding safely in a tiny crevice.
Since I'm most often the GM, I tend to have the highest potential of having player knowledge that is beyond my character's. My ways to mitigate that lies in choosing appropriate (knowledge) skills, acting cautiously as a default and often playing the devil's advocate (i.e. I'm doing it whether I suspect to know what's going on or not).
As a GM, I also often make good use from what a player _believes_ to be 'meta-knowledge' when she's actually completely mistaken. E.g. one of the players in my group is a walking monster encyclopedia, but he only knows the monsters from D&D 2e. So, in my 3e and 4e campaigns he's a useful source of 'rumours'. Some of his information is accurate, some of it is completely wrong. He also doesn't always correctly identify monsters. Monster templates work wonders in obscuring what the party is actually encountering.