DM to the player running Bubba : "He's a theif."
Bubba the Brainsmasher : "SO?"
DM : "Theives backstab for quadruple damage. One strike and he'd kill you."
Bubba the Brainsmasher : "WHAT!?!?!?!"
who me wrote:way later the paladin player found out that there had been three bags of gold in the wardrobe and then a big debate broke out, because he found out, his character did not yet he brought it in to play and wanted to hang the thief.
That's where the DM needs to let the player know that a player and a character are not the same. Just because a player knows something, doesn't mean his/her character knows it.
It's difficult. The player can get mad when you tell them that. Even though they wanted to spend attribute points on strength and dexterity instead of intelligence or wisdom to get a bad a$$ fighter.
"Sorry, Bubba, but you failed your perception role. You didn't see anything."
"But I know they did it!"
"Yeah, you know, but Bubba the Brainsmasher doesn't know."
A good player won't let it go any farther than that. He/she will let it drop. A bad player will get angry and the gaming experience of all will suffer as either they'll be p.o. the rest of the night, or the other player will "break character" and (in your example of the bags of gold) fess up to the bags of gold.
That's another thing the DM must determine before game play. How the gamers want to go about their gaming experience. I once had a group in which I would often write things down on pieces of paper and hand them to different gamers for what their characters would see, hear, or detect that the rest of the party couldn't (like a character with night vision seeing a trap in the dark) The player would then decide whether or not to tell the rest of the party. This group of gamers had no problem with this and understood the reason why. I had another group of gamers in which one of the gamers got very angry over this. Thinking I and other some of the other players were conspiring against his characters, so I didn't do the note thing. Even though it made the gaming experience more realistic, everyone decided it was better that everyone was having a good time.