2 men guard 2 doors...

Backstory: This is in a Reign of Winter campaign that I am running and have modified a bit. The players have entered Baba Yaga's hut, which contains an almost labyrinth within it.

The party comes across a door, and upon the door is a sign:

Inside this room are two brothers guarding two doors.
One door leads to Baba Yaga's vault, the other leads to certain death.
You can ask of either guard a single question before choosing which door to take.
However, one guard always tells the truth while the other always lies.

The players know this riddle already, but have a discussion before deciding how exactly to proceed. When they enter the room they see 2 halfling guards, one burly and the other handsome. Directly behind them is a large and intimidating looking door. Also behind them but off to the side is a more mundane looking door.

The barbarian wants to ask the guards who they are, but stops himself in time knowing that they only get one question.

The face of the group steps forward and asks the brawnier guard "If I were to ask your brother which door leads to the vault, what would he say?"

Looking confused, the guard answers "He would say it's that one." while thumbing at the large door directly behind them.

Other guard: "Yeah, it's obviously that one." He says, pointing at the same door.

The face, looking pleased with himself, asks "Can we go through that other door then?"

Still looking confused, the guard says "Uh... sure. Go right ahead."

DM: "So, are you all just walking through the mundane door?"

The party is about to, but the more cautious ones stop the group and elect that the dwarf should go first while everyone else stands 10 feet back.

Dwarf: "I go up and open the door."

DM: "Okay. Make a fortitude save~"

What the party, and even the guards, didn't know was that the sign was put there by a third brother as a practical joke. The mundane door just opened up to the latrine. It smelled like death.