You approach 2 guards and 3 doors. You know that 2 of the 3 doors will lead you to inevitable death and the other leads you to safety. You also know that when asking the guards about the doors, they will only indicate one of the doors and that you may ask one question to both guards or 2 questions, one to each guard.
This is all written on a tablet before you. However as you approach the guards to begin asking questions, they speak and give information not on the tablet.
The first guard, named 1, says, "One of us only tells the truth..." To which the second guard says, "... and the other only tells lies."
They then command you to begin your questioning.
How do you pick the safe door with absolute certainty?
Clarification: A guard, in this situation, is either a knight (who always tells the truth) or a knave (who always lies).
You ask guard 1, "If I were to ask guard 2 if or not door 1 is safe, what would he say?" Flip whatever answer you get and that's the status of door 1. If you conclude it's safe, walk past. Otherwise, ask the same question for door 2 and repeat the process.