There are two rooms, empty, with identical features. They share a ~3’ thick common wall. The only entrance to either room is on the wall opposite the common wall. Ideally, there should be no obvious path from one to the other. After players find the first room, they should need to pass through several unrelated areas before they can reach the second one.
The common wall between the two rooms has a slit in it. It’s 1’ long horizontally, and 2” wide vertically. Big enough for an arrow to fit into perhaps, but not an arm or a spear. The slit goes all the way through to the adjacent room.
When characters look through the slot, there appears to be a chest sitting in the center of the other room. However, when they reach that room, they’ll find it just as empty as the first room was. But if they look through the slot from this room, they’ll see that the same chest now appears to be in the opposite room. If there are players in both rooms, they’ll both see the chest in the room opposite the one they are in, but will not be able to find it in their own room.
Spells such as “True Seeing” or “Detect Magic” will reveal nothing, because there’s absolutely nothing magical about any of this. The chest is a sophisticated, but completely mundane, optical illusion. In the center of the slit between the two rooms is a tiny card with a chest painted onto both sides of it. If the card is fished out and examined, the ‘lid’ opens like an envelope. Inside is a 2” x 3.5” white card. In bold printed letters it reads “This card is proof that the task has been completed.”
If the card is given to anyone who has assigned a task to the players, that person will accept it as incontrovertible proof that the task was completed. No amount of logic will ever convince this person that the players failed. Their brain will fold over on itself to find ever more ridiculous explanations for evidence to the contrary, and they will go completely insane rather than believe the task remains incomplete. Further, anyone they show the card to will similarly agree that it incontrovertible.
If the king points to his adviser standing 5’ away and says “I want you to bring me my adviser’s head on a plate,” and the players instead hand the king this card, the king will say “Excellent! My adviser’s head on a plate! Exactly what I wanted. Here’s the reward I promised you.” All the while, the adviser is still standing 5’ away shaking in fear. If this is pointed out to the king, he’ll assume he’s seeing his adviser’s twin brother, or his adviser’s ghost. He may be annoyed that he now needs to hire a ghost hunter, but that’s not really the PC’s fault, now is it?
The card only works once. If the players endeavor to steal the it back, then whomever they give it to next will still view it as proof of whatever first task it was used for. So if the players were to give it to a farmer who needed his lost child rescued from orcs, he would be horrified when the players handed him the royal adviser’s head on a platter.