It was rather early in our AD&D campaign, and the characters were around 3rd-4th level. One of them was Nightspell, an elf magic-user/thief who had got a truly valuable treasure: the Shadowring. This magic item allowed the wearer to make short dimension-door-like jumps, and it was most useful to get behind enemies and to backstab them, for example; the Shadowring worked three times a day, and was considered a great asset by the whole group.
The adventurers were following some clues which led directly to a mysterious tower on the moors. Just before they got inside, I asked my players to secretly write on a scrap of paper the reason why their characters were entering the tower. Everyone but one answered truthfully: the player of Nightspell suspected instead some form of mind reading, and trying to be smarter than the DM (always a bad move) wrote down that he was just looking for a way to ally himself with the dark powers that ruled the building.
The tower was no doubt ruled by a dark power, but its enchantment was such that only those who had fulfilled their stated mission could ever come out. The characters were eventually successful and managed to reach their goals, with the notable exception of poor Nightspell, who remained trapped inside the accursed tower! And with him the prized Shadowring!