The brave knight approached the pair of doors.
Each door sported a gargoyle, and as he approached, they came to life. One spoke, and growled “One of us will lie and one of us will tell the truth. One door leads to the treasure and one door leads to certain death. You may ask one question.”
The knight eyed the speaking door carefully, thought a moment, and then asked:
“Which door would the other gargoyle say would be the door that leads to the treasure.”
After a moment of eye-twitching, the gargoyle points to the other door.
The knight grins knowingly and opens the unindicated door, stepping through triumphantly.
The doors slam shut behind him and the tiny room is dark, but he can still hear the gargoyle voices on the other side.
“One class in logic and they all make the same mistake.”
“Right — we hadn’t decided who was going to be the liar yet.”
“Nor what we were lying about.”
Just as the brave knight realized he had not considered the superset of conditions, the blood worms dropped on his head and he knew no more.