There was a lot of tongue-in-cheek with that not too serious question. The statement I commented on was "I usually carry two compasses. One might be wrong, two aren't." and in fact hikermor's post above is correct, two compasses can be wrong and for the same exact reason. I carry a compass, but the course taken from a GPS when walking is independent of the local phenomena that could affect a magnetic compass' accuracy/correctness.

Your point about carrying a back-up compass in case the primary is damaged or lost is valid.

Ron Hood demonstrated a good direction finding technique using a sun-dial: The Ottomani Sun Compass & Sun compass and direction finding