This is the door by which I exit the dorm every morning:
This is what Geoffrey K. Pullum of Language Log calls “nerdview”. By what definition is this not an exit? Fire safety! This stairwell does not have an exterior door at ground level, but only leads back into the building (right next to the lobby and main entrance). Therefore it is not an exit.
But for the ordinary mind, this sign is crying wolf. You'd think they could have said “Do not use in case of fire” or “Not an emergency exit”.