Missing red key on Doom II level 12, The Factory:

There is no red key on this level.

If you are in the large square "Cacodemon Room", and the
door to this room closed behind you, there is no way back
out into the rest of the level, unless you use cheat codes.
By activating the 5 switches in the room, a path to the
level's Exit is revealed in the northeast corner of the
room.  This Exit is the only way out of the level without
cheating.

Background:
Dominating the northwest corner of level 12 is a large,
square structure.  On the outer east side of the structure
is a door which can be opened without a key.  This door
leads into a small chamber.  On the west side of this
chamber is another, special door.  From the chamber side,
this door can only be opened with a yellow keycard.  From
the opposite side, the door can only be opened with a red
key.  If the player is on the "red" side of the door after
it closes, there is no way back out to the rest of the
level (unless the player uses one or more cheat codes).

Rumors suggest that the red key appeared in early test
versions of PC Doom II, but was later removed.