Broken lift and switch in The Citadel:

In the Doom II level The Citadel, there is a nonfunctional
switch located just above and behind the blue key.
To the left of this switch is a short lift/elevator, which
is in the lowered position next to the blue key.  The
switch was originally set up to drop the lift temporarily,
providing access to the key and courtyard.  This would have
also allowed the player to grab the key and ride back up to
the area where the switch is.  But due to a flaw in map
design the lift is permanently down; and so the switch that
was meant to control the lift does nothing useful.

PC Doom II players can click here to access a demo showing
the location of the broken switch and elevator.