Trapped monster on PSX Final Doom level 19, Deepest
Reaches:

In the southwest quadrant of this level is a large square
room.  On the Automap, this room is diagonally oriented.

In the middle of the room are some low, thick wooden walls
which partially surround a switch in the center.  When the
switch is activated, the outer walls of the room open up,
allowing access to a raised walkway that surrounds the room
on three sides.

Behind that switch in the middle of the room is a wide,
rectangular wooden pillar that stretches floor to ceiling.
An Imp is standing on top of this pillar, at its south end.
(Next to the Imp will be a rocket launcher or chainsaw.)

This wide pillar was intended to drop down to ground level
when the player picks up a weapon from the small square
platform on the northeast end of the raised walkway.
However, an error in this map prevents the pillar from
dropping.
(If you have stepped across that low square platform and
hear a continuous low grinding or thrumming noise, that is
the pillar attempting to drop.)

Although the Imp will remain trapped, it is still possible
to kill it.  To do this, face the southern-most end of the
wide pillar, then stand a few paces back, and fire your
pistol, chaingun or shotgun at the pillar.  Some bullets
or pellets may penetrate through the crack at the top of
the pillar where it meets the ceiling, and strike the Imp.
Listen closely, and you should be able to hear the Imp
being hit and/or killed.