Trapped monsters on PSX Final Doom level 23, Ballistyx:

On this map there are monsters which are trapped in an area
that is inaccessible to the player.  This is due to a minor
error made while converting the original PC map for use in
this game.

The player begins on this map in a small outdoor area.  A
few paces west of the starting location is a large room.
Just north of that large room is a hidden area where a few
Imps are located.  When those Imps hear the player use any
weapon, they are supposed to teleport into the large room.
However, some of the Imps will be unable to teleport out,
and will remain trapped in that inaccessible area for the
rest of the game.  Because of this, it will be impossible
to get a 100% Kills score on this map.

When playing at the lowest two Difficulty levels, 4 Imps
will begin in that inaccessible area, and 1 can teleport
out; When playing at higher Difficulty levels, 6 Imps will
begin in that area, and 2 can teleport out.