Trapped monsters in PlayStation Doom level 45, Tenements:

You begin this level in a small chamber at the southern tip
of the map.  Directly in front of you is a large room with
a slime-covered floor, and a raised walkway running north.
As you move into this room, sliding panels in the east and
west walls rise up, revealing a pair of elevated chambers
with Imps inside.

When playing at Ultra-Violence skill, 2 of the 5 Imps in
the west chamber will be trapped behind its back wall, and
are essentially unkillable.  Because of this, a player's
Kills score is normally below 100% when playing at the
highest Skill level.