Trapped monster(s) on PSX Final Doom level 21, Lunar Mining
Project:

In the large room where you pick up the red keycard there
is a rectangular chamber behind a secret panel in the
northwest corner.  This panel opens when you step into a
small enclosure that's located a few paces east of the key.
Just behind that rectangular chamber is another, shallower
rectangular chamber that can only opened when playing the
game in Cooperative mode.
That shallow chamber contains 1 or 2 humanoid monsters,
depending on the selected Difficulty level.
Because this chamber is inaccessible in Single-Player games
it will be impossible to get a 100% Kills score.
This problem is due to a minor error made when converting
the original PC version of the map to PlayStation format.
(In the PC version, there are no monsters in that shallow
chamber in Single-Player games.)