Inaccessible Items/Secrets in level 20, Unholy Cathedral:

In the main, central courtyard, there's a secret panel in
the east wall.  (It's to the lower right of the red, square
indentation high up in the wall.)  Behind this panel is a
small chamber which contains a Soul Sphere ("Supercharge")
and a chain saw.
Unfortunately, in PSX Doom this chamber is inaccessible due
to a flaw in how the map was translated from the original
PC Doom game.
In computer-based versions of the game, this secret panel
can be opened with the yellow key.
In PlayStation 1 Doom this panel can't be opened with the
yellow key, or by any other means.  Because of this, the
maximum Secret score for this level will be under 100%.
For better or worse, this is normal for the PS1 version of
this map.