Missing Item(s) on PSX Doom map 55, Fortress of Mystery:

In single-player games, a player will typically exit this
level with 75% Items (or less), even when the player has
picked up or touched all accessible items.
This is due to the fact that two Computer Area Map items
exist on the level, but a player is only allowed to
retrieve one of them.
(A Computer Area Map item is intangible to a player who
already possesses one.  A player who tries to pick up both
Computer Area Maps will pick up the first, but only walk
through the second.)
On level 55 a remaining Computer Area Map item will account
for as much as 25% of all attainable Items on the level.