Inaccessible secret room in map 10, CONTAINMENT AREA:

Near the middle of the south edge of the level is a secret
room that is inaccessible to players.  This is due to an
error that was introduced when the original PC Doom map was
re-edited for inclusion in the earliest console Doom games.
In the original PC map, this secret room was accessed from
a corridor.  In the conversion process, that corridor was
removed, but the room was not.

This flawed version of the map first appeared in Doom games
on Sega Genesis 32X and Atari Jaguar, and was later used in
Doom 1 games on Panasonic 3DO and Nintendo GameBoy Advance.
Because that secret room cannot be reached, a SECRET score
below 100% is normal for this map in those game versions.
At high Skill levels the room also contains score-affecting
monsters, which can also affect a player's KILLS score.

A revised version of that map, where the inaccessible room
was removed, appears in Doom games for Sony PlayStation 1
and the Sega Saturn.