Inaccessible second exit/"secret exit" in PC TNT/Evilution
level 26, Ballistyx:

This Final Doom level has 2 exits, one that's accessible in
Single-player and Cooperative games, and another exit (just
outside of a caged area which overlooks a walled-in field
of pillars) that's accessible only in Deathmatch games...
What you're seeing is the Deathmatch exit.  (In Deathmatch
games only this outside part of the map is accessible.)

Both exits lead to level 27, Mount Pain.

For more information including maps and screenshots, access
http://classicdoom.com/bal-dmx.htm