In the Fire cave mission on Bajor, just before the 3rd forcefield door, if you go around the stalagmite left of the door and follow it close near the wall and then go past the alcove that isn't lit and then go into the first alcove on the right that is lit you will see a "crack" where sections of the wall, ceiling, and floor are not lined up and you can see into the "void"
Well, it's gray...
In Counter-Strike / Half-Life 2 / Source Engine level development, we call it seeing into the "void" where if someone forgets to line up the ceilings and walls and leaves a small gap where the player can see into the darkness outside the playable area.
It's either the 2nd or 3rd door, I can't remember exactly because I was in the mission yesterday and now my memory is all fuzzy. ALSO, the last door I was at wouldn't drop the force field because the group of cardassians spawned "inside" the forcefield and I couldn't kill them to get the forcefield down. I know this because after killing the last group on the walkways, I was looking for a group to kill to get the door open and that's how I stumbled onto the "crack" in the wall.