The vanilla "Dark Hole" door types ( door_load_darkness00 ) lack collision. This tends not to be a problem, as they're typically placed in small places that players can't path through. TR though uses this vanilla mesh in several places where the opening surrounding the dark plane is wide enough to pass through, allowing the player to walk into the void behind the spaces. Raathim Ancestral Tomb, Lichgate is one notable example, but I'm sure there are other places, too. Ideally we replace this mesh with our own which has collision, or we slap collision boxes behind the worst offenders.