In underground missions, in caves, the walls or sometimes open spaces turn black. My captain and bridge officers disappear and I can't see what's in from of me.
Sometimes it's a wall and I don't know that I've stopped moving until a sound occurs or sometimes it's the enemy and I walk into an ambush. The black section in front of me is marked by a straight border not shaped by the terrain. This only happens in caves. Not during boarding parties or in buildings.