"After the area with the crushers, you come to a room with demons at either side. After this room if you turn around and press use, it opens the sector as a door and then closes after. It can be opened again from that side but if you open the sector and go back towards the start, you will get trapped and have to kill yourself or load a saved game, or in online play restart the game completely. This can in some rare cases be triggered by a monster. " Can it really be triggered by a monster in vanilla? I remember that in Doomsday, it can; maybe in ZDoom also.