In Map04 of Alien Vendetta, when picking up the Red Key, a bunch of pillars with Chaingunners from the water below will instantly rise, ambushing you (if you don't react within a split second, you will lose a considerable amount of health). In Doomsday, the platforms rise up and the Chaingunners ambush you, but the platforms they're on are totally invisible.
These platforms are underneath the surrounding body of water, and instantly rise up using a dummy sector. Since the floors of these platforms are low enough underwater that you can't see them from your view, they don't bleed (the back lower textures are missing, when they would be normally required for this). These platforms are, however, assigned a different floor (a brown floor) and have front lower textures for when the pillars rise.