In levels with rising walkway puzzles such as E2M2: Containment Area and MAP12: The Factory, sometimes the rising walkway segment doesn't change the sector property to a normal floor. This is not an actual bug in the engine, but rather an event unaccounted for in the level design. The bug occurs when the player steps on the platforms out of order. In this case, the bridge segment is completely surrounded by blood, so the "highest" floor has the same properties as that which surrounds it, and as it rises it will retain its old properties of a damaging sector with the blood flat.