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.
What is meant to happen is the player will walk directly from one segment of the bridge to another one. The sector they are stepping into will rise up 24 units, and change their floor properties to that of the highest adjacent floor. In this case, clearing the damaging sector special and changing the flat from blood to a normal one.
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.