I hit this bug as well in the Chemical Labs, while
Being lagging like mad is a bad combination when being attacked by two of them, and needing to barricade a door. I don't see what exactly would make it do that. There shouldn't be any AI running (they're just idling on the other side of the door until you hit the trigger point) and it's certainly not an intensive room.
Rest of the game ran perfectly fine with everything on (except motion blur) at 1280x1024.