Continuity mistake: The amount of blood on Alison's padlock in the first of the group traps changes instantly. (00:22:00)
Continuity mistake: In the beginning of the movie, Strahm reaches out into the hall to grab the tape player. The string it is hanging from comes with it. Strahm exits the room, and we get a reverse angle and the string that held the tape player is hanging from the ceiling, but empty. The string should not be there to begin with.
Continuity mistake: When Ashley is decapitated, a pool of blood appears under her severed head. Later when we see the head from the second room, the pool of blood is not present. (00:26:50 - 00:33:30)
Continuity mistake: When entering the second room, the position of the door, before it gets closed, changes depending on the camera angle. (00:33:15)
Chosen answer: Assuming the woman in the bathtub was able to be moved safely the amount of blood gained from her, while still being helpful, would probably not be enough to save the final two survivors from self harm. Without the heart pumping blood you would only get the blood that was in her arm at the moment. You would have to lift her or raise her so gravity would get other blood moving to get enough. Which the final two possibly could have done, but stress of the situation clouded their judgement.