Continuity mistake: When Strahm has just discovered the hidden door near the start, the position of the drips from the glow-in-the-dark gunk changes between shots. (00:06:45)
Continuity mistake: The shotgun that is positioned to Hoffman's throat changes position between shots. Note the gap between the barrel and Hoffman's throat. (00:41:05)
Continuity mistake: When Jigsaw's lawyer puts the opened envelope down on the desk, having taken the video out, he puts it horizontally on the folder. Camera cuts and it's now rotated 90 degrees instantly. (00:12:40)
Continuity mistake: When we first see Hoffman looking at the CCTV screens, we can see Ashley is lying down. When it shows her waking up, however, she is sat up. (00:20:20)
Visible crew/equipment: When the police cars arrive after Strahm's cube trap, there is a very quick shot of the black police car passing before the camera. When it does, a white screen and black stage light are reflected on its surface. This is just before it shows the SWAT going in. (00:10:45)
Continuity mistake: Spoiler: Right at the end, we find out that Saw III and Saw IV were happening simultaneously, as demonstrated by Strahm walking in and killing Jeff. We then see Hoffman close the door on Strahm, locking him in the room. Saw V continues from the exact moment when Hoffman shuts the door on Strahm. In Saw V, Strahm quickly kicks the door with his foot just seconds after Hoffman has closed it. However in Saw IV, we see Hoffman walking away. In this shot we should hear Strahm kicking on the door.
Revealing mistake: When Strahm is being crushed by the "closing walls" at the end, watch closely during the last few seconds. There is a brief shot from above Strahm, looking down as he screams and the camera is spinning. If you watch closely, you can tell the "closing walls" that are crushing him aren't moving during this shot.
Continuity mistake: Jigsaw puts the razor down on a piece of paper and it is in a near right angle. When Hoffman picks the razor up, it is still at an angle. However, as soon as it's in his hand it's perfectly straight. There is no time for Hoffman to straighten the razor without us seeing him do so. (00:47:00)
Continuity mistake: When the five victims are in collars ready to start their first test, Ashley gets decapitated at the end. There is a shot of her head on the ground in a pool of blood. But in another scene later on Mallick goes through the door into the next room where you can see the head on the ground, and the pool of blood under the head has vanished. (00:26:50 - 00:33:15)
Revealing mistake: When Stahm is in the cube trap, there is one shot just before he sees his phone, gun, etc. and you can see a massive gap down the side of the cube where it wasn't shut properly. With the size of the hole, it would be extremely hard to keep water in the cube. (00:09:35)
Continuity mistake: Just as the second room trap comes to a close we see 9 seconds on the timer and hear it ticking down. We hear 4 ticks and then there are 2 seconds left. There should be 5. (00:37:20)
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.
Character mistake: When Erickson is telling Strahm that he no longer has any jurisdiction over the Jigsaw case, he mentions that over a dozen dead bodies were recovered at the meat packing plant, including two cops. There were in fact only 10 dead bodies: Danica Scott, Judge Halden, Timothy Young, Jeff Denlon, Lynn Denlon, Jigsaw, Amanda, Art Blank, Officer Rigg, and Detective Matthews. (00:20:28)
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)
Visible crew/equipment: When Jill Tuck is talking to Agent Erikson in his office, there is a shot with a boom arm's shadow visible across a picture and bit of wall, behind Agent Erikson. (01:03:40)
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.