Visible crew/equipment: Several times in the movie you can see a stunt double for Donnie Wahlberg, like when he tries to hang himself. Art runs over and there is a wide shot focusing on both of them. No slo-mo required, but you can see it's a stunt double.
Continuity mistake: The blood stain on Riggs' white shirt around the neck has blood under the collar, then at the end it is over the collar and has increased in amount.
Deliberate mistake: When Agent Hoffman writes the letter to Amanda, he puts the envelope so her name is pointing down, yet when Amanda opens the drawer her name is at the top. The director had to do this or it would have revealed Hoffman was the new apprentice all along.
Other mistake: In Saw II, Jigsaw shares a story with Detective Matthews about how he attempted suicide after his cancer diagnosis by driving his car off a cliff. We then see Jigsaw pulling a metal rod out of the left side of his stomach. However, no puncture wound/scarring is visible on Jigsaw's stomach when his autopsy is being performed.
Continuity mistake: When the guy with the mouth stitched shut has put his legs against the machine to avoid being pulled anymore, a few seconds later you see him with his mouth open, enough for him to easily say something. At the end of the scene you see him being able to open his mouth easily, something he could've then done much earlier. And even if say his tongue was cut out to explain him not being able to say anything with an open mouth, he wouldn't be able to scream at the end either.
Visible crew/equipment: As Jill is being interrogated, when it cuts to a shot where the two cops are looking at her through two way glass, a camera man is visible between them and the boom mic in seen at the top left hand corner, very faintly.
Plot hole: When Rigg is listening to the tape in the first hotel room, the tape instructs him to wear the pig mask in order to conceal his identity from the hotel cameras. Rigg wasn't wearing the mask when he entered the hotel, so putting the mask on after the cameras have already seen his face would do absolutely nothing to conceal his identity.
Revealing mistake: When Rigg shoots Art, just before that you can see Art shouting. Look at his chest. The bullet wound is already there (no slow-mo needed).
Revealing mistake: When Cecil's face is being cut by the Knife-chair, the first time he comes into the blades, it is painfully obvious that the blood is being pumped through the knives, and that his face is not being cut. You can even see his bloody, yet cut-free, face almost come through the knives for at least two full seconds, before there is a flash-frame jump-cut, and a number of deep cuts magically appear on his face.
Revealing mistake: When Art is hitting the Trevor (the blinded man) character with the hammer/hatchet in the beginning, if you look closely, you can see blood coming from Trevor's head, yet there is no wound or cut. The blood is actually coming from the prop that is hitting him in the head.
Factual error: Several mistakes about the clock in Jigsaw's workshop: around 300 years ago the first pendulum clocks were built. They were completely different by size and design than this one, so the claim that it is so old is completely wrong. Real pendulum clocks like the one seen in the movie never have a second hand from center. You can also see by the movement of the second hand that it is a modern, battery operated quartz movement. This also means that the ticking you hear is not created by the clock itself but was added later.
Continuity mistake: When Rig opens the door the clock says 00:00 time left, but when he is talking to Art on the floor the clock says 00:01.
Continuity mistake: Rigg pulls the red scarf off the blonde woman and we don't see where it goes. Then there are two closeups of the woman where the scarf is nowhere in sight, and she has nothing on her left shoulder. Then in the next shot, suddenly the scarf is covering her and the machine behind her, and part of it is draped over her left shoulder.
Continuity mistake: When Jill went to get Cecil's jacket, she didn't lock the door behind her, yet, when Cecil forced himself in, he had to unlock the door to get into the same room his jacket was in.
Continuity mistake: If you go back and compare the ending of "Saw III" with the ending of "Saw IV", you can tell the lighting is different in certain areas (specifically when Strahm sees the dooor swing shut on its own), even though it is supposed to be in the same time and place.