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.
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.
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.
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.
Continuity mistake: *SPOILER* When Agent Strahm breaks into the Jigsaw's room in the end, he literally comes in less than a minute after "Saw III"'s conclusion. Yet, if you look, Amanda's dead body is in a completely different position than the end of "Saw III", which makes no sense, as there was no time or reason for it to be moved. At the end of III, her body was facing towards Jigsaw, and there was only a small amount of blood around her. Here, her body is facing in a different direction, and there is a ridiculously larger amount of blood.
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: The timer for Ivan's trap is messed up. The first shot we see it at 3 seconds. 12 seconds later we see the time at 42 seconds, and we know the 12 seconds are continuous as Jigsaw's tape plays during this time. 23 continuous seconds later we see it at 37 seconds (this would have been right if not for the second shot). This is when Jigsaw's instructions stop and his one minute should begin.As the first scythe descends into his right eye the timer is at 55 seconds. We see it at 57 seconds just before he gets his limbs ripped off.Two seconds is not enough time for him to scream after the injury to his right eye and recover to convince himself to take out the left so it must be a 1 minute 2 second interval, meaning he had at least 83 seconds to complete the task rather than 60 stated in the tape.