Visible crew/equipment: In the crushing room at the end of the film, Strahm is trying to open the box where Hoffman is. As it's descending into the ground, on the side of the box you can see the feet of a crew member with white sneakers moving around. It is not Strahm's feet: he's wearing black boots. No slow-mo is required.
Continuity mistake: The air hoses at the end of the film keep changing position. There is a shot from above: the air hoses are below actually resting on top of the glass box. Then when Strahm is trying to get out through the roof, there is a small rectangle cut out of the grated mesh and the hoses are above the roof and are not resting on the box anymore.
Other mistake: Virtually all the text in the various articles and reports in this movie are fake. Look at the John Kramer article at the very beginning, even at first glance you can tell it's an article about pilgrims or something and has nothing to do with John Kramer. Also, when Staham is going through the articles he found on Hoffman's sister, you can very plainly see that every article (there are a few) is just the same typed out story copied and pasted into various formats. (This doesn't even require slow motion, just pay attention.)
Revealing mistake: When Strahm removes his jacket at the end of the movie and starts pushing the walls with his hands and uses his legs, when he takes his feet off the wall, the wall moves. Now considering these walls are supposed to be solid steel, this doesn't make sense. The walls are actually wood.
Plot hole: Hoffman is able to implicate Agent Strahm as the mastermind behind the main series of traps in this film by planting Strahm's cell phone at the scene. However, Hoffman is repeatedly shown to be touching things at the scene without wearing gloves, so a forensics sweep of the crime scene afterwards would show Hoffman's fingerprints all over the place and reveal him to be the actual mastermind. Agent Erickson arrives at the crime scene before the two survivors of the traps complete their tests, so the forensics team would have been called in before Hoffman had a chance to remove any evidence that would incriminate himself. As a forensics officer himself, Hoffman should know better.
Add timeSerious B
Other mistake: Seth is told that in 30 seconds the pendulum will drop low enough to cut his body and in 60 seconds it will cut him in half. We then see the clock start ticking. After Seth has crushed his hands, we see the clock - 46 seconds have passed and yet the pendulum hasn't cut him yet.
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.
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.
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.
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.
Add timeA Demon
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.