Continuity mistake: SPOILERS: During the final scene, when Samuel L. Jackson is "puppeted" to lift his arm up to make it look like he's pointing a gun at the SWAT team, the number of laser-sights visible on his chest changes between cuts. In the wide-shot, there's only three, but when it punches into a close-up, suddenly there are five.
Factual error: It is later revealed that Detective Banks partner faked his death by killing a meth-head junkie with the same tattoo as him. He included the junky's filleted skin with the same tattoo in a box with his badge to make it look like the other kills. Police, firefighters, first responders all provide DNA into a data bank to be identified in case something tragic happens and their remains are recovered. William Schenk's DNA would not have matched that of what was in the box.
Continuity mistake: When Zeke is messing with the drug dealer, he lays on his back and snaps a picture with the dealer's broken leg. When the camera cuts to a new angle, suddenly he's sat almost all the way up. (Presumably this was done to quicken the pacing of the scene, but it's still a mistake regardless).
Continuity mistake: When Zeke and William are investigating the scene of Bozwik's death, pay attention when Zeke says that's not sure the body was a bum. The forensics agent behind William goes from walking to the side, to walking to the back, to standing still and taking a photo instantly between cuts. It's out of focus in the background, so it can be hard to notice... but it's there.
Character mistake: Zeke makes the bold claim that "John Kramer didn't target cops." This is completely and utterly false, and a cop like him should know better. Kramer repeatedly targeted cops. In fact, both "Saw II" and "Saw IV" have officers (Detective Mathews and SWAT Commander Rigg) as Kramer's main target.
Continuity mistake: A few times during the opening trap, the barbed-wire around Bozwik's wrist disappears and reappears, or swaps wrists between cuts. Ex. When the train rounds the corner before he's hit, in the shot behind him you can see the barbed-wire flopping around in front of him... but when it cuts to the front angle, suddenly it's gone.
Continuity mistake: Bozwik's teeth bizarrely change shape during the opening trap. In the first closeup, he appears to have slightly small teeth with gaps between them. But about 30 seconds later, there's another closeup and his teeth are suddenly larger with no gaps. All I can assume is that the two closeups were shot with different prosthetics or body-doubles.
Audio problem: During the final test, at one point Marcus says "Shoot this piece of s**t Ezekiel, now." In the first shot, it's clear he's saying something entirely different as his mouth movements don't match the dialogue. The scene then cuts to a closeup mid-line, and the closeup does match... but the first shot doesn't match in the slightest.