When the detective turns the corner and shoots the hooded killer (John) he is carrying a Shotgun, but when he ejects the bullet casing the sound it makes is from a metal casing. The only long gun that would make that noise would be a rifle. The noise the shotgun shell should make is a hollow plastic noise. The ejecting of the shell and the actual motion of the reloading must have been separate.
When Detective Sing asks Dr. Gordon to stay and listen to Amanda, he says "I wanna know if you wouldn't mind sticking around and listening to her testimony." What she was doing was giving a statement, not testimony, and any detective simply wouldn't make that mistake.
In the car chase scene, Zep is seen driving a late 1980's Ford Bronco II (could be a Ranger pickup because we only see the front). When he arrives at the warehouse, the truck has changed to an early 1990s Ford F-150.
After filming was over, Leigh Whannell played more parts to fill in some gaps in the movie. When Sing is entering the wear house, it is really Leigh and if you watch the full screen version, you can see Leigh, not Sing entering. Also, when Amanda is getting the key from the dead person, it is actually Leigh's hands not Amanda's.
In the scene with the victim trying to open the safe in the room with glass shards covering the floor, there is a close-up shot of his bare foot stepping on the broken glass. In the next, speeded-up scene that shows him walking all over the room trying to figure out the safe's combination, he is clearly wearing shoes.
Dr. Gordon has two cigarettes in the box, one of which he throws towards Adam as part of the doctor's plan to make Jigsaw think that Adam is smoking a poisoned cigarette. Adam lights the cigarette and pretends to die. It cuts back to the doctor where we see the two cigarettes lying in front of him again.
When Dr. Gordon is taking the box from the hole in the wall, the lock on the box is visible to Adam; after Dr. Gordon has put the box down on the ground, the box is turned around and the lock is not visible any more.
When Adam pounds on Zep with the toilet seat, he grabs his shoulder in pain. In the first shot of him holding his shoulder, his hand is soaked in blood. In the second shot, however, his hand is much cleaner.
After Adam searches in the toilet bowl, his hand is covered in brown slimy water. However, after he removes the lid from the cistern, his hand is completely clean, and he clearly used his other hand to remove the hacksaws from the water.
When Dr. Gordon is using the box to try to reach the phone he gets angry and tosses it to his left side. When we see his right side, the box is still lying there. A few moments later, when he is tugging on his chains in frustration, we see the box next to the door, in the direction that he threw it.
While Tapp And Zep are battling for Zep's gun. Tapp has pushed Zep to the wall and Zep is turning his gun upwards to shoot tap from over his shoulder, the gun changes from a Springfield Armory .45cal to a 9mm Beretta M92 whilst Zep shoots and after Tapp dies, it changes back into the .45cal.
When Zep is listening to Diana's heart, he pulls out a gun in an attempt to raise her heartbeat. While he's doing so, the shot from behind him shows the gun against Alison's head, even pushing a few strands of hair. The next shot shows the gun at least 6 inches from her head.
When Adam first remembers what happened before he was kidnapped, we see him making his way back to his apartment after snapping photos of Dr. Gordon. The camera Adam has slung over his shoulder changes from scene to scene. In the hallway outside the apartment, Adam has a black camera with a long lens and a mounted flash unit. As he enters the apartment, he's got a silver camera that appears to have a very short lens and no mounted flash. Once inside, he's got the black camera with long lens and mounted flash again.
After Amanda survives her reverse bear trap test, the Jigsaw puppet is seen wheeling towards her. If you look closely you can see the cabling controlling the puppet. This is in fact fishing wire which was used to control the puppet. (Possibly deliberate.)
Both Adam and Lawrence receive electric shocks through their chains. However, for the shocks to occur, the current must have a path to ground through the body, and both men are lying on a floor made of ceramic tile, which is an insulator. Even if the floor were somehow able to conduct the shocks, the pipes to which the chains are attached also contact the same floor; any current applied to the pipes would ground directly to the floor, not through the chains and victims.
After Gordon and Adam find the box with the cell phone and cigarette, Gordon tries to call 911. When he tries you hear 3 different tones and he says that the cell phone has been blocked for outgoing calls. However, FCC rules require every telephone that can access the network to be able to dial 911, regardless of any reason that normal service may have been disconnected (including deactivated or blocked phones).
Right at the end when Adam tried to shoot the real psycho and he got electrocuted he throws his gun away. But the firemen character taught us that if you got an electrical shock your muscles contract. He should have uncontrollably gripped that gun instead.
When Adam asks Dr. Gordon if he's going to have any more kids, there is a small, white spot on the right side of the screen visible for less than a second. This is likely a single-frame anti-piracy watermark or a defect on the film. They appear several times in the movie, especially in the final reels.
When Tapp and his partner go after the Jigsaw Killer on their own, they do not take a SWAT team or uniformed officers because they have no warrant, and it is thus an illegal search which the police would not provide support for.
Close-ups of the newspaper clippings lining Tapp's apartment walls show headlines about the Jigsaw Killer's murders...but the articles have nothing to do with the headlines, and the datelines have the news stories in different cities, ranging from Virginia to California.
When Dr. Gordon is about to leave after tucking his daughter in and his wife is confronting him, the lack of a ceiling and unfinished wooden rafters are visible in the set of their otherwise elegant home.
After Lawrence leaves the bathroom Adam leaves a bloody hand print on his Lawrence's right shoulder, but when Adam plays the tape recorder there is a flashback of Lawrence saying to Zep, "Why?" then Zep say, "It's the rules!" the hand print Adam made on Lawrence can be seen on his shoulder when it wasn't there when it actually happened.
When Tapp is fighting with Zep, Tapp is holding on tightly to Zep. The gun Zep has in his hand is a completely black gun, but when he slips out of Tapp's arms and accidentally shoots him, they both fall on the floor. The gun Zep has turns into the silver and black gun that he used throughout the movie.
When the camera is switching back and between Zep and Alison when Alison gets free and has the gun. The gun switches between Alison's right and left hand. When the camera is looking at Alison, she has her right hand outstretched, waiting for the phone and the gun is in her left hand. But when the camera is looking at Zep (over Alison's left shoulder), the gun is in her right hand with her left hand outstretched waiting for the phone.
When Gordon picks up the revolver to shoot Adam, and opens the chamber, there is no spent casing in the chamber. As it is later revealed the man is neither dead nor shot himself, he was actually the Jigsaw Murderer merely getting a front row seat, and so therefore it is possible that the gun was never even loaded prior to Dr. Gordon's use to try to kill Adam.
After Lawrence saws off his foot and Zep is down, he starts to crawl out of the room. When he is leaving and a shot shows up his pant leg, you can see his foot hidden inside the pant leg for one split second.