Continuity mistake: In the car chase scene, Zep is seen driving a late 80'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 90s Ford F-150. (01:23:40)
Continuity mistake: When Tapp and Sing go to Jigsaw's place and find the guy strapped to the chair, they uncover him, then re-cover him to see what Jigsaw is going to do. When Jigsaw pulls the sheet off, one side is still caught on the drill bits. When he starts the drills the sheet is completely clear. (00:48:00)
Continuity mistake: Adam grabs the bat with both hands right after he sees the laughing doll in his apartment (the view is from behind him). The very next shot shows the front of him, but he has the bat in one hand, and the camera in the other. (01:07:10)
Continuity mistake: Tapp is watching the Amanda tape. Jigsaw says "look" and then Tapp rewinds the tape for 2 seconds. He presses play and Jigsaw says, "Look around" He should have said the word "mate" at that point. (00:43:40)
Other mistake: When Larry is taking the cigarettes out of the box, look at his hand. It has a white paint drop or white dust on it, but throughout the scene it disappears and reappears. (00:55:10)
Continuity mistake: Jigsaw's thumb moves between shots after Adam gets the mini cassette recorder out of his hands. You can see Jigsaw's thumb, in the closeup shot, slide down to a position where it's lying horizontal and flat on the floor as the mini cassette recorder is being slid out of his hand. In the next shot, an overhead, you can see that his thumb has moved and is now more curled and in a more vertical position to where just the tip is touching the floor. Jigsaw isn't dead, so he could have moved, but this shot is an *instant* cut, he had no time to move.
Continuity mistake: Adam swishes his hand in the filthy toilet, getting brown liquid over his hand. Adam's hand suddenly becomes completely clean a very short time later. (00:12:50)
Revealing mistake: Adams shirt is dry, after being in a tub full of water. (00:10:20)
Continuity mistake: When Singh is following Jigsaw, he shoots him in the back and proceeds to close in on Jigsaw. In one shot from behind him he is next to a wooden spool. Camera changes to in front of him and now he's further away. (00:51:20)
Continuity mistake: When Dr Gordon is breaking the wall to get the box, there is 1 tile at the top that is complete on the outside shot and broken on the inside shot. (00:54:10)
Continuity mistake: When Tapp and Singh work out where the warehouse is Tapp has his hand out on the table over a map. The camera cuts and now they're on the arm of his chair. (00:44:50)
Continuity mistake: When Adam finishes throwing stuff at the camera and Dr Gordon explains about every angle being thought out, the shard of glass at Adams feet keeps changing position without being touched. (00:30:40)
Continuity mistake: When Dr Gordon is called to his office to speak with the two detectives, his chair is facing away from him as he approaches it but it suddenly turns, so he can sit down and talk with the detectives. (00:20:20)
Continuity mistake: When Dr Gordon is reading the note that was in the box with the phone, the way he holds it differs between shots. It goes from 2 handed to single-handed. (00:55:50)
Revealing mistake: When we see Tapp watching Dr Gordon's house, there are clips of newspaper headlines that flash up on the screen. However, the articles themselves have nothing to do with Jigsaw or even the headline above them. (00:41:00)
Audio problem: When Dr Gordon is cutting off his foot, the screams from Adam are just a replay of the screams he does at the end of the film. (01:27:05)
Continuity mistake: When Tapp and Sing discover the Billy puppet in Jigsaw's warehouse, the puppet's head changes direction at least once. (00:47:10)
Continuity mistake: The bloody bullet wound is seen on Tapp after he's shot. When he is shown after he falls on the ground, the bullet wound is not there. (01:27:50)
Continuity mistake: The bloody handprint on Lawrence's shoulder that Adam makes at the end is visible before he actually makes it. Second, when Adam finds the tape recorder, it flashes back to Lawrence saying "why?" before Zep tells him it's the rules and the handprint is also visible, but shouldn't be there as Adam made it after he killed Zep. (01:29:40 - 01:33:10)
Continuity mistake: We see Adam on Dr. Gordon on the screen that Zep is watching. By Adam's foot there is nothing, yet when we see Adam, there is a piece of glass. (00:30:00)
Answer: Amanda was supposed to put the key around Adam's neck rather than just on his chest. If she had done so as instructed by Jigsaw, he would've been able to unlock his shackle and leave the bathroom. Adam was a voyeur, the irony being that if his key had been placed correctly around his neck, all he would've had to do would be to look around to find it. Adam's tape states, "what do voyeurs see when they look into the mirror, " so, again, it's a hint that all Adam would've had to do was look at himself. I noticed as well, Jigsaw says, "the key to that chain is in the bathtub." /IS/ in the bathtub. Not /was/. So Jigsaw probably thought Adam could still get out. He says, "so many people are ungrateful to be alive. But not you. Not anymore." So it's like, 'I have taught you this lesson, now you can go free because now you'll appreciate your life unlike you did before'.