Continuity mistake: Simone wraps the tube round her arm just as the camera does a flash change. When the camera shows Eddie briefly, you can see Simone in the background with her arm on the table and a knife to her elbow. However, less than a second before she was tying the tube. (00:03:50)
Revealing mistake: There is one shot of Simone cutting off her arm where you can see it is a fake arm. It's noticeable by the change in colour. (00:04:50)
Continuity mistake: During the noose trap, William looks down the monitor's with Alan's pictures. We see a picture of Alan sitting in front of a fence, then William turns to look at Addy's monitor. In the next shot, William turns his head again to look at Addy's monitor. (00:38:10)
Factual error: The license plate on the back of Jill's car does not specify the state. It only has the number listed and "Fairweather State" written at the bottom. While it's never specified what state the Saw films take place in, every U.S. state and Washington D.C. require vehicles to have a state-issued license plate on the rear of the vehicle, so it can't be a vanity plate. (01:00:31)
Audio problem: Simone's scream at the end of her game is the same as one used earlier in the scene. (00:06:00)
Revealing mistake: In the opening game, the same shot of the needle moving on the scale is used twice when Eddie drops some flesh onto the scales. (00:05:25)
Continuity mistake: When Hoffman goes to see Jill Tuck at the clinic, she unlocks her door and opens it. Camera changes and the hand she is opening the door with changes instantly. (00:20:00)
Continuity mistake: When the power goes out at Umbrella Health Corp, Williams puts the folder down. Camera cuts and he puts the folder down again. (00:24:00)
Continuity mistake: When William walks into the steam room, we see the tape player from William's point of view. The camera then looks at William and the tape player has rotated 180 degrees instantly. (00:50:25)
Revealing mistake: When Hoffman strikes the matches, he throws them in the petrol. However, if you look carefully you can see they are not lit when Hoffman throws them. (01:10:30)
Continuity mistake: In the opening game, Simone's arm alternates from wet to dry depending on the shot. (00:03:30)
Visible crew/equipment: When Jigsaw says to William, that "success lies in the failure of your adversary", a boom mic can be seen reflected in the TV screen. (00:27:50)
Continuity mistake: At the very start, the position of the meat cleaver on Simone's table changes between shots. (00:03:40)
Chosen answer: The trap wouldn't have worked if William stood on the other side of the cage. When William walks towards the mother and son, you see a closeup of his feet stepping onto a pressure-pad that activates the trap. If William stood on the other side of the cage, the trap would never have turned on because the pressure-pad would not have been set off, and nobody would have died. Also, even though Jigsaw pre-plans his traps, there certainly are circumstances where the mother and son could have gotten hurt. One must remember, Jigsaw is insane, and it's shown repeatedly in the series that even though he intends to help people, he is not above sacrificing people or putting innocent people in harm's way to teach his subjects a lesson. (The most prominent examples being the mother and daughter from the original film and Joyce from "Saw 3D" who were innocent but placed in direct danger to be a motivation for the subjects).