Continuity mistake: Jill opens the 6th envelope to show Hoffman. When she does, the envelope instantly vanishes from her hands. (01:18:25)
Continuity mistake: When Hoffman wants the envelopes, Jill's collar is turned up. Camera changes and it's lying flat. (00:20:25)
Continuity mistake: When William sees the way forward, he puts his hand on the glass. There is no blood on the glass from the previous shot, and in the next shot, when he tells Addy to be careful, the blood is back. (00:39:20)
Continuity mistake: The amount of Petrol that Hoffman chucks over the FBI lab, Perez, and his boss, is more than what could be in the single container we are shown. (01:09:00)
Continuity mistake: When Simone has a go at Hoffman in the hospital, someone puts their hand on her shoulder. Camera cuts and the hand has gone. (00:16:00)
Continuity mistake: When Erickson is offering full disclosure to Hoffman, Erickson's badge is uncovered. Camera changes and now it's half under his jacket. (00:14:20)
Continuity mistake: When we see Eddie cutting the flesh in the opening game, the amount of blood on his shirt decreases and increases between shots. (00:04:40)
Continuity mistake: At the start when Simone is telling Eddie to wake up he has his hand on his chest. Camera changes and it's not. (00:01:50)
Continuity mistake: When Hoffman says to Amanda "Are you sure about that?" his mouth is right next to her ear. Camera cuts and it's a lot further away. (00:44:10)
Continuity mistake: When Hoffman leaves the hospital room, the male doctor has his hand on Simone. Camera changes and now he is gesturing with it behind her. (00:17:10)
Continuity mistake: When talking to Pamela near the start, her notebook is open or closed depending on the angle. (00:15:05)
Continuity mistake: When Simone is cutting her arm off, the amount of blood on the table differs between shots. (00:04:40)
Continuity mistake: When Eddie puts the first bit of flesh through the fence, he has his face leaning on the fence. Camera cuts and now it's not. (00:04:20)
Continuity mistake: At the start, Simone puts the rubber tube round her arm. Camera cuts - it's no longer round her arm and she puts it round again. (00:03:35)
Continuity mistake: When Hoffman talks to Pamela, he turn round and leans against the door. Camera changes, and he does the same action again. (00:14:35)
Continuity mistake: We are shown a clip at the end of someone putting the reverse bear trap on Amanda in Saw. However, the device is not the same one as seen in Saw. Note the mouth section. It is a lot thinner than in Saw. (01:17:50)
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)
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)
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)
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).