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: When Hoffman wants the envelopes, Jill's collar is turned up. Camera changes and it's lying flat. (00:20:25)
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: 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 Simone is cutting her arm off, the amount of blood on the table differs between shots. (00:04:40)
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)
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).