Factual error: We see Hoffman has to use pliers and force to remove Strahm's hand from the grill. However, since the hand is freshly severed, rigor mortis would not have set in, and he should have been able to remove it with ease. (01:09:30)
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)
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).