Revealing mistake: In the opening scene, the helmet closes on the guy's head. Inside there are spikes on all sides, but when his head falls to the ground the blood only comes out the bottom. The spikes should have also gone into the other side of his head and blood should have run down his face to the floor.
Continuity mistake: When Glenn Plummer gets the spiked baseball bat in the head, the camera shows two shots of the blood coming out of his head on the floor, and there's not much there. When the guy who killed him walks over to read the number on the back of his head, there is now a huge amount of blood, not enough time has passed in the 2 seconds since the previous shot for this to happen.
Continuity mistake: When Emmanuelle Vaugier walks into the room with the clear box trap hanging from the ceiling, in the middle of the two hand openings is a syringe with the antidote. It is filled with significantly more in the close up shots, than there is in the full shots which show a lot less in the syringe.
Other mistake: In the crematorium trap, near the end of the scene where the doomed character dies, his jeans should be entirely burnt due to the flames, yet they are not.This was corrected however i have personally tested it. For the amount of time that this character was in flames his jeans would have turned black from the soot created when jeans burn. Test it yourself.
Continuity mistake: In the opening scene when the informant is about to cut into his eye they show a close up of the knife just centimeters away from his eye ball. There is minimal blood only in his eye but nowhere else, where in the cut before it the whole area around the eye is covered in blood.
Continuity mistake: In the very beginning of Saw 2 when the informant awakes and realizes where he is and suddenly gets up from the chair there is a brief shot of him from the rear. If you look closely the leather waistband for the trap is completely loose, but when the camera quickly switches to his front, it shows the leather waist band tight around his waist.
Continuity mistake: In the part right before the girl is thrown in the heroin needle pit, Jonas takes the envelope with the drug dealers tape. When the cop's son pulls the cover off the pit, you can see that the envelope that Jonas just ripped off is still on. It's right near the bottom of the screen, and in the middle.
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Revealing mistake: When Amanda is slowly getting up right after being thrown into the needle pit, if you watch very closely, in one or two quick shots you can see a small pad on the actress' back used to hold the needles in place. Slow motion helps, but is NOT required. (Seen in Uncut version, though it may be in the theatrical version as well.)
Continuity mistake: We figure out at some point or another in the movie that the people in the game are supposed to have numbers on their necks. (In the back of their minds). But in the scene where Amanda gets thrown in the needle pit Franky G's neck is seen as he walks past Glenn P. and as he walks out the door, with no numbers in either shot.
Revealing mistake: When the group finds the picture of Daniel and his father Eric in the house, look closely at the photo - it was obviously very poorly patched together with photoshop. The lighting on Daniel/Eric does not match the lighting in the background at all, and it's clear that they have been crudely "cut out" of another photo and inserted over the background image. The extremely poor, obviously fake quality of the photo is even jokingly mentioned in at least one of the DVD commentaries.
Continuity mistake: After Xavier kills Jonas with the spiked baseball bat, we see Amanda, Daniel, Addison and Laura in a hallway. Laura collapses to the ground due to the nerve gas that they've all been inhaling. Between shots her left arm moves from her stomach to across her right shoulder.
Continuity mistake: In the scene when Eric Mathews goes to confront Jigsaw (after he first looks at the monitors), the chain of his badge gets twisted when he gets held back, but as he's about to leave the chain appears, untwisted. But when he gets back to the monitors, his chain is twisted only to be untwisted by him a few seconds later.