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.
Trivia: In the room with the razor blade box, if you look at the top of the frame, you can see a lock with a key in it. If the actress walked a few feet further, she could've unlocked the box and gotten the syringe. Obviously this would've defeated the purpose of her dying, but interesting to point out. Confirmed in DVD special features.
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.
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 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.