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: 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.
Factual error: After falling into the pit of needles, somehow all of the needles in Amanda's skin stay intact and do not break, despite her fall and her rapid digging through them. Very, very unlikely at best.
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 Mathew's 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.
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.