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: 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.
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.
Continuity mistake: When Amanda is thrown into the pit of needles, in some shots there is only one, while in others, there are two or more stuck in her back. Also, in some shots the needles are pointed at an angle, whereas in others, they are pointed almost straight up. (Uncut Edition)
Other mistake: The drip-meter that is supposed to administer Jigsaw's medication is not turned on. Anyone who has spent time in a hospital and seen one for real knows that a lot of numbers are shown on the displays when it is running to show medication status.
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.