Continuity mistake: There is a closeup shot of Matthew's foot right before he starts to smash it with the broken piece of the toilet bowl lid. He's placing the flashlight on the floor next to his foot and you can see that the sock is back on his foot, even though he took it off previously (right before he first attempted to saw through his leg).
Continuity mistake: The guy that is in chains stands up, looks around and takes in what he sees. There's a shot of him from the front and his hands are semi-clenched in FRONT of him, around waist level. The shot cuts to behind him and his hands are a good 4 or 5 inches OUTSIDE of his hips.
Continuity mistake: When Lynn says "Please, please let me go" there are tears in both eyes and tear marks on her cheeks. 2 shots later (maybe 7 seconds) you see a close-up of her face and there are no tears or tear marks. (Jigsaw was holding her hand, so she couldn't have wiped them away, plus you can see Lynn the whole time (in the background) and she never wipes them off).
Continuity mistake: In the 'Pig Vat' scene, the slide that the pigs slide on into the blades is clear with no pigs stuck on it. The pigs fall in one after the other with none getting stuck on the slide. However when the fourth pig drops onto the slide, you can see the a pig stuck on the slide and hanging over the edge of the slide. The next couple of pigs that fall don't knock another pig in and you cannot see the pig overhanging on the slide.
Continuity mistake: Jeff lays his bloodied hand right in the center of Jigsaw's chest (pointing towards Jigsaw's chin) when he says "I forgive you", but pretty much only his fingertips touch Jigsaw. Right before Jeff slices Jigsaw's neck you can see the handprint, but it's MUCH more defined than it should be AND the fingers are pointing towards the right instead of straight up.
Continuity mistake: When Amanda says "I'm standing right here" her hair is hanging down, but brushed back from her right eye, in the previous and subsequent shots her hair is hanging down and covering her right eye. She was holding the gun in her right hand the whole time and all the cuts were instant, there was no time to move her hair.