Continuity mistake: When Alison gets the gun from Zep and aims it at him, the gun is in her left hand. In the next shot it's in her right hand, and then it moves back to her left hand again. This is not an artifact of changing camera angles, you can clearly see the different hands holding the gun.
xx:xx:xxGrigory the Wanderer
Plot hole: Det. Kerry says at the scene of Paul's trap, "He had two hours." There is no way she could have known that. The clock simply said 3:00 and the tape specifically says "you have until 3 o'clock or this room will become your tomb". No way to know that's two hours after the fact.
Visible crew/equipment: In the scene where Adam walks around his apartment with his camera (because the lights were out) the filmmakers put lights in the set so the actors wouldn't walk into walls, and also letting the audience see what's actually happening. In the movie, though, there are supposed to be no lights on at all. However, just before Adam opens the closet door with Jigsaw in it, you can see the reflection of one of the set's lights on the top of Adam's camera.
Continuity mistake: In the scene where. Lawrence and Adam find the 'X' on the wall, there is a continuity error. Right after they find the 'X', the lights are turned on and there is the shape of a square on the wall (the cavity where the box is hidden). Before they find the 'X' there is no box or any dirty lines where the cavity is.
Continuity mistake: When Dr. Lawrence opened his envelope he got a key in it. When he tried it on the lock on his chain and it failed to open, Adam then asks to try it. Right when Adam asks for it, he puts his tape and envelope down separate, but when reaches for his tape again, it has ended up inside the envelope.