Continuity mistake: If you go back and compare the ending of "Saw III" with the ending of "Saw IV", you can tell the lighting is different in certain areas (specifically when Strahm sees the dooor swing shut on its own), even though it is supposed to be in the same time and place.
Continuity mistake: *SPOILER* When Agent Strahm breaks into the Jigsaw's room in the end, he literally comes in less than a minute after "Saw III"'s conclusion. Yet, if you look, Amanda's dead body is in a completely different position than the end of "Saw III", which makes no sense, as there was no time or reason for it to be moved. At the end of III, her body was facing towards Jigsaw, and there was only a small amount of blood around her. Here, her body is facing in a different direction, and there is a ridiculously larger amount of blood.
Continuity mistake: When Riggs arrives at the shop, the clock shows 1:16. But, at a second look, it's 3:46.
Continuity mistake: The timer for Ivan's trap is messed up. The first shot we see it at 3 seconds. 12 seconds later we see the time at 42 seconds, and we know the 12 seconds are continuous as Jigsaw's tape plays during this time. 23 continuous seconds later we see it at 37 seconds (this would have been right if not for the second shot). This is when Jigsaw's instructions stop and his one minute should begin.As the first scythe descends into his right eye the timer is at 55 seconds. We see it at 57 seconds just before he gets his limbs ripped off.Two seconds is not enough time for him to scream after the injury to his right eye and recover to convince himself to take out the left so it must be a 1 minute 2 second interval, meaning he had at least 83 seconds to complete the task rather than 60 stated in the tape.