Continuity mistake: When Seth has been sliced by the blade, he spits blood that goes all over his face. It then cuts to Hoffman looking at him, then back to Seth, and now the blood on his face has gone. This happens three or four times. (00:05:10)
Continuity mistake: The air hoses attached to the glass box at the end of the film are hanging straight down in some shots, and are wrapped around themselves in others, without anybody touching them.
Continuity mistake: The air hoses at the end of the film keep changing position. There is a shot from above: the air hoses are below actually resting on top of the glass box. Then when Strahm is trying to get out through the roof, there is a small rectangle cut out of the grated mesh and the hoses are above the roof and are not resting on the box anymore.
Continuity mistake: Throughout the movie, the clocks show incorrect time and at times changes from 8:20 to 18:20 and the countdown clock shows wrong sequencing. (01:01:00)
Continuity mistake: When Hoffman has the orange blanket put on him by the paramedic after escaping the Gideon Building, she puts it on him twice. It is also at one point not covering his right shoulder, but in the next shot, when Fisk talks to him, it is. (00:11:00)
Continuity mistake: When Strahm looks at his phone while locked in the sick room, the signal bar is on the right and the battery signal on the left. When Hoffman plants Strahm's phone in his hideout to frame Strahm, the signal bar is now on the left and the battery on the right. (00:06:40)
Continuity mistake: The tape recorder changes position in the final game. After the walls start moving in, Strahm drops the tape recorder and it is facing the door, completely straight. When he goes and gets his gun to shoot the box, it is on a slant now. Strahm didn't move it.
Continuity mistake: Spoiler: Right at the end, we find out that Saw III and Saw IV were happening simultaneously, as demonstrated by Strahm walking in and killing Jeff. We then see Hoffman close the door on Strahm, locking him in the room. Saw V continues from the exact moment when Hoffman shuts the door on Strahm. In Saw V, Strahm quickly kicks the door with his foot just seconds after Hoffman has closed it. However in Saw IV, we see Hoffman walking away. In this shot we should hear Strahm kicking on the door.
Continuity mistake: Jigsaw puts the razor down on a piece of paper and it is in a near right angle. When Hoffman picks the razor up, it is still at an angle. However, as soon as it's in his hand it's perfectly straight. There is no time for Hoffman to straighten the razor without us seeing him do so. (00:47:00)
Continuity mistake: When the five victims are in collars ready to start their first test, Ashley gets decapitated at the end. There is a shot of her head on the ground in a pool of blood. But in another scene later on Mallick goes through the door into the next room where you can see the head on the ground, and the pool of blood under the head has vanished. (00:26:50 - 00:33:15)