Continuity mistake: After the first room bombs go off, all the people are crouched on the floor and there is a load of dust. Camera shows Billy, then the people again. Now, there is no dust and everyone is stood up again. (00:35:05)
Continuity mistake: Just as the second room trap comes to a close we see 9 seconds on the timer and hear it ticking down. We hear 4 ticks and then there are 2 seconds left. There should be 5. (00:37:20)
Continuity mistake: The shotgun that is positioned to Hoffman's throat changes position between shots. Note the gap between the barrel and Hoffman's throat. (00:41:05)
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: 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 Jill is talking to Erickson, the height of the key on her chest keeps changing. (01:03:10)
Visible crew/equipment: When Jill Tuck is talking to Agent Erikson in his office, there is a shot with a boom arm's shadow visible across a picture and bit of wall, behind Agent Erikson. (01:03:40)
Visible crew/equipment: In the crushing room at the end of the film, Strahm is trying to open the box where Hoffman is. As it's descending into the ground, on the side of the box you can see the feet of a crew member with white sneakers moving around. It is not Strahm's feet: he's wearing black boots. No slow-mo is required.
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.
Plot hole: Hoffman is able to implicate Agent Strahm as the mastermind behind the main series of traps in this film by planting Strahm's cell phone at the scene. However, Hoffman is repeatedly shown to be touching things at the scene without wearing gloves, so a forensics sweep of the crime scene afterwards would show Hoffman's fingerprints all over the place and reveal him to be the actual mastermind. Agent Erickson arrives at the crime scene before the two survivors of the traps complete their tests, so the forensics team would have been called in before Hoffman had a chance to remove any evidence that would incriminate himself. As a forensics officer himself, Hoffman should know better.
Revealing mistake: When Strahm removes his jacket at the end of the movie and starts pushing the walls with his hands and uses his legs, when he takes his feet off the wall, the wall moves. Now considering these walls are supposed to be solid steel, this doesn't make sense. The walls are actually wood.
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.
Visible crew/equipment: When Strahm goes to get his gun in the crushing room at the end of the film, a boom mic is reflected on his jacket.
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.
Revealing mistake: When Strahm is being crushed by the "closing walls" at the end, watch closely during the last few seconds. There is a brief shot from above Strahm, looking down as he screams and the camera is spinning. If you watch closely, you can tell the "closing walls" that are crushing him aren't moving during this shot.
Continuity mistake: In the beginning of the movie, Strahm reaches out into the hall to grab the tape player. The string it is hanging from comes with it. Strahm exits the room, and we get a reverse angle and the string that held the tape player is hanging from the ceiling, but empty. The string should not be there to begin with.
Answer: The games of Jigsaw and his followers were always intended to be extremely difficult, but with a small chance for survival based on the actions of the player. Hoffman probably wouldn't have cared either day if anyone else survived, but likely anticipated that most of the others would die. (Strahm's line about everyone being supposed to die with Hoffman being the soul survivor was more conjecture than anything.) In terms of killing Jeff- both Jeff and Strahm were emotional hot-heads, so in all likelihood one or both was going to die if they encountered each other. Strahm's water cube was intended to be an execution tool for breaking the rules. That's why Hoffman seemed so shocked that he miraculously survived.