Deliberate mistake: During the entire movie nothing is mentioned about the rape of Merrin until the very end, even though it is quite obvious for a forensic investigator to determine she was raped, and something like that would be very important to help find a suspect. This is done deliberately to continue the plot as rape would make Iggy less of a suspect (being her boyfriend for years) and someone like Lee more (known by Iggy and others he had feelings for her).
Suggested correction: Boyfriends (and even husbands), are routinely accused of and found guilty of raping their partner. Given that they were seen arguing and told by the waitress that she saw Ig dragging Merrin to his car, there's no reason to think the police wouldn't suspect Ig of rape. We don't see any of the proceedings and we are told only a little bit of information about how the characters got to where they are at the start in order to build suspense, it has nothing to do with continuing the plot.
Deliberate mistake: Per his autopsy in Saw IV, John Kramer was 52 years old at the time of his death, and this film takes place between Saw and Saw II. Tobin Bell was around 80 years old at the time of making this film, and thus looks far too old to convincingly play someone in their early 50s. Amanda Young also looks a lot older in this film compared to other entries in the franchise that are chronologically later.
Deliberate mistake: When they are listening to the tape, Cheryl yells out and shuts it off. Scott says he wants to hear the rest of it, but then deliberately fast-forwards it a bit before starting it playing. If he wanted to hear the rest of it, why does he skip part of it? This allowed Cheryl to shut it off as soon as she finished her line/action, but then Scott could fast-forward it past a mark on the visible reel-to-reel tape so it would start immediately on the chanting rather than in the middle of the lecture where Cheryl stopped it.
Deliberate mistake: When Ben is discussing boarding up the windows to Tom, he says he still needs to get a few spots upstairs. Why would he need to board up the upstairs windows when the zombies have no way of reaching them?
Deliberate mistake: Several times, you see an exterior shot of Nathan's house, and it is very clear that it is the same shot being used over and over, sometimes reversed.
Deliberate mistake: After Venom eats the head of one of the Drake's henchmen there is no blood from the wound.
Deliberate mistake: While walking bare-footed in the barn looking for RJ, Wayne does not raise his feet very high - that is, until he is about to tramp on the nail that is perpendicular to the floor. If Wayne had kept walking like he had been, he might have bumped into the nail or object holding it up, but not got punctured by the nail. (01:06:40)
Deliberate mistake: When Margo is in the Superstition exhibit, she thinks she hears/sees something and runs for her life. She gets into the women's bathroom and tries catching her breath before the mirror. Hearing a noise outside of the bathroom, she goes into a stall, locks the door, and stands up on the toilet so her feet don't touch the floor. It was just a little old maid washing her hands outside the stall, and the strange sound Margo heard was the maid using her inhaler. However, how is this little old lady - with asthma, we can assume - supposed to have gotten all the way from deep inside the exhibit to the women's room within seconds, pushing a big heavy cart? Why would she even be in such a hurry? We can assume, however, that perhaps this was a deliberate mistake to show that, after all, maybe Margo wasn't just freaking out over nothing...that perhaps something was in the exhibit with her.
Deliberate mistake: After trying to get away from the monster, Costello finally unties the boat as Abbott rows away from the burning dock. The next shot of the boat has Abbott slowly rowing the boat, but you can see it's not moving as there's no wake, and when they jump off the boat after the Invisible man speaks, it doesn't rock, meaning the boat was mounted to something to prevent any movement, so they could make the jumps into the water.
Deliberate mistake: The disco ball doesn't start reflecting the sunrise until after the lead characters get out, even though it should have began to reflect light right when the door opened.
Deliberate mistake: When Larry and the two moving men are first bringing the mattress through the doorway and it gets "stuck," the camera placement makes it pretty obvious that the mattress isn't really stuck. There's far too much wiggle room on either end for it to be "stuck." (Obviously it needs to be stuck for the scene to work... but still a weird inconsistency).
Deliberate mistake: In the jump scare at the end of the movie, the killer has both hands despite the amputation suffered earlier - however, since it's a confirmed non-sequitur dream sequence, it does not matter. (01:36:40)
Deliberate mistake: The surviving passengers are all awoken from a deep sleep after Dinah realises the other passengers have gone. Yet none of them appear drowsy or tired and none of their eyes appear strained, despite having just woken up so suddenly. (00:09:20)
Deliberate mistake: Frequently during the movie, British surgeons are referred to as "Doctor". In Britain, surgeons are always referred to as "Mister"; this is a hold-over from the surgeon's non-medical past (barbers, etc). This would likely be confusing to American audiences.
Deliberate mistake: The legion of hellspawns during the film's climax are merely copied and pasted throughout the screen, as evidenced by the fact that many of them perform the exact same distinguishable body movements, such as pacing right and left from a crouched position.
Deliberate mistake: In the whole of the 'Psycho' series, there has never been a closet door next to the dress cabinet in Mother's room, but for this movie there is suddenly a closet.
Deliberate mistake: When Kyle goes through the windshield, the windshield shatters like tempered glass. Windshields at the time were made of laminated glass.
Deliberate mistake: When the scanners escape the apartment in the van and the assassins are in pursuit there is a street view shot of the assassins driving up next to the scanners' van. If you watch the street on the right side of the screen you can see people stood watching the filming in groups, out of harms way. (00:50:28)
Deliberate mistake: After Ryan and Michelle finish changing the flat tire and are surprised by Leatherface, he chases Ryan around the back of the car and smashes the windshield. At the moment the camera is in the backseat shooting out, we see the chainsaw with the chain popped off and hanging. My best guess is that it was done intentionally (for safety reasons) because the saw was close to Ryan. The crew must've been too lazy to completely remove the chain and just popped it off the track instead. (00:25:25)
Deliberate mistake: When Tiffany is reading the self-help book for recovering addicts, there's a closeup of a page. While the headline for the chapter makes sense, the body text is completely unrelated and is about corporate work culture. (Likely just some generic free text the prop department copy/pasted into the book prop.)