Factual error: There is one scene in the movie where Heath Ledger reads a Latin sentence in a book. As he is translating the sentence, he goes over the words one by one with his finger. This would never be possible in a Latin sentence, because words in Latin are never - I repeat: never - in exactly the same order as in English. Words that actually belong together can be placed on opposite ends of a sentence.
Continuity mistake: When Helen puts the trashbag down, with the teddy on top, the teddy's scarf hangs down over its face. But when Gypsy picks it up, the scarf is no longer hanging over its face.
Continuity mistake: Marion isn't wearing any clothes as they enter the convenience store. However, as they are standing in the doorway you can see he is wearing boxers.
Continuity mistake: Curtis is helping Molly seal the windows by screwing them shut. He hands the drill to Molly and she holds it up by her chest. When Claudia walks into the room Molly lowers the drill down in front of her below her waist. Then there is a quick cut back to Molly and Curtis as he says, " well that should do it", and the drill is back up in front of her chest again as she lowers it down a second time.
Factual error: Sean is tailing his victim, taking photos with an old Polaroid camera (the kind that prints instant photos). In one scene, he is sitting on a bench and takes several shots. When he shows these pictures later, some are zoomed in on the subjects, when it would not be possible to zoom with this camera. He would have had to get up and walk toward them, thus giving himself away.
Factual error: Throughout the film, most of the characters talk clearly underwater with the SCUBA mouthpieces in their mouths, which is impossible.
Revealing mistake: When Maggie is chasing after the truck in the light aircraft what appears to be a "pasted-on" face in the cockpit, on closer inspection (with zoom feature on DVD players) is a male pilot flying the plane.
Other mistake: When "the killer" is pumping gas, if you count the clicks associated with a liter going into the tank, they equal a total of 33 liters of gas, not 27 which is what the pump stops on.
Deliberate mistake: When the teacher walks over the floor towards the blackboard, he walks through a lot of blood. However when it cuts, he leaves no traces of blood where he steps.
Continuity mistake: When the monster truck rams the back of Adam's car the shot immediately after shows no damage at all to the back of the car. This happens in several scenes throughout the film until the car is eventually destroyed. (00:11:50)
Factual error: The movie takes place in Des Plaines, Illinois, with a couple of scenes from downtown Chicago. However, throughout the movie you can see both palm trees and mountains in the background, none of which actually exist in Illinois. The producers also point this out on the DVD commentary.
Revealing mistake: When Frank yells stop to Marion, you can see the marks on the road from a previous take. (01:05:20)
Factual error: When Don records the flight log, he says that they have been in the storm for 5 hours. That does not seem to match the timing of the events in the movie, but on top of that, it's a Vegas-Phoenix flight. That's a 250 miles distance that would take little more than one hour. (01:07:30)
Other mistake: Marvin survives the explosion by hiding in the refrigerator, though despite the massive blast, there is no singeing, burn marks, or even smoke around the holes that were torn into the front of the door before the explosion.
Character mistake: The Cadillac is identified by Robby as a '57 Eldorado. It's actually a '57 Series 75 Limousine.
Revealing mistake: Near the end of the DVD version, one of the main characters (I think Deke) is chainsawing the spiders. In two of the shots, you can see the man's legs through the spider.