Revealing mistake: When the shark first attacks and bites off Jim's arm, you can briefly see his real arm under his shirt as he's screaming in pain. (00:34:10)
Factual error: When Janet Leigh is shown lying dead on the floor of the shower, there is a close-up of her open eye. The pupil is contracted to a pinpoint (obviously due to the bright lighting) where it should have been dilated. After the film was released, Hitchcock heard from several ophthalmologists who pointed this out and suggested he use belladonna eye drops in the eyes of "dead" people in future films, as the chemical prevents the pupils from contracting. (00:48:30)
Continuity mistake: At the start of the film, the dog is seen going from room to room looking for food. Just before he goes into Dana's room, someone can be seen through the crack in the door, moving from left to right.
Continuity mistake: James takes off his tie to cover the cut on Kristen's hand, but when they run outside to escape in the car James is wearing his tie again, then after the failed escape they run back inside the house and James tie is missing. In another scene Kristen goes into the woods to look for James and falls into a ditch, then as she climbs out the tie is missing from her hand, but the next scene it's back in place.
Revealing mistake: When Eric Matthews is smashing his foot, he hits it the first time and it shows a close up. If you watch the small amount of blood when he hits it again they have used the same shot. (00:04:00)
Visible crew/equipment: In the scene after Patrick Bateman's second encounter with the detective in his office, we see Bateman having sex at Courtney's apartment. After he rolls off her, Bateman approaches the large, stand-up mirror in the bedroom. In the upper right hand corner of the mirror, a boom microphone can clearly be seen moving around to pick up Christian Bale's lines. The next time we get the same long shot, the microphone is gone. (00:52:45)
Continuity mistake: When John is driving to work and he hears Anna on the radio, he swerves to avoid a car. From inside the car you can see that there are no cars in the rear window, yet when the shot changes to an outside shot, there are 3 cars. (00:33:35)
Other mistake: Max says that Michael is the "Man of the house" and won't come in until he's invited, which Michael sarcastically does, except Michael isn't the man of the house - its actually his grandfather who owns it.
Continuity mistake: When the old car blows up both of the dead zombies are gone. (00:21:40)
Revealing mistake: When Ripley first burns the egg plants inside the nest, the Queen screams and moves her head. The steel rod used to hold the Queen's head to the rest of her body is easily visible.
Visible crew/equipment: When Marcus attacks Simon at the loading dock, as Simon flies through the air after Marcus kicks him, you can see the face of Johnny Lee Miller's stunt double. (00:57:15)
Continuity mistake: When Scott gets his throat cut, blood pours out. Yet when Terri spins him around, there's no blood anywhere on him except a small amount on his shirt sleeves.
Other mistake: When they encounter the first ant, the policeman runs out of ammo, but the FBI agent fires at least nine shots. Not bad for a snubnose revolver that holds 5 or 6 rounds.
Plot hole: After his mother's death, Peter sets a camera up outside her therapist's office, so he can see who goes in and who comes out; he thinks his mother's killer might be one of the doctor's patients - a tall woman with long blonde hair. All he sees is people going in and people coming out, and then he sees her - the tall blonde. Problem: He never saw her going in, only going out, which is critical, because the killer is the therapist - a tall man - wearing a long blonde wig and make-up. Of course no-one says anything about "We never saw her going in, only out, so it must be the therapist!"
Other mistake: When Sugimura is looking for Kotohiki, he activates his scanner to see if she is nearby. The scanner shows her location, but fails to show either Mitsuko or Kiriyama, who show up quickly after.
Visible crew/equipment: Before Lorna has her throat slashed, you can see a tube pumping the blood.
Other mistake: When Laurie has her episode in the bathroom about killing Annie, the clock stays at 7:03 the whole time.
Revealing mistake: Just before Hoffman knocks Jill out, there is no blood on her head or the table. As he grabs her head blood appears on her head just before he smashes it on to the table. (01:14:40)
Other mistake: Whenever Helen Hyde turns back into Richard Jacks, none of the clothes ever seems to rip or stretch - they both have different body types.
Deliberate mistake: When the kids are about to jump start the trap Phoebe hides behind the school bus door. Half a second later, when the trap opens, she's nowhere to be seen, not even jumping or stepping inside the bus. Then half a second later she has managed to get inside, turn around and protect her head while the bus windows burst. It all happens in real time for their hairs are still fluttering. It sure paces up the scene, but it's really awkward.