Audio problem: When Emily picks up the tooth from the box, she says "ew" but if you look in the mirror her lips don't move.
Audio problem: In the scene after Norman goes for a late-night swim, when he's telling Maureen that he slipped in the mud, you can see that his mouth never moves, yet he speaks the words "I slipped in the mud."
Audio problem: Right before Charley gets thrown into the closet doors, Jerry has him by the throat threatening to kill his mother if she wakes up. In the shots of Charley trying to free himself of the grip, Jerry's lips are moving but no sound can be heard. (00:28:15)
Audio problem: In the scene where Rose is talking to the demonic form of Alessa, watch her mouth as she says "Christabella will find Sharon." Alessa's mouth doesn't match every word she says.
Audio problem: Nancy shouts, "Yay Chris! Go long! Go for the pass, baby!" at football practice, and her voice is so distorted that the viewer can easily tell the line was added post-production (with too much gain).
Audio problem: As Krogh fires his revolver at the monster, the sound is out of sync.
Audio problem: When the mysterious lens flare creature possesses the microwave oven, its door opens with a disproportionate "creaking door hinges" sound effect. Inherent phoniness aside, the noise is also continuous, but if you look at the door, it did not open in one fluid motion; the actor's pelvic thrusts as he was boinking the stewardess got in the door's path for a moment putting it to a momentary halt. (00:14:05)
Audio problem: At the police station, Helen and Jake are talking. When the camera is on Jake, Helen says her lines but her mouth and the words are out of sync.
Audio problem: When Bugs says "Out there in the audience" when being held by the neck by the red monster, his mouth doesn't move.
Audio problem: When the hero asks the Coastguard to get him quickly back to London, it is followed by film of a single seat Hawker Sea Hawk jet fighter landing, which sounds just like a piston engined aircraft.
Audio problem: When Daniel is on top of the tree house, his screams don't always match up to his mouth. (01:23:20)
Audio problem: Jennifer and the Captain exit the plane and stay there for several minutes while sirens from airport services howl, obviously to help them after such a chaotic flight. However, wider shots show that there's not a single car or person nearby.
Audio problem: In the scene where Maggie and Steve are running outside and come across Richard standing on a mine, he says something along the lines of 'This whole place is covered in mines'. As Maggie starts her next line, a voice in the background shouts 'no its not! haha!'.
Audio problem: During the final test, at one point Marcus says "Shoot this piece of s**t Ezekiel, now." In the first shot, it's clear he's saying something entirely different as his mouth movements don't match the dialogue. The scene then cuts to a closeup mid-line, and the closeup does match... but the first shot doesn't match in the slightest.
Audio problem: This problem exists in the extended Director's Cut. When Narcisse is first introduced in the hospital, when we cut outside to Decker and the police walking to the room, they suddenly hear Narcisse grunting which makes them rush to his room. However, they are quite the distance from his room, and Narcisse's grunts are fairly low, so it's obvious they shouldn't have heard him from that distance. The audio mixing tries to make his grunts sound louder, but it doesn't sound right at all... they're way down the hall from the room, but his low grunts sound like they're coming directly right next to them. This is all the more confusing, as in the theatrical edition, they are alerted to a distant scream by a nurse, which was much more believable. (As the Director's Cut is currently the most readily-available version of the film on Blu-Ray online, and is the director's intended edition, I believe it should count as a mistake).
Audio problem: When we see a close up of the LP playing on the gramophone, you can occasionally see the needle is not actually touching the LP but sound is still heard.
Audio problem: In the scenes inside to cabin where Angela keeps her victims you can hear flies, but there are no visible flies anywhere.
Audio problem: When the unnamed Asian man sells Frank the box at the beginning, watch when he says the line "it always was." The words don't quite match the lip movements, revealing the line was looped in post.
Audio problem: When Susan's cutting the cake in the kitchen she drops the plates upon discovering David's body yet there's no sound of them breaking. (00:04:25)
Audio problem: When Patrick Bateman is exercising and watching The Texas Chainsaw Massacre, the audio does not match up with the actual film. (00:37:05)