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 eyedrops in the eyes of "dead" people in future films, as the chemical prevents the pupils from contracting.
Continuity mistake: When Marion arrives at the Bates motel, she sounds the car horn for the first time, the windscreen wipers are switched on. She gets out of the car, looks towards the old house, then goes back to the car, as she sounds the horn for the second time the windscreen wipers are now switched off.
Continuity mistake: In the shot in which Norman goes into the bathroom and puts the pail behind the door, the shower curtain rings are spread out. In the very next shot in which he emerges from behind the door, the rings are in tight groups. As he leaves the bathroom, the rings are spread out again.
Other mistake: In the shower scene, Marion Crane's palm looks water wrinkled as if she's been in the shower for quite a long time even though she was only in for a few moments before Norman stabs her. Also, when Norman drags her out of the bathroom, Marion's legs are stiff and together and she has her toes pointed.
Continuity mistake: After seeing Marion's dead body in the bathroom, Norman accidentally knocks down a framed picture of a bird (with the picture facing up and it is a few inches away from the wall). But when Norman is spreading the shower curtain on the floor, the picture has changed position (with the picture now facing down and it is against the wall).
Audio problem: When Norman runs down from the Bates house to the motel and meets Marion, he says "Gee, I'm sorry I didn't hear you in all this rain. Go ahead in, please." But you look closely, it looks like Norman is barely moving his lips. In fact it looks like Marion is saying something instead.
Continuity mistake: In the scene after Norman kills Marion, when he goes back into cabin 1 to move her body and clean the bathroom, look on the floor between the bathtub and the toilet and you'll see a pair of bathroom slippers. As he mops the bathtub and then dries off the floor with a towel the slippers magically disappear.
Continuity mistake: The shadows made by the sunlight streaming onto the hotel office wall completely change, from the scene in which Sam grills Norman about the $40K, to the shot in which Norman runs out to find Lila. While Sam and Norman were in the hotel parlor and Lila was in the house, who pulled the office window shade half way down - Mother?