Best horror movie mistakes of 1960
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 the boy slides down the banister, money falls to the ground and the boy doesn't see it. In the next shot, as the boy is walking away, behind him the money is lying differently than when it landed. Later, when the boy finds the money, it is again in a different position.