Continuity mistake: Marion gets into the bathtub/shower, and unwraps a bar of soap, throwing the wrapper down into the tub. Later when Norman is cleaning the bathtub, the wrapper has vanished.
Continuity mistake: When Marion searches for a place to hide the money, the lamp on the nightstand is over to the left side. However, when Marion places the newspaper on the nightstand, the lamp has moved to the center.
Continuity mistake: When Norman is spying on Marion through the holes in the wall, the long shot shows Norman's face about a foot away from the smaller hole. In the closeup, his eyeball is right up against the smaller hole on the outer sheetrock wall. When we return to the long shot, he's back to a foot away.
Continuity mistake: When Norman shows Marion her room, the bed that is reflected in the mirror (that Norman walks past) doesn't have a nightstand next to it. However, when Norman and Marion stand in front of the mirror and talk, there is now a nightstand next to the bed.