Continuity mistake: In the scene after Janet Leigh is killed Norman goes into Cabin one and starts mopping. He puts the mop in front of the bed for a second. Through the mirror you can see the mop fall when he put it down, but when he goes back to get it, the mop is up again.
Continuity mistake: Seymour watches in horror as the wino is run over by the train. In the long shots, Seymour's hands are waving up and down at his sides. In closer shots, however, he's holding his hands up to his face and screaming.
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.