Other mistake: When Norman and Mary are at the swamp, the sheriff hands him the ashtray they found and bates explains that it belonged to Mr Toomey. Mary say "Who"? and the sheriff explains who he is. This makes no sense as Mary knew full well who Toomey was and had no reason to play dumb.
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Suggested correction: She wasn't playing dumb. She says WHO because she didn't hear the name correctly.