Revealing mistake: The first morning the new Mrs De Winter is at Manderley, she goes to have breakfast, but when she attempts to pour milk into her coffee, there's no milk in the jug; she's just pretending to pour it.
Revealing mistake: In the scene where Cary Grant punches Jimmy Stewart: Jimmy starts falling back (and you hear the sound effect of the punch) when Cary's fist is a good 1-2 feet from Jimmy's face.