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.
Other mistake: In the windmill scene, the windmill reverses directions twice to signal the airplane. However when Joel McRae enters the windmill and we see the machinery it is obvious that there is no mechanism to reverse the windmill against the wind.