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.
Continuity mistake: Near the beginning Harriet Vane produces tokens for herself & Lord Peter Wimsey of miniature handcuffs with one cuff "open to symbolise we're giving up on crime". She puts hers on as a bracelet. Immediately after & several times throughout the film it's clear hers is a set of closed handcuffs.
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.