Twice during the movie, Maxim leaves Manderley to travel to London, once by train and another by car. On each occasion, he manages to get back before dark. Manderley is in Cornwall, as far from London as you can get, and even with 21st century roads, cars and trains, that feat would be impossible. This error is copied from the novel.
The large map on the courtroom wall is a map of the Americas. It is grossly implausible that such a map would be on the wall of an English courtroom.
When the new Mrs. De Winter first enters the room to do her correspondence, there are three books on the table. An establishing shot shows that the book on the left is labeled "menus", and the one in the center is labeled "addresses". Mrs. de Winter picks up the one on the left, and it is now revealed in a close-up to read "addresses" on the cover.
The word pamplemousse (French for grapefruit) is incorrectly spelled as 'pamplemouse' in the Princesse Hotel Monte Carlo menu toward the beginning of the film.
When de Winter is showing the honeymoon film, he tells Mrs. de Winter "this is where I set the camera up on the tripod". However, the image they're watching zooms in (this is before the cinematographic image zooms in on the entire film-viewing scene in the movie, itself). Since the camera was on a tripod, it wouldn't have been able to zoom by itself since de Winter was posing with his wife at the time, not adjusting the camera.
When the Narrator and Maxim marry, he gives her a large bunch of flowers. She repeats "Perfectly lovely", but the second time, her lips do not move.
At the inquest, when Ben begins to testify, the clock reads 11:48. A few minutes later when Mr. de Winter is recalled, it reads 1:53. But then when Mrs de Winter faints, the clock reads 11:48 once again.
In the outside take of Manderley seen in the scene where the Narrator stares at one window being closed, it's a miniature, as is the 'Mrs Danvers' dummy dressed in black. You can realize this by the motion of the window as it's being closed, not in a continuous way, but by little fast jumps, which look too unreal.
George is driving on the left-hand side of the road outside Monte Carlo.
In the Monte-Carlo Hotel Lounge, the Narrator and Mrs Van Hopper are having coffee. The Narrator's coffee cup, then a newspaper are in her right hand between shots without sufficient time or obvious motion to change them.
After the luncheon, de Winter and the Narrator are standing on the steps waving goodbye; Jasper the dog moves from one side of the steps to the other between shots.
Frank's hand holding the letter jumps from in front of the filing cabinet to resting on top of it.
During the dining scene after the second Mrs. de Winter knocks over the vase, the neckline of her blouse goes from untidy to perfectly neat between shots.
Impossible police procedure: immediately after the inquest, the Chief Constable starts a possible murder inquiry, on his own, taking the chief suspect with him. This error is copied from the novel.
Walking back from the beach, the background moves much more quickly than the characters.
When the de Winters first arrive at Manderlay and have got wet in the rain, Mrs de Winters's hair alternates between being almost dry to being rather wet.
Camera dollying back from Mrs. de Winter sitting on a chair brushes some flowers on a table.
The Narrator follows Jasper to the cottage. When she sees Jasper, he is standing up, barking in front of the door. She approaches the door and the dog is now lying to one side.
When Mrs. Van Hopper begins to put out her cigarette into the cold cream jar, the jar already has a cigarette in it. At the closeup of the jar, the original cigarette is gone. She puts in a cigarette at an odd angle but when the close up is over, we again see only the original one.
When Reginald Denny's character is in the study with Joan Fontaine's character he pulls out a file folder from a file cabinet and it's in the American format for 8.5 x 11 inches rather than the English A4 size. A4 is a bit narrower but longer than the US letter size standard and its paper would not fit the file folder Denny is holding.
When Mrs. Danvers draws open the draperies in Rebecca's room, she tugs very slightly at the draw cord causing the huge drapes to open several feet.
When Mr. and Mrs. de Winter first arrive at Manderley, it begins to rain -- but the sun remains shining while it's raining.