Continuity mistake: When Mr Stringer picks up Miss Marple from Akinthorpe Hall in his car, Alexander, as a harmless joke, has tied a string of old shoes and empty cans to the car and put a sign 'Just Married' on the back of the car. As the car first drives off, the sign is seen falling away, but when we get a different shot, the sign is well secured.
Other mistake: Just before Miss M sees the murder on the passing train, there is a shot of the express train overtaking hers. At the relative speeds, the express would have passed her window in 4 seconds, but she sees the murder for nearly 14 seconds.
