When the police inspector says he is going up to Robert Montgomery's flat to try to trap him, his assistant protests, "But you haven't got a warrant." Yet the inspector has already had the flat searched in Montgomery's absence without a warrant, and has told this to the assistant, who has said nothing.
The stolen diamond is about the size of a doorknob, much larger than the Cullinan, which is the largest clear cut diamond in the world.
When Robert Montgomery lists the crime scenes, he says that X always strikes in a deserted area of London. However, one of the crime scenes mentioned is Marble Arch, which is a very busy area in central London. Also, the first murder shown takes place on the bank of the Thames, but none of the crime scenes mentioned is anywhere near the river, and Gates End Lane, the scene of the last murder, is a fictitious street, although the other streets named are real.