Continuity mistake: Hannay is talking to the milkman inside the door. Outside the door you can see across the street, the two guys walking down the sidewalk. A moment later, Hannay goes toward the door and now there's the milk wagon on the street right in front of the door. When Hannay pointed out the two guys across the street to the milkman, the milk wagon wasn't there and the milkman is inside with Hannay so he couldn't have pulled it up.
Revealing mistake: The serial number of the autogyro has been reversed, showing that the stock shot has been reversed for effect.
Continuity mistake: When Pamela takes off her stockings in the hotel room, Hannay dries them by placing them under two china ornaments over the fireplace. When Pamela removes the stockings from the shelf a little later on, the ornaments are now much further apart, with another ornament placed between them.
Other mistake: When we see the Church Hymnary being opened, the date on the left-hand page is signed by Margaret in 1928, but the book is dated 1932.
Character mistake: During Mr Memory's act at the beginning of the film, he gives the answer regarding the last British heavyweight boxing champion and then says, "Am I right, sir?", but the question was asked by a woman.
Revealing mistake: During Mr Memory's act at the beginning of the film, a woman asks, "Who was the last British heavyweight champion of the world?". As she is saying this, you can see the man sitting next to her mouthing her exact words.
Revealing mistake: During Mr Memory's performance at the end of the film, he stops speaking and flinches before the shot is fired.