Character mistake: As Poirot reconstructs the facts in the flat (and he explicitly says "consider the FACTS"), he lays out that "a letter which was found at the scene of the crime with John Frazer written on the bottom." But the letter simply said "Frazer", the first name was just Japp's random conjecture based on the initials on another evidence being J.F. (00:27:40)
Character mistake: Somehow, Poirot is the first person to get to the basement - he obviously would have slowed down terribly the policeman and the young and spry Jimmy rushing down the stairs with him.
