The Disappearance of Mr. Davenheim - S2-E5
Other mistake: When Japp and the bobby arrest the pickpocketer, the dialogue says "It was only last week that we put him inside for three months" which obviously does not make sense. In the original story he did the 3 months of jail the year before, and this has to be a flub in the lines of this TV version (it is specified also that he did those in the winter, so the Sergeant can't possibly mean that he was released last week). (00:35:00)