The Disappearance of Mr. Davenheim - S2-E5
Plot hole: The safe is behind a painting that acts like a real door. When Mrs Davenheim spots it, it's wide open. But Japp questioned her in the same room and the painting was in place back then. It really makes no sense that with a story depicted as being so big (it makes the first page of the national newspapers) and with an investigation in place, nobody in a week has checked the safe at all. (00:27:50)