The Adventure of the Cheap Flat - S2-E7
Continuity mistake: Hastings, Japp (for some reason) and Poirot are at the movies; when the camera pans through the room, there's a guy sitting behind Hastings, while behind Poirot there are a red haired woman and a man, as confirmed by the close-ups. When the music stops and Hastings tells Poirot that he can open his eyes, the couple have moved behind Hastings. (00:02:50)