Factual error: In Poirot's study, Lord Pearson shows the big piece of mineral, saying "You're left holding a a nugget of top-grade 24-carat silver." That's a laughable blunder, from a top level banker and investor about to do a massive mining deal even. Silver purity is expressed in millesimal fineness. (00:11:30)
Factual error: The episode is set at a precise date (it's even a plot point); 8/2/1935. But at the beginning Poirot and Hastings are playing Monopoly, and right there on the board you can distinctly read; "Trade Mark - Pat.app. For Nº 3796-36." As signaled by the last 2 digits, it's a 1936 edition (the UK localization of the game). (00:01:40)
Factual error: One of the two police cars identified as Unit 10, is appropriately enough, a Sunbeam-Talbot Ten. The episode is set in 1935 though (with amazing accuracy for some details, including the poster for the movie "George White's Scandals 1935" shown before the car tailing starts), and that car was manufactured only after 1937. (00:19:55 - 00:22:00)
Factual error: Japp in this episode uses a Humber Snipe, a 1937 model. The story is set in 1935.
