Factual error: The opening assault of the movie bears the date October 31, 1914. Poirot's squad attacks the Germans using the cover of gas delivered through canisters. The first documented use of poison gas in warfare was in April of the following year by the Germans, but it possible that the Belgians would attack using non-lethal gas, which they employed starting August of the same year. They wouldn't have the gas masks that they are sporting during the assault, though, including a full headed hood designed by Cluny MacPherson in 1915 (worn by one of the extras that wanders behind Poirot as he takes off his respirator). (00:04:10)
Factual error: When Linnet walks to her room never to wake up again, she walks past a newspaper ("Daily Sketch") with the date of Monday, May 17, 1937. Since the cover story is the opening scene in the night club, and the wedding happens exactly six weeks after that scene, the movie has to take place in July at most. However, in the hotel lobby in Aswan, during the introduction of Ms. Van Schuyler and her companion (right after cousin Andrew tells the clerk that "not a soul" must see the telegram), one of the guests is reading issue 700 of the French magazine Candide, which came out only mid-August of the same year. (00:20:30 - 01:00:10)
Factual error: A .22 shot would be 120-140 dB, muffled with a pillow down to 100-120 dB. Only one person heard the second shot? No one heard the third? Plus at the end there's no way a .22 could pass through a male chest cavity, and halfway into a female chest cavity - it's too low power.
Factual error: The movie takes place in 1937: the new and improved Salome Otterbourne of this version has a repertoire of blues songs that are a few years posterior to that date, many years if we count the Rosetta Tharpe versions actually used in the movie. An argument can be made that they are all 'live performances' from a fictional character regardless of the vocalist who actually performed, and so the only song truly 'impossible' in 1937 would be "Shout sister shout", written in 1941, but we don't actually see Salome sing it. She does perform "Up Above My Head" and no recorded version of it exist before 1941, but it comes from a traditional gospel song.
