Factual error: The Jarrow crusade was 5-31 Oct 1936. The sign posted at the hostel stated Poirot was speaking 5 April. (00:15:30)
Factual error: Poirot gets on the train to London. The engine has an SNCF plaque on it. The video then cuts to the 1936 cycle race. The BCNF was not formed until 1938.
Dumb Witness - S6-E4
Factual error: Spoiler. Police Sergeant Keeley tells Poirot that Doctor Grainger died of carbon monoxide poisoning. The killer turned the natural gas in the bedroom and did not light the heater. This would result in the room filling with natural gas, not carbon monoxide. (01:17:30)
Suggested correction: Surely it was coal gas at that time, before we all converted to natural gas?
The product of coal gas is still methane, which is CH4. It may contain tiny volumes of CO but the gas asphyxiation would still have come from methane, not carbon monoxide.
Factual error: The prologue shows the dubbing of a newsreel by a narrator. Ten years later it's 1936. But in 1926 Hollywood was still experimenting with sound film, and a UK news reel would've been silent. (It was 1929 before any sound films were made in Europe).