Poirot's lecture is scheduled for Thursday April 5th. Although the movie is supposed to take place in 1936, the only April 5th that lands on a Thursday in the 1930's is in 1934.
The American student is studying on a Fulbright scholarship. This would fit with the original date from the book but the Fulbright program only started in 1946 which makes it anachronistic with respect to the Jarrow march.
When Poirot and Japp are reading the newspaper at breakfast (an hour into the film), the back page of the section held by Poirot is clearly identical to the back page of the paper bought at a newsstand by Japp at the beginning (approx. 4 minutes in), even though a few days have passed in the story.
When Patricia Lane removes the photograph from the album, the page should be darker where the photo was and lighter in the open areas. Yet it is all the same dark shade, revealing that it is in fact a new album, not the long-held family record it is supposed to be.
At the end scene where Nigel Chapman is confronted with his father's letter,the policeman in the background has 5217 on one lapel and 2175 on the other.
Sally Finch's cover story is that she was a student on a Fulbright scholarship in 1936. The Fulbright program did not begin until 1946.
At approximately 6:45 in, a train car pulls into the (fictional) "Hickory Road" station on the London Underground. The train is seen to have big stickers/decals on the windows which read "NO SMOKING." The episode is presumed to take place in either 1935 or 1936. Smoking was not banned on the London Underground until 1984.
As the students introduce themselves, Sally Finch states, "I'm here on a Fulbright scholarship...". The year is 1936 as that was the year of the Jarrow Marchers who figure prominently in the story line. However, the Fulbright program did not commence until 1946.
Although set at the time of the Jarrow march of 1936, the rolling stock of the London Underground is clearly the "38 Stock" first introduced only in 1938.
During the fashion show there is a gramophone playing a blue label HMV record, which weren't produced before the 1950's. In the 1930's it would have mainly been a red or burgundy label.
Mrs Nicoletis pronounces her cousin Giorgios's name incorrectly - Giorgios is correctly pronounced 'Yor-gos' in any Greek dialect, while she calls him 'Georg-yos' with a hard 'g'. This is a mistake no native Greek speaker would make.