People smoking filter tip cigarettes. I can't find any evidence (google, etc) that tips were readily available in GB in 1933. Maybe imported, but would be very expensive.
When Cust is being shown round the guesthouse the landlady refers to a heavy drinking resident as a "topper". This is a mispronunciation of the, now little-used, word "toper".
At about 18 minutes in the graveyard, the sun is shining brightly, even though it is supposed to be raining. Crew must be creating the supposed rainfall.
At about 14 minutes when Cust is about to go out, he looks in the mirror. The viewers see his face, which must mean he's looking at the camera, not at himself.
When the letter of 31st March is being typed, it says 'I trust you are well?'. When Poirot reads it is says 'I trust you are well'. No question mark.
Bexhill's De La Warr Pavilion was not constructed until 1935, while the ABC Murders is set in 1933.
The series is set in 1933 and we see a scene showing the De La Warr pavilion in Bexhill on Sea which was not built until 1935.