Poirot says at 28 min 27 sec that he doesn't like odd numbers, only even numbers
yet at 34 min 12 sec into film, he puts caviar on his 9 crackers, an odd number.
When Poirot meets Rufus Van Aldin's wife the nun's cell bed has Royal Navy bedspread on it.
Playing "Sing, Sing, Sing (with a swing)" as the jazz dance music for the party in Nice would have been difficult as the album was not released until 1950.
When the train travels from Marseille to Nice it is shown travelling west instead of east and then arrives at the westbound platform in Nice (with an impossible view of the sea).
In the convent, Poirot addresses the nun as "madame" instead of "sister".
This is not really an error: at this moment, he does not anymore address the nun as "Sister Dolores", but rather "Madame" Dolores, as he understood that she is actually the mother of the daughter she had with Rufus Van Aldin.
In the convent, Poirot addresses the nun as "madame" instead of "sister".