As Poirot drives up to the house (in the limousine); the reflection of a 'modern' car can be seen parked off set.
When the group gathered after dinner, Rosamund makes a comment referencing a revival of the play "The Miracle Worker", however that play was first adapted by William Gibson for a 1957 Playhouse 90 production, while this Poirot adaptation is set in mid 1930s England. The Miracle Worker is the English title of a 1937 Russian film, Chudesnitsa, but there was no stage play.