When the doctor's sister is seen in the opening scenes she has on a blouse with intricate cutwork near the neck. The blouse is cream color and the cutwork a dark brown. The work is so beautiful it's memorable. In another Christie/Suchet episode, Cat Among the Pigeons, the headmistress of the school is shown wearing the same blouse.
The picture in Ackroyd's hallway, of a mother nursing a sick child is the one that provides the vital clue in the episode, Dead Man's Mirror.
The 1979 film, "Agatha," about the mysterious disappearance of Agatha Christie for eleven days in 1926, begins with the publication of "The Murder of Roger Ackroyd."
At the beginning of this episode, the newly-retired Hercule Poirot is not using his trademark silver swan-handled cane in favor of one with a black tusk-shaped handle. When Poirot returns to London, the swan cane is once again in his hand. No explanation is given for the swap.
When perusing the unopened letters from Roger Ackroyd's desk, the top one is from The Hudson's Bay Company, the venerable Canadian retailer.