We see young Amyas painting with his left hand. As an adult, he uses his right hand.
Talulah Riley has light hazel eyes whereas Sophie Winkleman has dark brown eyes and they both play Angela Warren.
Every time Elsa drops her tea cup (in multiple replays), which smashes on the ground, there is no sign of any contents in the cup - it is bone dry.
When Poirot reads a newspaper article in the Reading Room of the British Library, the article is seen to consist of the same sentence repeated several times over, in a different typeface from the rest of the page, and with several spelling and punctuation errors.
After accepting Lucy Crale's case, Poirot starts his investigations in the reading room at the British Museum. Though the series is set in the 30's, the modern glass ceiling is clearly visible in the background over Poirot's shoulder.
In one scene a collared dove can be heard calling; collared doves were not present in the UK until the 1950s nor widespread until the mid-1960s.