Continuity mistake: Mrs Pengelley is outraged by Hastings' question about the niece. Poirot opens his umbrella and begins to move forward to ask a different question. The left side of his coat is mostly spotless, but in the next close-up that side is instead covered in water stains, including a quite visible streak on the chest. (00:05:40)
Character mistake: The rude doctor says that Edward Pengelley "wouldn't poison his grandmother's dog", to which Poirot irately blurts out "but it is not the dog of Madame Pengelley's grandmother that has been poisoned." He should have said "of Monsieur Pengelley." (00:12:50)
Continuity mistake: When Japp does the comical fistwave at Poirot at the end, Poirot's carriage is passing by a line of people (woman with red coat, young woman with greenish one) that already passed by when the constable was giving the 'bad news' to Japp. (00:48:20)
Continuity mistake: Hastings sits down when Japp is having his meal at the inn, and eats one of Japp's sandwiches. In the rest of the scene, the sandwiches on the plate change position and Japp's beer has been replenished a bit. (00:35:00)
Continuity mistake: The level of sherry in Radnor's glass changes after Hastings smiles at him when Poirot is laying the trap. (00:38:40)
Continuity mistake: After reading the will of the deceased, the lawyer folds back the first page twice in two different shots. (00:26:05)
Revealing mistake: As he scolds Hastings for his pancreatic hullabaloo, Poirot drinks his tisane from an empty cup. (00:02:55)
