Other mistake: Investigating Poirot's investigation, the clumsy constable crushes a tiny egg under his foot. Poirot crawls to him and points with his cane at the other eggs in the bush. But he points practically at his shoe, and the eggs are in a different spot to the left. (00:20:10)
Other mistake: After Katrina is arrested, Mr. Delafointaine is reading about it on the newspaper. The newspaper is dated Thursday 23rd May 1935. The date is correct, the whole investigation does happen in a day, starting around 10 AM; Poirot is greeted by the couple with "Good afternoon" and to imagine that by the time he gets to their home Katrina's news (complete with picture of the arrest!) is already in a late edition of the newspaper (a local one, at that) is a bit too much of a stretch, with all the steps taken for the investigation, everything in broad daylight. (00:43:20)
Other mistake: When Japp and the bobby arrest the pickpocketer, the dialogue says "It was only last week that we put him inside for three months" which obviously does not make sense. In the original story he did the 3 months of jail the year before, and this has to be a flub in the lines of this TV version (it is specified also that he did those in the winter, so the Sergeant can't possibly mean that he was released last week). (00:35:00)
Other mistake: Lowen allows Hastings inside his Bugatti, but then leaves to join a member of his staff who beckoned him. We actually see him in front of the staffer and the person with him. Cue Japp, who has then a little comical interlude with Hastings for about 20 seconds. Cut then to Lowen who.is just now being introduced to the guy he came over to greet. Surely he has not been standing in awkward silence all that time. And even if we assume that this conversation is supposed to happen simultaneously to the Hastings and Japp show, it is half as short, meaning it would have cut into their antics when he ends it by yelling at them. (00:23:15)
Other mistake: Japp manages to coordinate the pincer action of the 3 police cars using a very broad map and no real information about the whereabouts of the suspect. The whole scene is rather silly, it would work in a modern thriller with a GPS, perhaps, not with toy cars and a street map of London.
Other mistake: Poirot, Hastings and The Veiled Lady go upstairs because the lobby is 'too public'. The man spying on them is hiding behind the same newspaper prop used in the previous episode of the season, where Hastings was reading it in Cornwall at the beginning. So must be a pretty old issue now (Peril at End House takes a few days, and they were on holiday). (00:07:40)
Other mistake: Miss Lemon reminds Poirot of the dentist appointment he has at 11. Poirot then receives a phone call from Japp. It turns out during the investigation that the police broke into the apartment at 10:45. Considering that at least a preliminary examination needed to be made and at the time of the phone call Japp was in his office and Poirot was still comfortably having his tisane, everything leads to believe that the dentist appointment should have been later in the day (or the events at the Mews sooner). (00:03:30)
Other mistake: This adaptation opens with a nice fake Pathe Gazette news reel. The voice says that "last year" Farley sold a record 5 million pies, and that the new wing of the plant is due to open. At the inauguration, there's a banner behind him saying "1885-1935" and he's saying that in 1935 they sold more pies than at any point in their history, and he gives the kind of speech you give at the beginning of a year, trying to set a new record while still on the hot trail of the previous. At the same time, Miss Lemon tells Hastings that the typewriter broke 'last Easter' and she burst out with Poirot that she has been complaining 'for the last six months', which would put the event at the end of the year.
Other mistake: The man in front of Poirot in the line to embark argues supposedly in Italian with the officer: he does it in broken Italian but trying to pronounce informal words (even an incredibly explicit and vulgar curse at the end!) with unnatural cadence and a very thick Spanish accent. (00:28:30)