Factual error: Poirot is visiting an art exhibition with the Countess, and expresses his admiration for a painting by Marc Chagall. Amazingly enough, that painting is "Les Plumes en Fleurs", something Chagall will create in 1943, years after the time when this pre-WW2 episode takes place. (00:24:50)
The Mystery of the Spanish Chest - S3-E8
Continuity mistake: During the party, the colonel expresses to Poirot his disappointment for the jazz record being played, music "from the inferior races." He puts a tart in his mouth, closing his lips past the piece of fruit. Cut to the wider shot, and he is still holding a tart that sports the strawberry he just chewed on. (00:19:00)
The Mystery of the Spanish Chest - S3-E8
Continuity mistake: Poirot enters Japp's office when the inspector is dealing with the tribulations of modern machinery. As Poirot sits down, you can notice the crumpled paper by the phone being different in the reverse shot, when you can also see Japp's hand being at the corner of the typewriter. But he brings it up from his lap in the next shot. (00:38:15)
The Tragedy at Marsdon Manor - S3-E6
Continuity mistake: Poirot and Hastings are in front of the corpse at Marsdon Manor. When Miss Rawlinson makes her comment about him taking advice, she is almost with her back against Hastings and in a dramatic light totally different from what shown in establishing shot. (00:17:45)
The Mystery of the Spanish Chest - S3-E8
Continuity mistake: In the flashback during Poirot's narration, the scene with Colonel Curtisstalking to the victim at the military club happens differently from the first time around, with Clayton switching hand position after a different line of dialogue, starting off in discontinuity with the previous shot (originally the continuity was flawless). (00:44:40)
Factual error: Hastings and Miss Lemon decide to investigate on their own. In the outside view of the first suspect they go question, a large contrail is visible on the left of his building (contrails were not a complete impossibility in the 30s, but it's rather odd to randomly see one in an establishing shot for this timeframe). (00:22:40)
The Tragedy at Marsdon Manor - S3-E6
Revealing mistake: In the reveal at the end of the episode we see a prolonged close-up of the newspaper "Nairobi Daily Press." Not only the fateful article linked to the murder is written in a font different and larger from the one used in the rest of the first page, but you can also see where the fake part abruptly ends, resuming mid-sentence the article of a real (or realistic) newspaper used as base for the prop, back to the right font and consistent style. (00:50:30)
The Tragedy at Marsdon Manor - S3-E6
Continuity mistake: The morning after his arrival at the village, Poirot sits with Hastings for breakfast (prime minister Baldwin is mentioned on the radio). Hastings is holding the newspaper with his left hand and the right is on his knee. New shot and both hands are on the table. It keeps happening during the scene. (00:14:25)
The Tragedy at Marsdon Manor - S3-E6
Continuity mistake: Poirot holds the widow's painting for Hasting and asks "You do not see?" In the reverse angle with Hastings (obviously) not seeing a thing, Poirot's hand in frame is holding the painting differently. (00:39:15)
The Tragedy at Marsdon Manor - S3-E6
Continuity mistake: The car with Poirot and Hastings enters the village. Right as the camera begins to pan to the left, notice a woman with a maroon tailleur and a powder blue hat. She walks in the same direction of the car but turns around a moment later. Several seconds after, when we see the two protagonists in the car, the same woman can be seen amongst the passersby through the back window. (00:04:15)
Plot hole: Much like the 1978 movie adaptation, in this version there is an absurdly short time elapsing between Linnet leaving the bridge table and the incident and murder; it's barely two minutes when she'd have to go to her cabin, do everything a proper lady of the time would do to prepare to go to bed, and fall sound asleep. That's because the witness character (Cornelia Robson here) in the novel is supposed to be droning about her boring life for a long time, but there are no gaps in her speech to imply that a long time passed - at most there's the waiter bringing a drink at the very beginning who sorta comes out of the blue.
The Tragedy at Marsdon Manor - S3-E6
Plot hole: It is revealed towards the end of the episode that the painting Mrs. Maltravers did when the murder took place was not painted that day and that time of the day, because it had the wrong shadows. But we actually saw what was on the canvas when the secretary was leaving to go to the bank, and it was that exact painting in an early state. She also came up with the idea for the murder the night before, making even more unbelievable that she'd just have the alibi painting ready and waiting the morning after. That without mentioning the fact that the murder itself as described is not something that would have required an amount of time needed to make a whole painting, and that she could have finished it later anyway (she gave it to Hastings a day later, after all).
Answer: He definitely says "Belgian", but the subtitles get it wrong and show him saying "American."
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