Factual error: At the party, Marcus Hardman tells Bernard that the Countess recently arrived from Russia, and she describes herself as being in exile. Which made sense in the source material, set right after the Russian Revolution, but less sense in this adaptation, set in the mid 1930s. If she stayed in Russia that long, she would have spent 15-20 years with zero privileges from her rank at that point, and nothing from her old wealth, seized by the communist government.
The Mystery of the Spanish Chest - S3-E8
Continuity mistake: When Major Rich is interrupted playing billiards, the shadows on the playing table have inconsistent angles between shots. (00:11:30)
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 Mystery of the Spanish Chest - S3-E8
Continuity mistake: When Poirot pulls out of the drill hidden in the handle, he holds it triumphantly in his right hand. In the next shot he mounts it on the handle starting the motion with the drill already in the left hand and the rest of the gimlet in the right. (00:40:00)
The Mystery of the Spanish Chest - S3-E8
Continuity mistake: Japp hands Poirot the phone in his office holding it with the left hand, which turns into the right hand at the cut. (00:40:10)
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)
The Adventure of the Egyptian Tomb - S5-E1
Factual error: In the scene after Poirot pretends to be poisoned, a person folds the modern flag of the Arab Republic of Egypt instead the flag of the Kingdom of Egypt as it was at that time.
The Underdog - S5-E2
Factual error: Lily looks over part of the procedure for the manufacture of Astroprene. There are several problems with the chemical structures shown in step two of the procedure. This step appears to be a simple acid catalyzed rearrangement of the molecule shown. The procedure should begin with a C inside the hexagonal ring and end with the C outside the ring with no other changes. The errors include, among other things, a carbon atom at the lower left of the ring with two lines (bonds) to it. The C should have four bonds (lines), the two shown plus two to hydrogen atoms (H). The bonds to H may be condensed so the C will look something like -CH2 - (the 2 would be a subscript). (00:10:30)
Factual error: Poirot smells and very carefully tastes a drink and says, "Potassium cyanide." It is impossible to distinguish between potassium cyanide and sodium cyanide (the other common cyanide) by this method. In addition, there are another less common cyanides that would smell and taste the same as potassium cyanide. (00:16:01)
Hercule Poirot's Christmas - S6-E1
Character mistake: After Japp's arrival, when Poirot walks up the stairs with the two male detectives, at one point he says "Mais no madame." ("But no mylady.")
Factual error: The Jarrow crusade was 5-31 Oct 1936. The sign posted at the hostel stated Poirot was speaking 5 April. (00:15:30)
Factual error: Poirot gets on the train to London. The engine has an SNCF plaque on it. The video then cuts to the 1936 cycle race. The BCNF was not formed until 1938.
Factual error: The prologue shows the dubbing of a newsreel by a narrator. Ten years later it's 1936. But in 1926 Hollywood was still experimenting with sound film, and a UK news reel would've been silent. (It was 1929 before any sound films were made in Europe).
Dumb Witness - S6-E4
Factual error: Spoiler. Police Sergeant Keeley tells Poirot that Doctor Grainger died of carbon monoxide poisoning. The killer turned the natural gas in the bedroom and did not light the heater. This would result in the room filling with natural gas, not carbon monoxide. (01:17:30)
Suggested correction: Surely it was coal gas at that time, before we all converted to natural gas?
The product of coal gas is still methane, which is CH4. It may contain tiny volumes of CO but the gas asphyxiation would still have come from methane, not carbon monoxide.
The Murder of Roger Ackroyd - S7-E1
Factual error: A few drops of acid are dropped on a penny and the liquid bubbles (colorless liquid) as the acid eats its way through the penny. The penny is mostly copper. Any acid that can react with a copper will also produce a bright green to bright blue solution of dissolved copper, which is not the color seen. (00:05:20)
Suggested correction: Not necessarily. It depends on the acid and its strength. A weak acid may only oxidise copper to a monovalent state (Copper (I)) (which is colourless) rather than its divalent (Copper (II)) state which produces the blue solution.
Revealing mistake: As the major tries to convince the Chief Inspector to leave the island, we see Hastings come down the stairs. However, prior to this we can see Hastings waiting just out of shot for his cue to enter. The edge of his hat is visible for several seconds, and when he's cued he begins walking into the shot. (01:15:27)
Factual error: In the beginning of the episode when Poirot eats with the daughter of the murdered man, on several occasions there are closeups of him. When in closeup, it is obvious the actor is wearing modern contact lenses, not accurate for the time.
Continuity mistake: Poirot gnaws the pearls Cornelia returned and deems them to be fake. He indignantly tosses them on the desk, where they show up in the following shot neatly arranged. (01:05:15)
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.
