The Mysterious Affair at Styles - S3-E1
Plot hole: Can't fault this massive plot hole to the adaptation, but to the source material; the culprit (forgetting the stupidity of writing an incriminating letter detailing the plan to murder someone, and put it in a desk he shares with her) since there are people outside the room that are about to enter, tears the letter in 3 neat vertical strips, rolls them, puts them in the vase on the mantlepiece, and then opens the side door to slip away...instead of simply pocketing the letter and going through that same door. Nobody was going to search him or anything and could have burned it, torn it into confetti, anything, later. It takes way way longer to do what he did, which needed him to stay there in the room increasing the chances of being found out. And of course he and his accomplice do not retrieve the letter after.
The Mysterious Affair at Styles - S3-E1
Plot hole: To enter Mrs. Inglethorp's bedroom, Hastings and John have to ram the door and certainly break the lock (it is replaced, so there was damage done). The doctor leaves the house very early in the morning just after ascertaining the death of the woman, and tells them he "locked both rooms" (the room has three doors but one is bolted internally, allegedly). But the door was broken, and in fact when Poirot arrives, which is early in the morning, as Hastings went to the village waking him up, he finds a brand new lock in the door. They would have needed to summon a locksmith at maybe 6 or 7 am, to repair an internal door in a country house, and he should have done it on the spot (also, the door frame itself is more likely to suffer damage than the lock itself, no real damage was shown during the scene). The keys are also all in the same bunch and not looking in any way different (one should be brand new and shiny). (00:28:45 - 00:37:40)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: Poirot is concentrating in London, making a house of cards. When he says "I steady my nerves, that is all", the 6 of hearts can be seen in his right hand, and he sets it as the diagonal piece. Cut to Hastings, and then Poirot supposedly still has the same house in front of him...but it is not. Where the 6 of hearts was, there is now a 6 of spades, and the whole castle is different, with the cards pulled tighter together. Later he rebuilds the house, and he sets horizontally in the close-up a 5 of spades, which turns into the 2 of spades immediately after. (01:19:40)
The Mysterious Affair at Styles - S3-E1
Factual error: Poirot is taking a break from the prosecution' speech and is walking with Hastings, explaining to him Mary Cavendish's jealousy. In the street, they pass by a truck (registration plate MR 5496) that is a Morris-Commercial 1-Ton model. Morris began productions in the early 20s, and we are in 1917 for this episode. (01:17:45)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: The guests of Styles Court manage to get inside the room of Mrs. Inglethorp and talk to her before the second fatal crisis happens. In the first wide shot you can see Hastings peek at Lawrence and then look towards the fireplace, which is an action he'll do in the close-up that follows - Lawrence does not seem to look in the direction Hastings is checking out, at all, but that is actually a nice touch considering the resolution. Moreover, in the second close-up of Hastings looking at him (the one that follows the view of the fireplace), Mary is directly in front of Hastings, you can see her insignia and arm. But instantly she is shown at the cut, creeping along the wall behind Lawrence. (00:29:00)
The Mysterious Affair at Styles - S3-E1
Factual error: At the local food store, Poirot is talking with the shopkeeper. In the background are plenty of food packages that show the great care for authenticity that is a mark of the series. I feel like disputing at least a couple; the Polly box with a parrot is a Taormina American-made can that should be a product of the 20s, and the Brown and Polson's Patent Corn Flour has a claim on the box that says "65 years world-wide reputation." The patent is a 1856 one and the annexed name is from 1859 (previous ads with established date show that year as the one they started counting from); in 1917 the box would have not said "65 years", it hadn't even reached 60. (00:19:20)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: At dinner, Alfred Inglethorp asks Hastings what he's going to do after the war. The person to his left is placing a glass on the table. The next cut happens mid-sentence and in that instantaneous timeframe the glass is perfectly still while the wine was still noticeably moving in the previous shot. (00:10:10)
The Mysterious Affair at Styles - S3-E1
Factual error: The movie opens with Hastings watching a news film about the "New Flanders Offensive." It says that on "7th June 1917 General Haig attacks Ypres", but at the beginning of the year Sir Douglas Haig was promoted to Field Marshall. A news propaganda reel would surely have called him with the appropriate title. (00:00:35)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: The coroner asks Mary to tell everyone about the quarrel. Frontal shot of Mr. Wells; he is pointing his hand with the spectacles at a certain height. Reverse shot; the hand is lower, resting on the other hand, and only then he raises it to the same level as it was in the other shot. (00:53:40)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: When Evie Howard leaves the house after the argument with Mrs. Inglethorp, she sits on the sofa explaining what happened to Hastings and the others. She has her hands on top of the purse handles. After a brief cut on Mary saying "oh no", one of the handles is free. (00:16:40)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: About the burnt will, the lawyer asks the obvious question to John Cavendish "Are you so sure it's a coincidence?" He's holding in his raised left hand a sheet. Next shot and he has both hands down on the desk. (00:48:55)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: Cynthia is sobbing with Hastings, and he asks her to marry him. The handkerchief in her hand changes position between shots (particularly easy to spot when you look at the black band). (01:13:05)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: After Hastings reminds him of the mantelpiece, Poirot shouts "Mon Dieu!" and pounds the table. The cards scattered on the table differ in disposition between shots when he stands. (01:21:05)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: When Hastings spots Poirot from the window as he arrives for the denouement, his hand is in a different position on the window frame (left or right of the vertical bar). (01:26:55)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: At the hearings, the doctor is on the stand and says "There was no strychnine in it." Mr. Wells closes his fountain pen. But then in the next shot he holds it in his hand. (00:52:40)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: Poirot locks the document case in a close-up. The shadow on the case is at the center of the object, but in the wider shot it is projected entirely on the left side. (00:38:15)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: Mrs. Inglethorp's death happens with her having a much messier hair and the neck tilted to an angle in the wider shot, compared to the close-up on Gillian Barge. (00:30:00)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: When Hastings tries the fake beard on, it sticks past his fingertips more in the shots from behind than on the front. (01:09:15)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: The larger broken porcelain pieces Poirot is examining in front of the fireplace change place between shots. (00:39:45)
The Mysterious Affair at Styles - S3-E1
Continuity mistake: Hastings has just met the Ingelthorps, and now is outside so John Cavendish can introduce his wife. In the wide shot it's evident that John lowers his arm, but in the closer view his arm is still held up high before Mary speaks. (00:08:10)