Continuity mistake: Poirot has a chat with Jackie before dinner after the murder. She says she is ashamed, holding the railing. Poirot tells her that now he, Simon, is available to her again, and Mia Farrow has just the back of her fingers against the railing. (01:40:45)
Audio problem: During the dinner scene when Poirot finds an eel in his plate, the quartet plays a song in perfect classical music, but they are using instruments typical of the local folklore. A rebec does not sound like a violin. (01:42:55)
Continuity mistake: The 'moray eel' on Poirot's plate is shown in close-up. The garnish is on the left side of the plate. In the wide angles though (and at one point Poirot lifts the plate, so we get a good view) the plate is turned differently, the garnish being on the side opposite Poirot's seat. (01:43:00)
Continuity mistake: The pool of blood by the maid's sleeve extends at a different length in the different shots when they find her body. (01:45:10)
Factual error: The body of Louise (the maid) is discovered when the room-service guy drops a towel on the floor and it lands in a pool of blood. The blood obviously ran out of her cabin from under the closed door. However, the cabin door thresholds are not at level with the floor (neither outside nor inside the cabins), but several inch higher, so her blood could not possibly have escaped through the cabin door. All along the movie you can see the actors carefully stepping over those irritating thresholds in order not to stumble, though I think it is hard to spot the edge from within Louise's cabin. But watch when Dr Bessner enters her cabin to investigate the body, it's pretty obvious he takes a step "down". (01:45:15)
Suggested correction: You're right, there are big thresholds. But I have just watched this film again, and when Poirot and Race enter her room after she is discovered, you can see vents in the threshold, which would explain how the blood seeped out onto the deck. I do think they were put into the film set for the room interior shot on purpose to explain how the blood seeped out, as I cannot see any vents when glimpsing the thresholds on the deck side.
Continuity mistake: When Poirot says "That's not quite right" in rebuttal to Race's frustration, he raises his index finger. The same gesture in the reverse angle has him with both index and middle finger raised. (01:46:10)
Continuity mistake: When the doctor arrives at the second murder scene and shouts "Killings, killings!", David Niven by the door is in pose with his arms crossed, and at the cut he has one hand on his knee and the other on the opposite hip. (01:46:40)
Other mistake: The flashback that shows when the culprit is able to inform the accomplice that Mrs. Ottenbourne is about to expose them differs in many small details from the original; when she enters the room, Ustinov by the door fixes his jacket with one hand, and in the original scene he did it with two, when Simon urges her to tell the story from the beginning she has her hand on her chest in the flashback and not in the original. And her last words are "I saw that it was" vs "I saw it." This is no subjective, hypothetical flashback. (01:48:05 - 02:10:05)
Continuity mistake: Salome Otterbourne says she knows who killed Louise Bourget with her own eyes. She waves dramatically her left hand as she says that, and lowers it. Next shot and her hand am raised again, and it's not even the left. (01:49:30)
Continuity mistake: Inside the saloon, Poirot says he knows who killed the three victims. Maggie Smith is looking at him, her left forearm raised. In the reverse shot, her arm is down. (01:52:55)
Continuity mistake: When Poirot calls the doctor's testimony "irrelevant", the doctor's left hand is on his leg, but he's holding his clock's chain in the next shot. (02:00:25)
Other mistake: During the denouement, there is a scene with the meeting between Simon and Jacqueline inside the doctor's cabin. Jackie opens the doctor's case where he keeps the surgical tools pulling it out from a drawer, and she does not place the box back in the drawer or closes it; we see the whole time she spends inside the cabin, so it'd be impossible for the doctor not to notice what happened. (02:08:25)
Factual error: When Jackie pulls out the unusually high amount of cash that Simon happened to have on him, you can read the banknote was issued in 1940. The movie is set in the 1930s. (02:08:40)
Suggested correction: 1) The shot is onscreen for a fraction of a second. 2) Even using freeze-frame, I do not see any visible date. Do you have a screen capture of this?
I did not, but I took one for you.;). I just uploaded a zoomed-in and straightened version you prolly can already see in the pending screenshots section. It says "Juin 1940", but you shouldn't need it; the note is on screen for a full second or slightly less, but the fact that allows you to read it without even pausing is that she holds the note straight, her motion stops just long enough to avoid stealing the moment with freeze-frame (in fact if you take screenshots on VLC like I do, the compression might make it harder to read than watching it in real time). And also pre-WW2 1000 Franc notes have a huge date, I circled it for you in the screenshot. You can also notice the signature of the "caissier general" that matches notes post-summer of 1937 at earliest, and the only date given in the movie is 1930. Serial numbers are consistent with a 1940 release as well, they definitely used a legit banknote from the era who happened to be a late print.
Continuity mistake: The streak of blood on Simon's neck changes from the first time we see it to the time when the second gunshot is fired. (02:15:25)
Continuity mistake: When Bette Davis' character bids farewell saying that the place resembles a mortuary, Maggie Smith next to her holds her bag to her chest, but in the next shot she is also holding her suitcase with the same hand. (02:16:40)
Continuity mistake: Mr. Ferguson and Rosalie Otterbourne say goodbye to Poirot; Ferguson is holding a suitcase in his right hand, but when Poirot says one last thing to them, the suitcase disappeared mid-stairs. (02:17:15)
Plot hole: Simon wasn't shot at the time everyone thinks - he shoots himself after killing his wife - so how did he get all that sweat, in his forehead, through his hair? It was all over him.
Suggested correction: He'd just been running around the boat, and he'd just killed his wife. That'd make anyone sweat.
What the OP says is that he's already sweaty before he does any of that; if you look at the first scene when he's tricking the others, and in Poirot's flashback during the denouement, it is true that the actor's face is already pretty sweaty, although not quite as much as it is later when Simon is actually in pain. Look at him when he closes the bottle of varnish, his hair is a mess sticking to his forehead. You could argue he was nervous, but in the rest of the movie he keeps his cool all the time without such displays of perspiration.
Continuity mistake: At the end, when people leave the boat, as seen in the wide shot, the wood plank is at a different distance from the stair's handrail than when Rosalie Otterbourne and Ferguson leave the boat.
Plot hole: It's not solely the adaptation's fault, but in the visual medium it sticks out more. We know that the killer used nail polish to simulate the blood the first time around, and then throws away the stained handkerchief and replaces it with a clean one which will be stained in real blood. The problem is, you see the fake blood, which is nail polish, drip down his pants and soak his hand; while the pants may not be too much of a problem since the doctor cuts them off in the proximity of the wound (even if we see a drip in the uncut part, it's likely that Poirot never took a look at them), Simon still has a hand covered in paint, which has also a characteristic odor as Poirot remarks in a different part of the movie, and wouldn't come out easily. Somehow that goes entirely unnoticed, and it wouldn't take the sharp instinct and senses of Poirot to pick up on the anomaly, you'd think. He was also of course mighty lucky that nothing spilled on the couch or carpet.
Plot hole: This movie adaptation adds an odd bit where an attempt is made on Poirot's life using a cobra. How in the world did the culprit manage to get a hold of a venomous snake, is never explained or addressed and goes against the carefully designed plot, since the culprit would have needed to obtain it through a third party (a member of the staff, perhaps) and that would expose them. Poirot actually says "you found another use for that serpent" but the idea that Simon actually obtained the snake before they devised the plan and somehow just happened to carry it around on honeymoon when the plan for his uxoricide was entirely different and carefully prepared, is even more absurd.
