Continuity mistake: When Ferguson helps Simon Doyle on the couch, his tie slides off Simon's non-injured leg once, then in the next shot it is on it again and re-slips. (01:00:55)
Continuity mistake: At the beginning of the scene when Jackie gets drunk in record time, David Niven's shadow on the curtain changes sharply when the angle switches the first time; in the closer view he casts a deep shadow right behind his head, while in the wide angle of the room there's barely any shadow. In the same transition, Linnet is touching the cards with both hands first, simply holding her cigarette in the second shot. (00:57:55)
Continuity mistake: When the doctor feels the need to make a precise assessment of the stone's age, the maid is suddenly close to the big rock between shots. (00:49:00)
Continuity mistake: Linnet is enraged because apparently Salome Otterbourne featured her in her fiction book and equated her to "a nymphomaniacal baboon." Nonsense aside, she gets up holding the book. Angela Lansbury is looking at her, left hand upturned. Cut, and she is staring at her with both hands in her lap, downturned. (00:35:35)
Continuity mistake: Mrs. Salome Otterbourne sits next to Linnet. Linnet is holding an object between her hands; when Salome says that writers can get flowery in their speech, Linnet replies drily, but the hands changed also position. They were at the same level in the first shot, they are one on top of the other in the follow-up. (00:35:00)
Continuity mistake: Bowers and Van Schuyler retire in the cabin leaving Linnet behind them. The deck is dusky, but in the reverse angle that follows, there she is, on the fully sunlit deck. (00:34:30)
Continuity mistake: During the conversation when Bette Davis' character is introduced to Lois Chiles', the background changes inconsistently between shots. (00:32:40)
Continuity mistake: In the first conversation between Linnet and Mrs. Van Schuyler, Bette Davis' hands are positioned differently depending on the camera angle - either the right hand or the left hand on top of the other. (00:33:00)
Continuity mistake: Simon and Linnet regain their wits after the unexpected encounter with Jackie on top of the pyramid of all places. They make out, and Linnet's hair changes radically position depending on the camera angle throughout the whole snogging. (00:14:35)
Continuity mistake: When Simon frees the horse's reins from under the stone, the saddle blanket all of a sudden folds between shots. (00:13:25)
Continuity mistake: On the top of the pyramid, Linnet whispers "Silly" to Simon's question about her happiness, and she holds him close. Her thumb is on display at the side of his bicep, but at the cut it's on the part of the arm that faces her. (00:12:20)
Continuity mistake: In the introduction of her character, Bowers tells Mrs. Van Schuyler "What do I care" to her threat. She sits straight, with the cigarette hand slightly below chin level. In the wide angle she sits back, with the cigarette hand resulting at a higher position. (00:09:45)
Continuity mistake: When Jackie tells Linnet that she has a favour to ask, she holds the box at different height and tilt during the conversation, and the position of her fingers on it changes as well. (00:05:40)
Continuity mistake: When the butler tells Linnet Ridgeway that the architect isn't there yet, she turns towards him dramatically in the first shot, and in the following she again turns towards him. (00:04:30)
Plot hole: Jackie supposedly doesn't have any money, but she seems to have enough to travel halfway around the world, following Simon and Lynette.
Suggested correction: As Jackie was plotting with Simon to kill Linnet the entire time, surely its the case that Simon had provided Jackie the funds to be able to do this so that they could enact their plan?
Suggested correction: It's not like "not having money" necessarily means lacking at that exact moment any cash, including the funds for one trip; it can simply mean your lifestyle is not sustainable and you're gonna run out completely dry or be forced to sell your remaining properties soon if you don't change anything. The novel makes it much more clear because the characters actually address it and express concern for her saying that she probably exhausted the small allowance she had for that crazy stunt. Poirot in fact in the novel suggests them initially to just let her go on until she'll eventually be unable to keep up. Back to just the movie, it's also worth noting that she is following them on that one trip, which brought them in Egypt from Venice through Brindisi, so 'halfway around the world' is relative too, it's not like she's been squandering the money in this endeavour for a long time.
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)
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.
Other mistake: When Poirot questions Maggie Smith, he repeats what she said about Linnet Ridgeway "three days ago." Besides the discrepancy in dates (it was the day before the murder), Poirot at the time was dozing off on the portside deck (mrs. Ottenbourne passes him by going across) so he could have eavesdropped on the conversation in the near area, but not on that particular one which happened on the starboard deck.
Other mistake: Poirot has uncanny hearing abilities, since he could pick up word for word the conversation Miss Ottenbourne and Ferguson were having several feet behind him at the temple of Karnak. He has also perfect memory, and yet he says that the girl asked the question about libeling the dead "two days ago." The murder (in the movie at least) happens the same evening of the visit at the temple. (01:14:50)
