Continuity mistake: When Carbury asks Poirot "Do you reaaaally believe all that?" as he reads Dr. King's testimony about the shooting, Peter Ustinov's hand is up to his breast pocket in a shot, down the side in the next. (01:17:50)
Continuity mistake: At the denouement during afternoon tea, Poirot opens his statement greeting "Ladies and gentlemen." Look at Lady Westholme's table; she and Miss Quinton in the wide shot turn towards Poirot to listen, but in the next shot they are in an entirely different position (Lauren Bacall has her hand under her chin, then on the table, Hayley Mills puts her hand in her lap, then she has her forearm on the back of the chair). (01:19:10)
Continuity mistake: When Poirot in his teatime speech addresses Raymond Boynton, there's a close-up on Dr. Sarah King looking at Poirot defiantly. There's no direct sunlight on her, but then there she is, sitting next to Raymond with sun all over the chest and left shoulder and cheek. (01:19:50)
Continuity mistake: Poirot runs through the hypothesis that Carol murdered her stepmother. In the middle of it there's a close-up on Miss Quinton smoking a cigarette, but in the wide shot immediately following that, she isn't. (01:21:10)
Continuity mistake: When Nadine gets up from the chair to confront Poirot about implicating young Ginevra, she puts in a haste her shawl on the plate in front of her, but it's perfectly rolled in during the rest of the scene except another shot. (01:23:35)
Continuity mistake: When Lennox Boynton scoffs "What motive, for God's sake" at Poirot's accusations on his wife, the position of his right arm and the position of his hat under the sun have changed from the previous shot. (01:23:55)
Continuity mistake: When Lennox picks the wristwatch from his mother's lap showing the actual events, the natural light of the scene is entirely different between shots - the skirt is sunlit in close-up, in the shade in the other angle. (01:25:45)
Continuity mistake: When Carbury mentions Wallis Simpson to Lady Westholme, he's holding his baton alternating an overhand grip with an underhand one. (01:26:40)
Character mistake: Poirot talks with the Boyntons about the solution of the enigma after the dances, sitting at the table with them. By them, the British flag is hanging upside down from the wall - they are at the coronation ball, so not exactly a place where protocol does not matter. (01:29:05)
Continuity mistake: During the coronation ball, Poirot explains that the murder must have not been intimate enough with the victim to tamper with the heart tonic. As he says that, his pince-nez are on his lap, but they are in his breast pocket a moment later, and it switches position again when it gets back to the same camera angle. (01:29:30)
Continuity mistake: When Jefferson Cope says "I admit" during dinner, the position of his hands changes in the mid-sentence cut. (01:29:50)
Revealing mistake: In the close-up of the killer's revolver firing, the revolver is...clearly not firing. It's just the sound dubbed in, but there's no flash or discharge of any kind. (01:33:10)
Audio problem: When Poirot stands from his seat after telling the truth about the spider, he looks at Dr. King. Her lips move but there is no audio. (01:33:50)
Continuity mistake: Just before Poirot moves to the other side of the dinner table after revealing lady Westholme's deception, Dr. King at first has both hands on the table, but in the next shot she is in a different position. Same for Jefferson Cope and his cigarette; he's smoking, then he is not. (01:33:55)
Continuity mistake: When Dr. King asks Poirot why would the culprit, of all people, kill Mrs. Boynton, Lennox next to her has her fingers crossed together, but in Poirot's reply he has the closed fist against the palm, and his wife's position is different as well. (01:35:05)
Audio problem: During the fireworks, Poirot's line about the "nice and royal" show giving him "confidence" is dubbed over, and you can in fact see Peter Ustinov in the shot of the crowd, not saying those words. (01:37:40)
Revealing mistake: When Poirot reads the newspaper detailing Lady Westholme's death, the font and the clarity of the print on the article stick out from the rest of the page like a sore thumb. (01:38:30)
Other mistake: Lady Westholme leaves the room to take a call from England from her husband, Lord Westholme. When she gets back to Poirot, she mentions with outrage the call from Colonel Carbury. It can't be both, it's not even where she lives or works.
Other mistake: Poirot realises that the champagne was poisoned because he happens to notice two cockroaches sipping the spilled champagne instantly died, in fact FIVE are dead when he looks again. This is quite cartoonish; besides the fact that roaches wouldn't die that quickly for that (if they would), we are in the middle of the ballroom/party hall of a luxury cruiser; if the moment something vaguely edible happens to fall on the floor there's half a dozen ravenous cockroaches jumping on it, people have better things to do than dance; that's one massive infestation.
Character mistake: On the way to Jerusalem, Poirot says that "that man, Weizmann, put it rather well the other day, before Lord Peel's commission", and gives a famous quote about the world being divided in 'two places' for the Jews (those where they are not allowed to live, and those they cannot enter). The movie takes place in May 1937, and Chaim Weizmann's quote is from November of the previous year. 6 months are hardly "the other day."
