Plot hole: The killer shows up at the scheduled appointment at 8 AM. They kill the idiot blackmailer with an overdose of morphine. Remember, that morphine that supposedly killed Thrombey in 10 minutes. Marta finds the blackmailer at 10 AM...alive, and does CPR on them, keeping them alive long enough for the ambulance to come and bring them to the hospital, even if in critical condition. So we went from "kills in 10 minutes, you can't even try to save him" to "after 2 hours, you are still hanging on"? (01:56:10)
Factual error: Opening the blank envelope, Marta finds the fragment of the toxicology report. It is signed "Office of the chief medical examiner, Norfolk County, Massachusetts." But then it writes also the address of said office, which is in Marlborough. Marlborough is also the city when Marta resides, and where the lawyers' letters we see come from. But Marlborough is also a city in the Middlesex county, not Norfolk. (01:28:25)
Factual error: The drug in Marta's bag is incorrectly labeled as Ketorlac, when it's Ketorolac, with an extra O. It's the name of the molecule, not a brand name that could have been altered for legal reasons - and they mention the commercial name just minutes later during questioning, with no alteration. (00:36:00 - 01:49:00)
Continuity mistake: The lawyer arrives at the mansion and gives the reading of the will at roughly 10:30 am (it's morning, he says he'll be ready "in 10 minutes" and a few minutes after you can see on the phone Jamie Lee Curtis' character holds in hand that it's 10:16). Marta gets immediately stormed by the angry mob of scorned relatives and escapes with Ransom, who drives her to a little roadside inn. It's not implied that they drove hours, but let's just assume it's noon or even past noon (which already seems a lot). Ransom feeds her beans, and Marta is forced by her odd gag reflex to spill the metaphorical ones. When she has just finished telling him how the "suicide" went... it's pitch dark outside, and at the mansion. (01:02:30)
Continuity mistake: After walking in the muddy path, Marta is greeted by very affectionate dogs. Daniel Craig approaches the dogs and the kneeling Ana de Armas with just his left hand in his pocket. Close-up, and he has both hands in his pockets all of a sudden. Return to the first wider angle and Craig's right hand is out of the pocket just as before. (00:58:10)
Factual error: The damning vials of medication are in close-ups. It is printed on the label that we're talking about 20 ml vials. The morphine vial is 5 mg/ml. When Marta says that she gave Harlan 100 mg of drug she is then wrong; unless she administered the full vial. The two vials have in fact different concentrations: the Ketorolac is in a 30 mg/ml solution, so Marta would have administered 600 mg of ketorolac tromethamine, not 100. The maximum dose for a geriatric patient is 60 mg per day; even the fact that she'd administer it in IV for days for just a pulled shoulder is definitely overdoing it anyway - and she puts even morphine on top of that. (00:36:00 - 01:49:00)
Visible crew/equipment: While Blanc talks to Marta about how long he has known her to be involved in the case and that he has an opinion on whether she should help the family, a person (most likely a crew member) leaves the house in a stooped position, as all the other inhabitants should have left it by now. (02:04:19)
Other mistake: During the revelation of the second death, when the killer opens the toxicology report they just retrieved, the date is revealed; it says it was issued on 09/27/2018. But the events of the movie happen in November (it's Sunday 18th Nov when Marta wakes up with all the reporters at her door). It should also be noted that when a close-up of the report was shown earlier (when Ransom was looking at the anonymous letter brought by Marta) that copy had a big stamp in a corner saying 11-17-18, that in this copy is totally absent. (01:56:55)
Other mistake: Marta sinks her white shoes in the mud to cover her tracks. We see that the shoes are, as they should be, all dirty to the sole and welt. They are clean again in the following scene though (on a sidenote, the shoes have been cleaned after the murder then, and she or her mom missed the bloodstain - it's very much possible, it just adds a little implausibility). (00:57:20)
Continuity mistake: Walking outside of the shack with the recording equipment, Marta is rubbing the tape with the magnet. Notice the position of her hand when she is walking - correctly, she is holding the magnet end against the cassette. The close-up that reveals the trick to the audience though, starts with her holding it the opposite way. (There is technically time for her to flip it around in her hand in the couple seconds when her hand is not on camera, but we see her put her hand by her side to conceal the magnet's presence, and then how she holds it tightly against the same spot on the coat; switching the side the magnet is facing would require a deliberate movement with no sense or purpose). (00:57:00)
Continuity mistake: As the camera is panned wide with an aerial view of the estate there are no leaves on any of the trees. But as Marta steps out of her car, the trees are full of very colorful leaves.
Audio problem: After Harlan knocks over the Go board, Marta says "I'm done with you." When she says that, which is when she puts the two vials on the table, her mouth isn't moving.