Other mistake: Dante and The Agency both have extensive photos and videos of the team's previous exploits. Trouble is that most of it is simply stills and footage from the previous movies in the franchise, often occasions when nobody was around with a camera to witness and document what was going on, certainly not at the angles we see.
Factual error: Poirot attends a Halloween party in Venice in 1947. Halloween was never publicly recognized as a holiday in Venice or Italy in general in the XX century, especially with the traditional pumpkin-centered iconography. One could argue that, for the most part, Poirot is attending a private party organized by an American soprano. But, besides how unlikely it would be for 1947 Catholic nuns to take part in such a pagan-themed event, there are scenes showing Halloween decorations in the streets of Venice far away from the party location, as if it were a public holiday and not a private gathering from an eccentric foreigner.
Suggested correction: Besides what Oliver says to Poirot in the beginning about Americans bringing over Halloween to Italy, it's not true. Halloween has a European and Christian origin. A day called All Hallow's Day on November 1st has been a national holiday in Italy since the 9th century. There have always been activities in Italy the evening prior to All Hallow's day. This includes games, themes of death, carved pumpkins (turnips before), and masks ("guising"), all of which are very old traditions done all over Europe.
Being Italian and having lived in Italy for the past decades, may not make me an authority over all things Italian, but I can assure you that Ognissanti (which is what you quote as being "All Hallow's Day," since we obviously haven't been borrowing English names for our festivities) has never been observed as a national holiday with anything remotely close to what is shown here, which is, like Ariadne Oliver says, plain and simple Halloween. Like I said in the entry, it'd be perfectly fine for Americans to celebrate it, but no pumpkin banners in the public streets and parades with people shouting "Happy Halloween!" with nuns in full garb, no less! Halloween has never been celebrated here with any mainstream fanfare until the very, very recent years.
Continuity mistake: Wick takes the lapel pin off and places it in his right front jacket pocket. Shortly afterwards he takes it out of his front left jacket pocket. (00:49:10 - 00:56:20)
Character mistake: When McCall put his gun to his head and pulled the trigger, he's saddened that it didn't go off. Most handguns (including the one he was using), after the last round is fired, the slide locks to the rear. He would not have made the mistake of thinking it was loaded.
Continuity mistake: Byron walks towards the maid and surrounds her waist with his arm. From the angle behind, he is repeating all previous movements.
Continuity mistake: When the first police officer looks at the body, Maggie's long hair is down her back, but is not when an officer takes a photo. (00:01:38 - 00:02:33)
Continuity mistake: While Lucky does push-ups on the floor, the positions of the pack of cigarettes and lighter on the coffee table change. In the side view, the cigarette pack goes from partially behind the glass to a couple of inches to the right, and the lighter goes from mostly to the right of the cigarettes to behind the pack. (00:42:08)
Revealing mistake: After Kristin tramples on the muffin, it should crumble/flatten, but it starts to inflate back into shape. (01:17:45)
Continuity mistake: They say Eden's former partner died in a diving accident in Barbados. Later in the movie, they are looking at statements from witnesses to the drowning from Bermuda.
Continuity mistake: The scenery view shots near the beginning of the movie apparently are not meant to represent the passage of time (such as "one year later"), so the various changes in foliage - from green leaves (Summer) to leafless trees (Winter) to orange leaves (Autumn) to green leaves (Summer) and back to Fall foliage - do not make sense. This inconsistent foliage continues into the movie. (00:05:30 - 00:06:34)
Plot hole: When James leaves the resort after finding his passport, he packs his bags and boards a bus headed for the airport. The bus is then intercepted by Gabi and the gang, and James gets off, leaving his belongings on the bus. The following morning, when he proceeds to leave the resort a second time, he is shown packing his bags in his hotel room, despite having left them on the bus.
Suggested correction: We only see him get off the bus, the next shot is him walking with the cars behind him. They could have got his stuff from the bus in between shots.
Continuity mistake: When Nick and Audrey arrive on the island, you see Nick holding Audrey's bag. But later on in their room, the photo shows Audrey holding the bag. (00:06:50 - 00:11:36)
Continuity mistake: In the hospital, Sebastian's head is centered on the pillow. When the camera shifts, the top of the pillow to the right of his face is angled too low towards the upper-right of the screen. (00:02:26 - 00:02:51)
Factual error: Eleanor Falco is mentioned as having substance problems. No way would the FBI recruit someone with such a problem.
Deliberate mistake: Ryan was cooking venison (deer meat) in a pan over hot coals. He speared a piece with his knife and held it up for his young son, Billy, to take, which Billy did with his fingers. But did not get burned or show any signs that the meat was hot. Billy was even able to take a bite of the meat. The meat could not have cooled off that fast, even outdoors in jacket-wearing weather. (00:07:04)
Factual error: It's not physically possible to remotely cause an elevator to fall to the bottom of the shaft. Even if the Heart somehow gained full control over the motor, there are emergency brakes on the elevator itself that engage if the elevator begins to reach an unsafe speed.
Continuity mistake: The dad is given a face tattoo on his right cheek just before going down the alley to be attacked. He is driving in his car, and the tattoo is gone.
Continuity mistake: At the bar, the amount of beer in Maria's "Corona Extra" bottle varies from at the neckline, to below, to above. After she takes a swig, it is still at the neckline; then it is about 1/2" above the neckline. (00:25:50 - 00:26:44)