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.
Factual error: Lenny, using a manual typewriter in 1889, would not be able to type that fast or the striking keys be able to produce a sound similar to that of an electronic typewriter (not available until decades later). A manual typewriter's single key had to have time to retract (at least partially) before the next key could strike to prevent the keys from overlapping and getting stuck together. Equally paced, rapid striking sounds were not possible for an extended period of time. (00:47:07)
Factual error: It is Halloween night, 2023, but there is summer tree foliage (and even flowering shrubs some time later). There should be peak fall foliage in Vernon, British Columbia, Canada by mid-to-late October. (00:05:40 - 00:14:53)
Factual error: Stoney sets his lit marijuana cigarette on the metal garbage can lid and walks away. The cigarette is still lit and does not burn down after 3+ minutes of screen time. A lit marijuana cigarette left unattended burns a short time and forms ashes before extinguishing itself. (00:55:35 - 00:58:50)
Factual error: Lieutenant Commander Sharp addresses the crew of the USS King. At the end, he shouts "Semper Fi." Marines say "Semper Fi," not Navy personnel. "Hooyah!" would have been more appropriate. By the way, I don't think bright metal collar insignias are worn with the blue camo uniform. They use subdued rank insignia. But I am not sure.
Factual error: The way Michael poured wine out of the newly opened bottle (standard 750 ml/25 oz?), the estimated amount he poured into six glasses plus drank some from the bottle, and the angles at which the six friends tilted their glasses for the toast, were not realistic. Also, the wine bottle appeared to have more than one-half (possibly two-thirds) of its contents left when Michael poured it into the fireplace. A newly opened bottle of wine would not be held at a downward angle to start pouring. (00:26:18 - 00:27:34)