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: Though admitting the artistic license of Dom's car freefalling and safely landing on the damms' spillway, after it gets engulfed in flames both inside and outside, the fire doesn't burn or even scorch any of the tyres, upholstery, nor a single inch of Bryan or Dom's skin or clothes.
Factual error: Eleanor Falco is mentioned as having substance problems. No way would the FBI recruit someone with such a problem.
Factual error: Kristin meets Lorenzo at the airport. He asks for her number, which she gives him. Later when he calls her, her Mafia family take the phone and smash it to pieces while she's talking. They give her a new burner phone. They say they did a background check on Lorenzo. How? The family never met him at the airport, the only time she saw him. Further, when they meet for dinner (how she found him is another mystery), she shows him a picture of her son on the phone. How did a photo get on her new phone?
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.
Factual error: Mr. Monk is crossing off days on a calendar until Trudy's passing anniversary. Three days before, he says "only two days left." Mr. Monk is too much of a perfectionist to say that. How did this happen?