Continuity mistake: At the reception, Miss Euphemia Bouc walks with Miss Van Schuyler and Bowers saying that money is the only friend a woman can rely on. Look at her champagne glass, a little more than half full. Her son jumps on the desk of the lobby and nods to her, and her glass filled up some. (00:22:00)
Other mistake: At the end of the origin story of Poirot's facial hair, we see the damage on his cheek is massive, extending well into the cheekbone. His girlfriend says he'll grow a moustache to cover it, and surprisingly, it works. At the end of the movie we see the scar without the moustache, and while smaller than the initial damage, it still extends into the cheek at the level of the nostrils in a way that the 'stache couldn't seamlessly cover it like in the rest of the movie and its prequel.
Factual error: The movie takes place in 1937: the new and improved Salome Otterbourne of this version has a repertoire of blues songs that are a few years posterior to that date, many years if we count the Rosetta Tharpe versions actually used in the movie. An argument can be made that they are all 'live performances' from a fictional character regardless of the vocalist who actually performed, and so the only song truly 'impossible' in 1937 would be "Shout sister shout", written in 1941, but we don't actually see Salome sing it. She does perform "Up Above My Head" and no recorded version of it exist before 1941, but it comes from a traditional gospel song.
Plot hole: This adaptation further exacerbates the problem of previous movie and TV versions; in the novel, Jackie would make the female designated witness talk over and over pretending also to get drunk after Linnet left to go to bed. In the Ustinov version it happened too, but the diversion didn't seem to last more than a couple minutes. Same in the Suchet version. Here, Simon even provokes Jackie practically the moment his wife exits the room. There's no time whatsoever for her to change and get in bed, let alone fall asleep.
Factual error: A .22 shot would be 150db, muffled with a pillow down to 120-130db. Only one person heard the second shot? No one heard the third? Plus at the end there's no way a .22 could pass through a male chest cavity, and halfway into a female chest cavity - it's too low power.