In the first flashback scene featuring the meeting between the writers, Josef Von Sternberg, and David O. Selznick in 1930, the characters mention Universal Studios as the "horror studio" and mention titles such as Frankenstein and The Wolf Man. Frankenstein would not be filmed and released until the following year while The Wolf Man would not be made until 1941; 11 years after the scene takes place.
Marion Davies had a slight stammer when not in front of the cameras. That's not evident in this film.
Mank excuses himself for vomiting in the dining room by saying, "It's all right. The wine came up with the fish." In fact, he said, "The WHITE wine came up with the fish." This is a joke on the rule that one should always drink white wine when eating fish. In reality, this event occurred at a very formal dinner party given in Hollywood by the producer Arthur Hornblow Jr., not at a Hearst party at San Simeon.
As L.B. Mayer leaves Thalberg's funeral, you can see he is wearing a monogrammed scarf, NOT a tallit. Tallitim (correct Hebrew plural) are not worn at funerals.
The Arroyo Seco Parkway did not open until Dec. 30, 1940, and it was not called the Pasadena Freeway until many years later.
Marion uses the word "bonkers" in 1933. It wasn't coined until the late 1940s.
Lily Collins character cites the temperature at the Battle of Hastings in Centigrade. Everyone at the time would have quoted it in Fahrenheit.
While the movie is meant to look and sound like it was produced in the period, in black and white and with a mono soundtrack, it's shot in an aspect ratio (2.20:1) that did not exist until TODD-AO 70 MM in 1953.
Mank says that George Bernard Shaw said of San Simeon "It's what God would done if he'd had the money." This was a remark that George S. Kaufman made of a friend's estate in Bucks County, Pennsylvania.