The film has almost too many factual errors to be mentioned and shouldn't be considered a "true" portrait of Monroe's life.
The film's title misspells Norma Jean(e)'s name by omitting the "e" at the end of her name. This might have to do with the fact that her name was incorrectly misspelled on her birth certificate which omitted the "e". Nonetheless all official documents throughout Marilyn's life have her listed as either Norma Jeane Baker, Dougherty, or Mortenson (her mother's second husband's last name). Ironically, however, during the film, when Norma Jeane slips Ted Lewis a napkin with her name name and phone number on it, we see that she has correctly spelled her name with the "e" at the end of it.
The film portrays Norma Jeane/Marilyn getting plastic surgery as a way of landing a contract with 20 Century Fox in 1947. In reality Monroe's plastic surgery occurred in 1950 consisting of a chin implant and a minor nose job to correct her somewhat bulbous nose. Monroe had made several films up to that point thus the explanation for her slightly different appearance in films before 1950.
The film portrays 20th Century-Fox executive Ben Lyon giving Norma Jeane her knew name Marilyn Monroe. In reality arriving at the name happened only after two other choices were rejected, and from a collaboration between the two of them. Lyon first suggested Carole Lind as her stage name, although she readily disliked it. Following her idol Jean Harlow, who choose her mother's maiden name for her stage surname, Norma Jeane suggested Monroe her mother Glady's maiden name. Originally Jean Monroe was chosen for her stage name but it was Lyon who then suggested she consider Marilyn. Lyon chose that name for Norma Jeane reminded him of famed 1920's Broadway star Marilyn Miller. At first Norma Jeane rejected Marilyn but Lyon told her that not only was the name alliterative but that the double MM's in her name would be good luck and thus she agreed to Marilyn Monroe.
Norma Jean and Marilyn wear earrings for pierced earlobes. The real Marilyn Monroe did not have pierced earlobes, and wore clip earrings.
Norma Jeane gets a nose job due to studio executives believing her nose is like a "bake potato." However, it is clear that Ashley Judd who plays Norma Jeane has a smaller nose then Mira Sorvino who plays Marilyn.
Norma Jean shoots the pictures for the Brown & Bigelow 1947 calendar. However, in a previous scene, she said she was 21 years old at the time, which means the scene is set after June 1, 1947. They wouldn't produce the 1947 calendar in the middle of the year.
We see a man smoking some cigarettes and there is a Surgeon General's tobacco packaging warning on the box. These warnings did not appear until 1964 and did not become common until the early 1970s.
The underwear underneath Marilyn Monroe's dress while filming The Seven Year Itch (1955) are Victoria's Secret panties. The brand Victoria's Secret was not established until 1977 and has only gained prominence since the early 1990s. In real life, Marilyn wore Hanro cotton briefs.
Eddie and Norma Jean smoke filtered cigarettes before filtered cigarettes were introduced.
When "Marilyn" backs up and runs over "Norma Jean", the car has a 1952 license plate. Theoretically this should have happened years earlier.
As Strasberg confronts an actress during an Actor's Studio session, you can see the shadow of a crew member's head to their left for a few seconds until he/she realizes that he/she is in the shot.