(at around 1:10:00) When Bobby can't start his car after the fight with Sandra, the car has only one headlight on. When he takes a golf club right after that and starts smashing the car, both sets of headlights are on, including the brights.
(around 32 minutes in) When Bobby starts singing "Beyond the Sea" to Sandra, the ground is wet. In the next shot it's dry again.
When Bobby is singing his first song at the Copa, his hair changes throughout the song. First it's all combed back then there is a strand that is on his forehead, then it keeps flipping back and forth.
When Bobby starts whaling on the driver's window of his car, the head of the golf club come completely off with the third stroke; however when he moves around to the front and begins hitting the hood, the iron is whole again.
Sandra Dee did not stay with Bobby Darin in the hospital as he was dying. She was in an alcohol induced denial at home. She was passed out on the floor from drinking and the family had to break into the house to find her and notify her. In fact, Bobby's second wife was banished from his room because she could not hold her tears.
The film omits several key people (including Bobby Darin's second wife), and some key events (including how he learned the truth about his background) did not take place as shown.
In reality Bobby Darin was with Robert F. Kennedy during the campaign when Kennedy was shot, and in fact in the same hotel where/when it happened. "Beyond the Sea" had him in his trailer at Big Sur when he gets the news on the radio.
In the first scene when Bobby sings 'Mack the Knife' he tosses in an ad-lib by throwing a karate kick and shouting 'ah-so, Madame Myook.' This is based on an actual ad-lib which can be heard on a 10 November 1963 recording of 'Mack the Knife' at the Flamingo Hotel in Las Vegas. However, the real Bobby Darin says 'Madame NHU' and not 'Madame Myook.' Madame Nhu was considered the first lady of South Vietnam from 1955-1963. And, she was well known to the American public in the early 1960s because of her notorious political comments, staunch conservatism, and high sense of fashion.
Robert F. Kennedy is depicted as having a Secret Service detail. Presidential hopefuls were not entitled to this. That was changed after his murder.
During the 1963 touring montage, leading up to his Oscar nomination, Bobby sings "Fabulous Places," a song was written for Doctor Dolittle (1967) four years later.
(at around 1:27:00) When Bobby is in hospital the first time, the heart monitor is a modern type with a flat panel addressable display. Monitors of this era used monochrome oscilloscope type CRT displays.
The song "Once Upon a Time" was published in 1962. In the movie, you can hear Bobby sing it on his wedding night to Sandra Dee in 1960.
The Revox B77 reel to reel recorder wasn't available until 1978.
(at around 33 mins) Camera shadow appears briefly twice during "Beyond The Sea" song and dance; on the back of a waiter as Bobby approaches Sandra at a table, and later on Sandra herself.
(at around 5 mins) At the beginning, a reporter wanted to ask Bobby a few questions for portraying himself in a biography film, by saying "No one hasn't done anything like this before.", yet celebrities like Jackie Robinson (The Jackie Robinson Story (1950)), Audie Murphy (To Hell and Back (1955)) had done it before.