Factual error: During the drag racing part of the film, they show an orange Plymouth lining up to race a red 57 Chevy. When the back of the Plymouth comes into view you can see a Direct Connection license plate with a Mopar logo on it. Chrysler's Direct connection line didn't exist in 1964, and the red white and blue Mopar logo wasn't around until 1972.
Factual error: When John Belushi, Wendy-Jo Sperber and Treat Williams are on the stolen motorcycle, Belushi gets mad, pulls a pin, and disconnects the sidecar from the motorcycle. This is not how sidecars are actually attached or they wouldn't go the same direction as the bike.
Factual error: According to the novel on which the movie was based, the "40" in the title refers to the number of players on a football team. But by the time the movie was made, football teams had 45 players. The coach even says Nick Nolte was just one of 45 players. So the title of the movie really makes no sense.
Factual error: The movie takes place in 1850-51, and yet when the Rabbi is challenged to a shootout in San Francisco, the weapon he is handed is an 1858 Remington New Model Army revolver.
Factual error: When Dave is "true"ing his wheel, he adjusts one spoke with 3 full turns. That wheel would not be rideable after that much adjustment on one spoke.
Factual error: There were no gourds in Ancient Judea, of that type. The 'gourd' mentioned in the Bible refers to a colocynth plant, which yields only a small fruit, 5 - 10cm diameter. The gourd in the film is from a pumpkin, and not from a watermelon, as others have suggested. Pumpkins originated in the Americas, and there weren't any in the rest of the world before Columbus.
Suggested correction: This is a movie where Brian falls inside a flying UFO with alien pilots. I seriously doubt that the filmmakers had any intention whatsoever to be historically accurate.
Regardless of the random UFO scene, this is still a factual mistake. It doesn't matter if a film maker sets out to intentionally make a historically accurate film or not. There's nothing to suggest this film was set in an alternate past or that it was a sight gag as if it was a Mel Brook's film.
Factual error: The TV remotes of the day wouldn't control more than one TV at a time, yet they all come on and go off at the same rime.
Factual error: Marie plays a horn solo on the beach. At times continuous sound is coming out of the horn while she is inhaling through her mouth. Horns only produce sound when the player exhales.