Revealing mistake: To save money during the beach picnic scene, Welles used stock footage from King Kong (1933) for the jungle background. You can quite clearly see pterodactyls flying in the background (confirmed on DVD commentary).
Citizen Kane (1941)
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Directed by: Orson Welles
Starring: Joseph Cotten, Orson Welles, Agnes Moorehead, Dorothy Comingore
Character mistake: During the dancing girl scene, as they are singing the praises of Kane, the girl to the immediate right sings out of time with the rest of the dancers during the line "His favorite son." Even though she forgets to sing the line, she can be heard singing it a bit too late. (00:41:00)
Charles Foster Kane: You know, Mr. Bernstein, if I hadn't been very rich, I might have been a really great man.
Trivia: The shot of the loud, squawking bird was purposely placed by Orson Welles to make sure the audience was still awake.