Audio problem: When Comicus is announced in Caesar's court, the trumpeters play "Hooray for Hollywood". Notice that the music starts before they get their instruments to their mouths.
History of the World: Part I (1981)
1 audio problem
Directed by: Mel Brooks
Starring: Dom DeLuise, Mel Brooks, Madeline Kahn, Gregory Hines, Harvey Korman
Revealing mistake: When Comicus et al are running from the Roman soldiers, Miriam pulls Comicus through a secret marble door. When it closes, you can see the glue residue revealing that it's Formica.
Count De Monet: Your Majesty, you look like the piss boy.
King Louis XVI: And you look like a bucket of shit.
Trivia: The Viking funeral gag was a Jewish in-joke, based on the libelous old myth that Jews had horns. (Making light of anti-Semitism has long been one of Mel Brooks' trademarks.)