Factual error: In the scene where Clark is working on the lights the full moon is shown. This is December 14th according to the advent calendar. On December 24th when Santa's sleigh goes sailing over the moon, it is still full.
National Lampoon's Christmas Vacation (1989)
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Directed by: Jeremiah S. Chechik
Starring: Chevy Chase, Juliette Lewis, Diane Ladd, Beverly D'Angelo, John Randolph, Johnny Galecki
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Trivia: When Clark smashes up the decorations when he can't get the lights working, Chevy Chase genuinely broke a finger when he punched Santa, but stayed in character, kicking the decorations instead, and that's the take used in the film.
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Chosen answer: Because there was cat food in her jello mould. He's pointing out the absurdity of someone adding cat food to jello.