Revealing mistake: In the scene where Cary Grant punches Jimmy Stewart: Jimmy starts falling back (and you hear the sound effect of the punch) when Cary's fist is a good 1-2 feet from Jimmy's face.
The Philadelphia Story (1940)
1 revealing mistake - chronological order
Directed by: George Cukor
Starring: James Stewart, Cary Grant, Katharine Hepburn, Ruth Hussey
Continuity mistake: Uncle Willy pours a glass just over half way full of vodka. With each shot the amount of vodka in the glass changes.
Elizabeth Imbrie: What's this room? I've forgotten my compass.
Macaulay Connor: I'd say, south-by-southwest parlor-by-living-room.
Trivia: Katharine Hepburn played this role on broadway, it was written for her. She had Howard Hughes buy her the movie rights so only she could play it at the movies. She didn't want any other actress playing Tracey Lord.