Continuity: In the gala scene, Zeppo explains that Groucho is always on time and will arrive at exactly ten o'clock. Everyone starts singing the first big musical number, "When the Clock on the Wall Strikes Ten." There is even a shot of a clock set to ten. In the middle of the song, the clock strikes... six.
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Mistakes
At the gala scene at the beginning Groucho slides down wearing a black tuxedo. Not long afterwards he is suddenly wearing a gray suit jacket, and after his musical number it changes back to the black tuxedo. See more...
Duck Soup mistakes
Continuity: At the gala scene at the beginning Groucho slides down wearing a black tuxedo. Not long afterwards he is suddenly wearing a gray suit jacket, and after his musical number it changes back to the black tuxedo.
Continuity: During the "Pop Goes the Weasel" dance number, Groucho's flute turns into a banner with a brass pole, which then disappears and is replaced by his trademark cigar. When he dances to a spot between the two uniformed "officers" for the finale, the cigar also disappears.