Spencer Tracy, playing a priest, makes a note to himself in one scene, "That Rooney kid skipped Mass again..." Two years later, he again plays a priest in Boys Town and is charged with reforming a boy played by Mickey Rooney.
Clark Gable and Jeanette MacDonald did not get along at all during filming, and avoided each other completely off the set.
Clark Gable hated the final scene where he breaks down, and insisted he should only be filmed from behind while saying the "soppy" lines.
Jeanette MacDonald's older sister, Blossom Rock, signed with MGM and was given the name Marie Blake. Jeanette's character in San Francisco was named Mary Blake. Her sister used the name Blossom Rock when she played Grandma Addams on The Addams Family.
Al Shean (born Adolph Schoenberg), who plays the Professor in the film was once half of one of the most popular teams in vaudeville - Gallagher and Shean. He was also the younger brother of Minnie Marx, the matriarch of The Marx Brothers clan, and was instrumental in writing many of the first sketches that his madcap nephews first performed on the vaudeville circuit before their enormous success on Broadway and in Hollywood.
D.W. Griffith directed several scenes without ever being credited.
Erich von Stroheim, who had been unceremoniously fired from MGM many years earlier, contributed additional lines in the script without studio head Louis B. Mayer ever knowing.
Debut of Robert J. Wilke.
In the film, Blackie Norton (Clark Gable) runs for the office of Supervisor in the city of San Francisco, the same job Harvey Milk was to hold many decades later when he was assassinated.
One of Mary's opera gowns was later used for "Glinda" in The Wizard of Oz.
The comment that Spencer Tracy makes about the "Rooney kid" is an ad-lib (watch Jeanette MacDonald's expression reacting to it). Tracy had worked with Mickey Rooney earlier that year in Riffraff and knew that director W.S. Van Dyke abhorred retakes, priding himself on bringing in productions fast and under budget - hence his nickname, "One-Take Woody".
The dress Jeanette MacDonald wears while singing "Would You" was re-worn by Judy Garland in For Me and My Gal (1942).