A Hollywood legend claims that Warner Brothers, which produced Casablanca (1942), threatened to sue The Marx Brothers for using the word "Casablanca" in the title. Groucho Marx wrote a letter to Warner Brothers in which he threatened to sue them for using the word "Brothers": "Professionally, we were brothers before they ever were." However, film critic Richard Roeper claims (correctly) that the story is fake. In fact, Warner Brothers never threatened to sue, but merely inquired about the story of the Marx Brothers' film, to make sure there was no copyright infringement. Groucho used the inquiry as an excuse for a publicity stunt. He wrote a series of comic letters to Warner Brothers (in which he told the studio, "Professionally, we were brothers before you ever were.") The letters were published in "The Saturday Evening Post" to publicize the movie.
According to The Marx Brothers biographer Joe Adamson, Harpo Marx was offered $50,000 to utter the single word "Murder!" in this film, presumably to add publicity value to the film by having him speak for the only time on-screen. Harpo declined the offer and never spoke publicly until a concert one year before his death.
Hoping to take charge of their film careers, The Marx Brothers financed this movie themselves, under the heading of Loma Vista Films. They even did a brief pre-filming tour of scenes from the movie, as they had done with A Night at the Opera (1935) and A Day at the Races (1937), hoping to sharpen the script's comedy.
Near the beginning of the film, the prefect of police gets on his microphone and says, "Round up all likely suspects." If you look closely at his lips, he really said, "Round up the usual suspects" - a famous line from Casablanca (1942) - but the replacement line was dubbed in later to avoid legal problems with Warner Brothers.
Rick never says, "Play it again Sam," in Casablanca (1942) and, contrary to a popular rumor, the line does not appear in this film, either. The quote was often ascribed to the dialogue from the Humphrey Bogart film.
Because The Marx Brothers (as The Marx Bros.) is credited first before the title, but is not in the more comprehensive end credits, it is listed first in the IMDb cast list, as required by IMDb policy on cast ordering.