While preparing their 1946 film A Night in Casablanca, the Marx Brothers received a legalgram from Warner Bros. noting that the forthcoming picture's title resembled Casablanca, which had been released approximately four years before. The lawyers demanded that the name be changed. Groucho Marx replied with a letter of his own:
Apparently there is more than one way of conquering a city and holding it as your own. For example, up to the time that we contemplated making this picture, I had no idea that the city of Casablanca belonged exclusively to Warner Brothers. However, it was only a few days after our announcement appeared that we received your long, ominous legal document warning us not to use the name Casablanca.
It seems that in 1471, Ferdinand Balboa Warner, your great-great-grandfather, while looking for a shortcut to the city of Burbank, had stumbled on the shores of Africa and, raising his alpenstock (which he later turned in for a hundred shares of common), named it Casablanca.
I just don't understand your attitude. Even if you plan on releasing your picture, I am sure that the average movie fan could learn in time to distinguish between Ingrid Bergman and Harpo. I don't know whether I could, but I certainly would like to try.
The entire letter is here, along with a description of the followup correspondence, in which Warner Bros. requested an outline of the film's story and Groucho replied that he would be playing "Bordello, the sweetheart of Humphrey Bogart."
[Hat tip: Bryan Alexander.]
Update: Snopes says that while Warners did contact the comedians with concerns about the movie, it didn't actually threaten to sue them, and that the Marxes spread the story as a publicity stunt. Meanwhile, Wes Ghering's The Marx Brothers: A Bio-Bibliography claims that Warners eventually did file a complaint, though "it was ironed out quickly in arbitration."