The trademark dispute between the Brothers Warner and Marx has become an iconic example of the little guy winning over the over-reaching corporation and doing so with wit. Here is a link to the first of Groucho's letters (and which also includes links to other sites about the Marx Brothers and other comedy teams). The tiff involved the Marx Brothers' "Night in Casablanca" and Warners' "Casablanca."
The Marx Brothers were sued a number of times for copyright infringement, once under a criminal count. That case, Marx Brothers v. United States, 96 F.2d 204 (9th Cir. 1938), involved infringing and aiding and abetting the infringement of a radio script by Garrett and Carol Graham ("The Hollywood Adventures of Mr. Dibble and Mr. Dabble"). The Marx Brothers had met with the Grahams, and expressed an interest in the script; changes were made at the Marx Brothers' request. The Marx Brothers testified that they subsequently forgot about the work and that when the copyright owner's "gag man" (one Boasberg) gave them the script to use on the radio, they assumed he was the author. A jury convicted them. One unique feature of the case is that it highlighted a flaw in the 1909 Act: the failure to specify the term of protection for unpublished registered with the Copyright Office under Section 12. The Ninth Circuit read into the act the term for published works (28+28), measured from the date of registration.
In the same year, in a civil case, Clancy v. MGM, 37 USPQ 406 (S.D.N.Y. 1938), plaintiff's play "Nuts to You" was not found substantially similar to the boys' "A Day at the Races," even though both works had in common a veterinarian who runs a sanitarium and owns a race horse: no dialogue or other plot elements were taken, however.
A Day at the Races was, however, found by a jury to have infringed a scenario called "High Fever," a judgment upheld the following year in a California state court, Barsha v. MGM, 32 Cal. App.2d 566. The appellate opinion contains a lengthy list of alleged similarities, which were found sufficient to uphold the jury's finding.