The "mudhole" in which the famous brawl took place wasn't actually made of mud. It was made of a material called bentonite, which is used in the drilling of oil wells and has the consistency of chocolate syrup. According to actor Leo Gordon (the first one to be knocked down it), that scene took a week to shoot.
When Batjac, John Wayne's production company, needed completion funds for The Alamo (1960), it borrowed the money from United Artists. The Batjac film library was used as collateral for the loan. Making "McClintock" profitably for U-A allowed Batjac to reclaim control of the films.
John Wayne insisted that the role of the weak, insipid Governor be called "Cuthbert H. Humphrey", with the intention that he be seen as a parody of liberal Sen. Hubert H. Humphrey, whom Wayne intensely disliked.
The failure of the original copyright holder to renew the film's copyright resulted in it falling into public domain, meaning that virtually anyone could duplicate and sell a VHS/DVD copy of the film. Therefore, many of the versions of this film available on the market are either severely (and usually badly) edited and/or of extremely poor quality, having been duped from second- or third-generation (or more) copies of the film.
In the scene where the Comanches are being outfitted with rifles it's easy to see that they're Krag Jorgensen carbines, meaning that this film takes place in at least 1896, as the Krag didn't come into service as military armory until 1894.
Although Stefanie Powers claims that John Ford came to the set to direct the movie for a week, Andrew V. McLaglen the director says that it never happened. He says he was there for the entire shoot of the movie.
I made a mistake with an earlier trivia correction. The instrument being played in the parade for Rebecca was a HELICON, NOT a Sousaphone. The difference being that the Sousaphone's bell points up, the Helicon points ahead, which Sousa didn't like. It made the sound of the "tuba" precede and overpower the other sounds of the band. The Helicon was introduced in the 1860s.