For the most part it seems to me that people behave as if literary and film genres possess firm boundaries. A mystery is a mystery, a Western is a Western, and never the twain shall meet. In truth this is often not the case, as most literary and cinematic works tend to cross genre boundaries from time to time. The Ox-Bow Incident isn't just a Western, it is also a psychological drama as well. Pscyho isn't just a horror movie, but also a mystery. There are very few books or films that can truly be said to belong firmly to only one genre. That having been said, there are those times when a work combines two or more genres that are so distinct that it becomes something utterly new. I refer to such works as being genre mélangé or, in plain English, "mixed genre."
In my opinion the perfect example of genre mélangé is the old TV series The Wild Wild West. The Wild Wild West was pitched to CBS as James Bond in the Old West. It was then automatically planned to be a combination of the espionage and Western genres. As the series got underway, however, it emerged with elements of science fiction of the Jules Verne variety and even outright fantasy. The Wild Wild West was then not simply a Western, but also a spy drama and a sci-fi series. Common sense would seem to dicate that such a duke's mixture would not work. Not only would such a series be poor in quality, but it would also probably not see much success. Yet The Wild Wild West is regarded by many as a classic TV series. It ran four years on CBS before going onto one of the most successful syndication runs of all time. In fact, it was so successful that Fox debuted another sci-fi Western in the Nineties. While The Adventures of Brisco County Jr. did not have a long run, it remains a cult series to this day.
Another example of genre mélangé is the movie The Crimson Pirate. Made in 1952 at the height of Hollywood's pirate craze, The Crimson Pirate was ostensively a pirate movie. Burt Lancaster played the title character, Captain Vallo, who decided to sell weapons stolen from a Spanish galleon to a rebel leader. Along the way he encounters Professor Prudence, an inventor whose inventions are decidedly advanced for the 18th century! The Crimson Pirate is then not simply a pirate movie, but a sci-fi movie as well. As it is also played tongue in cheek and has a great deal of humour, it could also be considered a comedy. The Crimson Pirate is then simultaneously a pirate movie, a sci-fi movie, and a comedy.
Another cinematic example of genre mélangé is Kronos, known in the United States as Captain Kronos: Vampire Hunter. This Hammer classic follows the adventures of Kronos as he battles a new sort of vampire. The movie is obviously a horror film, yet as Kronos wields a sword to great effect, it can also be counted as a swashbuckler. Director/screenwriter Brian Clemens also incorporated elements of John Ford Westerns. Kronos is then a combination of horror, swashbuckler, and Western.
Of course, at times when enough works combine enough elements of enough genres, a whole new genre can be born. This was the case with cyberpunk. Cyberpunk combined film noir elements with elements of science fiction with a focus on computers and other information technologies. What originally began as genre mélangé became a subgenre of science fiction in its own right. Oddly enough, cyberpunk would spawn yet another subgenre--steampunk. Steampunk is a subgenre of science fiction using a Victorian or Victorian style setting, but with decidedly advanced technology (usually powered by steam, hence the name). The early steampunk works used the noir elements of cyberpunk and the same anti-authoritarian attitudes, although they eventually grew to resemble the scientific romances of Jules Verne and H. G. Wells more than cyberpunk. Arguably, both The Wild Wild West and Adventures of Brisco County Jr. could be considered precurssors to steampunk.
Anyhow, genre mélangé as probably existed as long as the whole convetion of genres has. Indeed, the past few years have seen Van Helsing, Hellboy, and others. I rather suspect as time goes by there will be more works of genre mélangé.