Author Existence Failure: The show's creator and producer/executive producer, Michael Garrison, died in an accident at his home during filming of the second season episode "The Night of the Ready-Made Corpse." Bruce Lansbury (younger brother of Angela), who was already on board as producer with Garrison above him, took over the reins from then on (although the series remained "A Michael Garrison Productionnote in association with the CBS Television Network" to the end).
Blooper: Robert Conrad fell from a chandelier and was nearly killed while filming "The Night of the Fugitives;" stuntman Red West was rather badly injured after going headfirst into a not-so-breakaway piano during "The Night of the Running Death;" Ross Martin stumbled over a prop gun and broke his leg in a fight sequence from "The Night of the Avaricious Actuary." At least some of the film from the first two accidents was retained in the final cut of their respective episodes.
The Character Died with Him: When Ross Martin died there were plans in the works for a third reunion movie or even, according to some sources, a revival of the series. Network executives proposed continuing without Martin by either writing out or killing off Artemus Gordon, but Robert Conrad refused to do the show without his old co-star. In a sense, the series itself died with him.
CBS did not want the show's creator Michael Garrison to be overseeing the show because of how much the pilot had cost, which led to Garrison having a legal battle with the Eye throughout season one while the show went through seven producers - some of whom never even got to do one episode - before Garrison got control back. Eventually, Garrison did get in a producer to his liking (besides Fred Freiberger, under whose reign Loveless was created - in fact, the first episode to be shown after the pilot was a Freiberger-produced one) in the form of Bruce Lansbury... but CBS still got a Garrison-less show in the end, though not in the manner anyone would have preferred.
Ross Martin was very much playing a Sidekick in the first season - he had relatively little screentime, rarely got the girl and his primary role in the final confrontation with the villain-of-the-week was either to watch or lie on the sidelines unconscious. Martin was reportedly unhappy with this as he'd been promised something a bit more substantial when he accepted the role after turning it down multiple times. He was quoted as saying, "Each new producer tried to put his stamp on the show and I had a terrible struggle. I fought them line by line in every script. They knew they couldn't change the James West role very much, but it was open season on Artemus Gordon because they had never seen anything like him before." Once the producer situation stabilized, so did Artie's characterization and Martin gradually moved up to true co-lead status with Robert Conrad.
Defictionalization: The creators of the series were apparently probed by the CIA, both because some of the gadgets in the show struck so close to actual gadgets used in espionage and because the boys at Langley really liked some of the other gadgets that weren't being used in Real Life at the time.
The Everyman: This was originally intended to be Artemus Gordon's role in the show; it was felt that a James Bond-type character in a continuing TV series wouldn't be relatable enough without some kind of audience surrogate. The surrogate could also be used to provide West with his supply of gadgets, killing two birds with one stone. However, as the series developed, so did Artie's character, and this particular trait went by the wayside.
The show was canceled due to content, not because of declining ratings. (At the end of The Sixties, CBS executives got nervous about fictional violence after all the Real Life violence of that decade.)
The show ended up being lucky to survive its first season. After the show was picked up by the network, and placed on the fall schedule, one of the biggest television corporate bloodlettings happened, taking out nearly all of the network executives who helped develop the show. The new regime, wanting to put the past regime behind them, changed the new schedule as best as they could in the short time available to them, dropping a number of shows that were developed for the new season. The Wild Wild West survived this purge, but barely, as the new executives didn't get the show, and were concerned about the show's cost, which was expensive for a show of that era. Had the show not become a hit from the get-go, it's likely it would have been canned faster than you can say Artemus Gordon.
Fake Nationality: All over the place, particularly Pilar Seurat as a very un-Chinese-looking Chinese princess in "The Night The Dragon Screamed," Paul Wallace doing an English accent that isn't even good enough to be called excruciating in "The Night of the Eccentrics" and Ricardo Montalban as a Confederate Army colonel in "The Night of the Lord of Limbo." (And this being Ricardo Montalban, he doesn't even bother with an accent.)
A large number of recogizable faces turn up in guest roles during the course of the series, often as the Villain of the Week or the Girl of the Week. Among those that appear are Michael Dunn, Richard Kiel, Victor Buono, Suzanne Pleshette, Robert Loggia, Jeff Corey, Leslie Nielsen, Katherine Ross, Elisha Cook Jr., Martin Landau, Burgess Meredith, Yvonne Craig, Richard Jaeckel... and that's just the first season.
Messrs. Loggia, Corey and Jaeckel turn up in later seasons as well.
Also, Jackie Coogan (The Addams Family's Uncle Fester) is a cowardly sheriff and a hypnotised Mayor ("The Night of the Cut-Throats" and "The Night of the Winged Terror, Part 1" respectively).
Yes, that's a young Richard Pryor as one of Count Manzeppi's henchmen in "The Night of the Eccentrics."
An odd case in "The Night of the Avaricious Actuary." Ross Martin was injured while filming a fight sequence and a substitute was therefore used to play Artie in the immediately preceding dialogue scene. The new actor's face and voice are very prominently displayed in said scene (in fact, he's the image for the act-ending freeze-frame/dissolve to drawing), making it obvious that he is not Ross Martin. Understandable confusion results for some in the viewing audience.
A more straightforward example is President Grant. In the pilot episode, he is played by James Gregory; in every future appearance he is portrayed by Roy Engel.
Playing Against Type: Don Rickles as evil magician Asmodeus in "The Night of the Druid's Blood." No blustering or brash hurling of insults here.
Real-Life Relative: Robert Conrad's father appears in a non-speaking role as a guard in "The Night of the Murderous Spring."
Recycled Script: Both "The Night of the Skulls" and "The Night of the Cadre" feature a villain who helps notorious murderers escape from prison so he can train them as a crack team of assassins to kill President Grant. In both episodes, West cracks the case by posing as a killer and getting recruited for the team.
Sensitive Guy and Manly Man: According to Robert Conrad, he and Ross Martin had this dynamic in real life and it greatly contributed to their on-screen chemistry.
Spell My Name with an S: Artemus Gordon's nickname is spelled "Arte" in various places, including the material accompanying the official DVDs. However, in the episode "Night of the Juggernaut" the character himself spells it "Artie."
Throw It In: Ross Martin had a tendency to ad lib. If it was funny, it was sometimes kept in the final version.
"The Wild West". The second "Wild" was added by second producer Collier Young.
"The Night of the Big Blackmail" was originally called "The Night of the Deadly Blades." Since the episode does indeed feature deadly blades but does not involve any blackmail, the change seems an odd one to say the least.