In "A Bullet for El Diablo", the title dictator's daughter is replaced by a double in an attempt to assassinate him. (It works.)
Steve McGarrett comes face to face with his double in "The Ninety-Second War, Part I".
And in "Welcome to Our Branch Office", where criminals have set up a phony Five-O office with simulacra of our heroes, three of the main four are impersonated by people with similar attributes — but the fake Danny Williams, like the real Danny Williams, is played by James MacArthur (in the end credits, "Fake Danny" is the only one of the four not listed).
Actor Allusion: The villain of "A Bullet for McGarrett" is an American communist mole who learned his Brainwashing skills from Wo Fat while he was a POW during the Korean War. Khigh Dhiegh, the actor who played Wo Fat, had earned his first fame as the master brainwasher Yen Lo, who turns American POWs into double agents during the Korean War in The Manchurian Candidate.
Banned Episode: The 1970 episode "Bored, She Hung Herself" was banned after a viewer supposedly died from imitating a deadly yoga technique that looked a lot like Autoerotic Asphyxiation. The episode was barred from ever airing again, not even in syndication, and it's not included on the second season DVD box set.
Directed by Cast Member: Jack Lord, of course; in addition to directing six episodes he was also more or less an uncredited executive producer (especially after creator and actual EP Leonard Freeman passed away in 1973).
Wo Fat, played by Anglo-Egyptian-Sudanese-American Khigh Dheigh. Yes, really.
Mexican actor Ricardo Montalban gets it twice: as the Japanese Tokura in "Samurai" and as European race car driver Alex Pareno in "Death Wish On Tantalus Mountain". At least they weren't in the same season (although seeing these episodes back-to-back in reruns can be jarring...).
Mark Lenard, alias Sarek, played a Japanese saboteur in "To Hell With Babe Ruth" with the intensely European Will Kuluva also cast as a Japanese. Oh dear. (Then again, Lenard's role was originally meant for Ricardo Montalban. As Karen Rhodes put it in her book on the series Booking Hawaii Five-O, "apparently a Japanese actor wasn't considered.")
Fictional Counterpart: Averted. There has never been a "Five-O"-like police agency in the history of Hawaii. Reportedly, phone books advised tourists to dial 911 instead of trying to find the number to Five-O.
Tim O'Kelly as Danno, Lew Ayres as The Governor and James Gregory as State Department official Jonathan Kaye (a recurring character on the series) in the pilot movie; Ayres later guested on the series (twice, in different roles), as did pilot cast members Andrew Duggan (four times!) and Leslie Nielsen. Kaye, meanwhile, was played by a total of six different actors.
Similarly, Albert Paulsen played villain Charley Bombay in season two's "Just Lucky, I Guess"; when the character returned in season eight's "McGarrett Is Missing" he was played by Charles Cioffi (making him The Danza). He also had his name slightly changed to Charlie.