• Acting for Two:
    • 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 1".
    • 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.
  • Fake Nationality:
    • Wo Fat, played by Anglo-Egyptian-Sudanese-American Khigh Dheigh. Yes, really.
    • In "Samurai", Mexican-born actor Ricardo Montalban played a Japanese mobster.
    • Spock's dad played a Japanese saboteur in "To Hell With Babe Ruth." Oh dear.
  • Forgotten Theme Tune Lyrics: The show's distinctive theme music is entitled "You Can Count On Me", and Sammy Davis Jr. at one point actually recorded it:

If you get in trouble, bring it home to me
Whether I am near you, or across the sea.
I will think of something to do.
I'll be on the lookout for you.
And I'll find you -- you can count on me.

  • Missing Episode: "Bored She Hung Herself", a 1970 episode about a deadly yoga technique that bears more than a passing resemblance to Erotic Asphyxiation, has been banned since its original broadcast, allegedly because a viewer accidentally died while imitating the technique. The episode was never syndicated, and it's not included on the second season DVD box set.
  • The Other Darrin: 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.
  • Playing Against Type: Andy Griffith is a con artist in "I'm a Family Crook -- Don't Shoot!"
  • Trope Namer: For Book'Em Danno.