!![[Franchise/TheHitchhikersGuideToTheGalaxy General franchise]] trivia:
* AccidentallyAccurate: Adams didn't write jokes in Base 13, but the fact remains that using THAT numerical base, 6X9 is indeed 42.
* AdaptationSequence: Radio Serial > Books > TV Series > Adventure Game > Movie
* BeamMeUpScotty: Say it with me, people. It's "The Answer to the Ultimate Question of Life, the Universe and Everything". Not the meaning of life. It doesn't help that ''Adams himself'' wrote a book entitled ''The Meaning of Liff'' (defining a number of words that didn't exist in the English language, but should have).
* LifeImitatesArt: Smartphones and tablets with access to Wikipedia mimic the capability and functions of the Guide with uncanny accuracy, right down to the Guide's questionable accuracy.
* RecycledScript:
** ''Literature/LifeTheUniverseAndEverything'' was based on an unused ''Series/DoctorWho'' script, ''Doctor Who and the Krikketmen''.[[note]]And you can tell: Trillian very much becomes an {{Expy}} of the Doctor, with the other characters behaving rather like the Doctor's Companions, although this is in keeping with Trillian's intelligence and generally kind-hearted nature.[[/note]] It might have been a second Hitchhiker TV series.
** The later radio series were essentially adaptations of Adams' last three Hitchhiker novels, retconning pretty much the entire events of the second series to being merely the delusional rantings of Zaphod Beeblebrox, instead of following directly from them.
* SelfAdaptation: Creator/DouglasAdams' level of involvement with each adaptation of ''Radio/TheHitchhikersGuideToTheGalaxy'' varies, but the novels and computer game are the ones he had the biggest (or, in the case of the novel, only) hand in, and he delighted in completely reworking the story each time he tackled it. The TV version also had his input, and his last draft of the film before he died was used as the final one with minimal editing.
* TechnologyMarchesOn: Digital watches sure are a neat idea, huh? (In the later radio series, and the movie, they were replaced by mobile phones.)
* TropeNamer:
** AuthorExistenceFailure, partially (as mentioned below, derived from "total existence failure")
** BitsOfMeKeepPassingOut
** DeconstructorFleet (pun on Vogon Constructor Fleet)
** FutureImperfect
** GargleBlaster
** InsignificantLittleBluePlanet
** LetsMeetTheMeat
** PuffOfLogic
** RealityIsOutToLunch
** SoLongAndThanksForAllTheGear
* UnfortunateName: Slartibartfast. Adams wanted him to have an immense sadness, so gave him a terrible name. He started with [=FartyFuckBalls=], and mutated it until it sounded rude without actually being rude.

!![[Literature/TheHitchhikersGuideToTheGalaxy Novels]] trivia:
* AuthorExistenceFailure: TropeNamer, almost; the third book mentions a "total existence failure". Later, of course, succumbed to the trope when Adams died while working on the sixth book; his last published collection of pieces, ''The Salmon of Doubt'', contains an early draft of a ''Literature/DirkGently'' novel that Adams was hoping to rework into a Hitchhiker book.
* CreatorBreakdown: Regarding the DownerEnding of ''Mostly Harmless'' and the mixed-to-negative reaction from fans, Adams conceded, "I just had a thoroughly miserable year, and I was trying to write a book against that background." He intended a sixth book to give the series a better conclusion, but succumbed to AuthorExistenceFailure first.
* ExecutiveMeddling: In ''Life, the Universe and Everything'', the Silver Bail of Peace is the Rory Award for "Most Gratuitous Use of the Word 'Fuck' in a Serious Screenplay". US censors were offended, and Adams, bowing to their wishes, promptly changed the offending word to "Belgium". Fortunately, [[RuleOfFunny the resulting passage was much funnier]], as Adams modified the surrounding conversation to fit the change, as well.
--> '''Arthur''': "Have you ever been to Belgium in fact?"

!![[Film/TheHitchhikersGuideToTheGalaxy Movie]] trivia:
* AuthorExistenceFailure: Douglas Adams died a year before the film was completed, though after he'd finished his parts of the script.
* GodCreatedCanonForeigner: Many of the differences from previous versions were Adams creations, from pre-death versions of the script, including the POV Ray, the flyswatters, Humma Kavula, and the romantic elements.
* InMemoriam: "For Douglas."

!![[Radio/TheHitchhikersGuideToTheGalaxy Radio]] trivia:
* Slartibartfast is not named for the majority of the episode he first appears in. WordOfGod explained that it was a joke at the expense of the woman who had to type the scripts - that she'd typed this name a dozen times and he simply said 'My name is not important'.
* The seventh episode was originally broadcast on 24 December 1978. Consideration was given, partly due to the date and partly due to the fact that all the plot threads had been tied off at the end of episode six, to making it a stand-alone Christmas special in which Marvin would have been both figuratively and literally the star (of Bethlehem), and by participating in a nativity scene would be cured of his depression. In the end they made it a normal episode, devoted to untying enough plot threads for the series to continue.
* CastingGag: The Tertiary, Quandary and Quintessential Phases had a lot of cameos by actors who had featured in non-radio versions of the story:
** Chris Langham, Arthur Dent from the 1979 Institute of Contemporary Arts stage production of ''Hitchhiker's'', pops up as Prak in the finale of the Tertiary Phase.
** David Dixon, the TV Ford Prefect, had a cameo in the second episode of the Quandary Phase, getting pissed off at Arthur for trying to donate to save the dolphins when he should know they've all vanished. (This case is lampshaded, as Arthur--still played by Simon Jones, who acted alongside Dixon in the TV series--asks if they've met before.)
** Creator/StephenFry, the [[Film/TheHitchhikersGuideToTheGalaxy film's]] Guide, cameos in the third episode of the Quandary Phase as Murray Bost Henson.
** The Quintessential Phase series had Sandra Dickinson, Trillian in [[Series/TheHitchhikersGuideToTheGalaxy the TV version]], voice Tricia [=McMillan=] (Trillian's alternate universe counterpart).
** In a non-remake-related example, Geoffrey Perkins, Douglas Adams's boss at the BBC, plays Arthur Dent's boss at the BBC in the second episode of the Quandary Phase.
* Creator/{{Hipgnosis}}: Did the cover art for the British releases of the soundtrack albums.
* RealSongThemeTune: The theme for the radio show was a snatch of the Music/{{Eagles}}' "Journey of the Sorceror", an instrumental from their 1975 album ''One of These Nights''. It would be re-orchestrated for the Radio [=LPs=], and that arrangement was later used for the TV series.

!![[Series/TheHitchhikersGuideToTheGalaxy TV Show]] trivia:
* PropRecycling:
** The background characters in the restaurant at the end of the universe are equipped almost entirely with costumes recycled from earlier episodes, and in particular from the various Guide entries, thus helping justify the expense of items which otherwise would have appeared on screen for only a few seconds each.
** A badge from one of the Golgafrincham hats would be worn by [[Series/RedDwarf Arnold Rimmer]].
* RecycledSet: The Vogon spaceship interiors were from ''Film/{{Alien}}''.
* TooSoon: Creator/TheBBC provided a content warning when the episode involving the air attack on the Guide offices (a giant H-shaped skyscraper) was aired shortly after 9/11 - to their credit they didn't postpone the broadcast altogether.
* WhatCouldHaveBeen: At one point a second series was planned which would've been based on the ''Series/DoctorWho'' story which later formed the basis of ''Literature/LifeTheUniverseAndEverything''.