This page is for the entirety of ''Franchise/TheHitchhikersGuideToTheGalaxy''. For trivia items that only pertain to specific adaptations, please see the subpages below:

* [[Trivia/TheHitchhikersGuideToTheGalaxy1978 The original radio program]]
* [[Trivia/TheHitchhikersGuideToTheGalaxyTrilogy The book series]]
* [[Trivia/TheHitchhikersGuideToTheGalaxy1981 The live-action series]]
* [[Trivia/TheHitchhikersGuideToTheGalaxy1984 The video game]]
* [[Trivia/TheHitchhikersGuideToTheGalaxy2005 The 2005 film]]
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* AccidentallyCorrectWriting: Adams didn't write jokes in Base 13, but the fact remains that using THAT numerical base, 6 x 9 is indeed 42.
* AdaptationSequence: Radio Serial —> Books —> TV Series —> Adventure Game —> Movie
** Complicated further by the books being an adaptation of only the first two radio serials, while the other three books are original. The final three radio serials, meanwhile, are adapted ''from'' the later books.
** Also the second radio serial incorporated ideas (such as the importance of towels) that first appeared in the first book.
* BeamMeUpScotty: 42 is not "the Meaning of Life". Say it with me, people: It's "the Answer to the Ultimate Question of Life, the Universe and Everything". It doesn't help that Adams himself wrote a book entitled ''Literature/TheMeaningOfLiff'' (defining a number of words that didn't exist in the English language, but should have).
* TheCastShowOff: When Music/MitchBenn joined the cast of the Hexagonal Phase as Left Brain, its introduction was reworked to give him an excuse to sing the teleportation ProtestSong from ''Literature/TheRestaurantAtTheEndOfTheUniverse''.
* {{Defictionalization}}: Many bartenders have had a go at reproducing the Pan Galactic Gargle Blaster; not surprisingly, most of the results are almost, but not quite, entirely ''unlike'' "having your brains smashed out by a slice of lemon, wrapped 'round a large gold brick". When asked about such drinks on {{Website/Slashdot}}, Adams said "Unfortunately, there are a number of environmental and weapons treaties and laws of physics which prevent one being mixed on Earth. Sorry."
* InspirationForTheWork: Creator/DouglasAdams claimed that the title came from a 1971 incident while he was hitchhiking around Europe as a young man with a copy of ''the Hitch-hiker's Guide to Europe'' book: while lying drunk in a field near Innsbruck with a copy of the book and looking up at the stars, he thought it would be a good idea for someone to write a hitchhiker's guide to the galaxy as well. However, he later claimed that he had forgotten the incident itself, and only knew of it because he'd told the story of it so many times. His friends are quoted as saying that Adams mentioned the idea of "hitch-hiking around the galaxy" to them while on holiday in Greece in 1973.
* 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. This might have been a deliberate homage on the part of smartphone and tablet makers as the communicators in ''Series/StarTrekTheOriginalSeries'' influenced the design of flip phones.
* 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.
* ReferencedBy: ''VideoGame/SniperElite5:'' In the "Spy Academy" level, scanning one of the enemy infantry brings up the note "Lukas Steinhäusl: Has a terrible pain in all the nerves down his left side." Marvin the Paranoid Android has a terrible pain in all the diodes down his left side.
* RevivalByCommercialization: Louis Armstrong's "What a Wonderful World," played at the conclusion of the first phase of the radio series, became popular again because of its use here. It was done again at the end of the TV series.
* ScienceMarchesOn: The first book has a joke about a planet where, to prevent erosion, the difference between what a tourist eats and what he excretes is surgically removed from his body right before he leaves, so if you go to the bathroom there, it is vitally important you get a receipt. Science has since worked out that most of the matter that a body expels after digestion comes out as sweat or exhaled carbon dioxide, so that wouldn't actually help any.
* SelfAdaptation: Creator/DouglasAdams' level of involvement with each adaptation of ''Radio/TheHitchhikersGuideToTheGalaxy1978'' 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 script before he died was used as the final shooting script with minimal editing.
** Probably the only adaptations in which Adams was not involved are the comics. This goes to show because they follow the novels pretty faithfully.
* TechnologyMarchesOn:
** Digital watches sure are a neat idea, huh?[[note]]One should remember that when the radio series and first novels were written, digital watches had power-hungry LED displays that needed to be activated by the opposite hand for the wearer to tell time, and the displays were unreadable in direct sunlight.[[/note]] (In the later radio series, and the movie, they were replaced by mobile phones.)
** Also, the quip about needing "several inconveniently large buildings" to carry around a copy of the ''Encyclopedia Galactica''.
** The fact that the Guide itself is no longer technologically impressive as it’s basically a tablet that can only access Wikipedia. If another adaptation ever gets made, the Guide would presumably now have to be an app.
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