Adaptation Displacement: Many who can quote the movie from end-to-end don't even realize there is a book. In the introduction to more recent printings of the book, William Goldman says flat-out that if you are picking it up for the first time, it's probably because you've seen the film.
Ass Pull: The holocaust cloak. It strains the limits of credulity as to why Miracle Max (played by 5'7", 130 lb. Billy Crystal) would have possession of a cloak big enough to fit Fezzik (played by 7'1", 500 lb. André the Giant). Part of the reason it works is that the movie plays the absurdity for comedy. (The book mentions it briefly beforehand, Goldman claiming that Fezzik had needed one to fulfill a task in the - nonexistent - longer version of the chapter of "S. Morgenstern's" work.)
Also the soundtrack, courtesy of Mark Knopfler. In the last sword fight, the timing of the music is perfect with the choreography, and the chords embody the dramatic tension and climax of the scene. This happens throughout the movie, but here it is most noticeable.
Captain Obvious Reveal: The Dread Pirate Roberts as Westley. Buttercup even tells him that Westley had "eyes like the sea after a storm." The camera zooms on "Roberts" sea-blue eyes as she's saying it.
Cult Classic: Only made $30.8 million on a $16 million budget, but has become so popular that it borders on being an outright (non-cult) classic.
Ensemble Dark Horse: Considering just how widely-quoted and iconic Inigo is, it's easy to forget that he's a good distance from the main character (he only has one really big scene in the first half), and his own subplot is largely self-contained.
"Funny Aneurysm" Moment: In his introduction to the 30th Anniversary Edition to the novel, William Goldman assures the reader that the sequel, Buttercup's Baby, will be in the reader's hands "in time for the 50th Anniversary Edition". While this was probably meant to just be a joke on how long it was taking, in real life Goldman died five years before the book's 50th Anniversary, with no sequel ever appearing.
Girl-Show Ghetto: This was part of the reason why the movie didn't do well on its first release. The marketing department at 20th Century Fox had no idea how to promote an absurdist swashbuckling adventure with a "girly" title and romantic arc and simply gave up and didn't release any promotional materials. Later, to compensate for the feminine-sounding title, some DVDs have a cover that puts the Dread Pirate Roberts in the front and center and emphasizes action in the synopsis, basically the exact same way the Grandfather talks it up to his grandson. On the other hand, some DVDs have Buttercup with a crown on the cover, with a very pink box, playing this trope perfectly straight.
In the 1973 book, gamers will appreciate Goldman deciding to skip over the long and repetitiveFetch Quest Inigo and Fezzik undertake late in the book to procure the ingredients for Max's miracle pill. In fact, this is just one of many things that was abridged that sounds an awful lot like Fake Longevity in an RPG game specifically, as opposed to just general story-telling Padding.
The priest mentioning "a dream within a dream" becomes this with the release of Inception 23 years later, which used the phrase as a key part of the plot.
Fezzik's discussion of "fighting half a dozen people" seems prescient when you consider the 1989 Royal Rumble two years later, in which Andre The Giant takes out the first few fighters.
The Dread Pirate Roberts/Westley fights Fezzik, and claims that masks are "terribly comfortable. I suspect everyone will be wearing them in the future." Fast forward to 2017, and hecouldwellberight. And then again in a darker context, when the COVID-19 Pandemic means that literally everyone is instructed to wear masks.
At the end of the original introduction to his book, dated December 1972, William Goldman laments that high adventure (one of the selling points of the book) seems to be in short supply at the time; "Nobody takes out a sword nowadays and cries, 'Hello. My name is Inigo Montoya. You killed my father. Prepare to die!" Fast forward fifteen years or so, the movie comes out, and that very line has become one of the most quotable movie lines of all time, usually said while playacting a swordfight. The 30th Edition has a preface in which Goldman "traveled" to Florin and said kids were all imitating Inigo's catch phrase.
The "Ancient Booer" is appropriately named given that her actress Margery Mason lived to be 100.
Hollywood Homely: Westley (among others) refer to Prince Humperdinck, played by the rather handsome Chris Sarandon, as ugly, specifically as a "warthog-faced buffoon".
Ho Yay: Humperdinck and Count Rugen anyone? And Inigo and Fezzik.
Informed Wrongness: Buttercup getting engaged to Humperdinck, even though a) it was three years after she believed Westley was dead (five in the film), b) Westley didn't bother to let her know he was alive or give her a reason to wait for him, c) her fiance was the crown prince of Florin and could have her imprisoned or executed for saying no (which he does threaten to do in the book).
The Man in Black is Westley, and he has developed an immunity to iocane powder. Even if you don't know going in, Cary Elwes' now quite noticeable voice will probably tip you off. To his credit, Elwes at least tries to help the deception along by speaking softly and slowly early on, and in a much louder, faster, and more confident voice once he shows up as "Roberts".
Prince Humperdinck is the true Big Bad of the story. He doesn't love Buttercup and wants to kill her in order to justify a war against the neighbouring country. This fact is so much common knowledge that it's easy to forget that, in the movie proper, it's a twist that's only revealed after the death of Vizzini.
Many viewers take the "True Love" aspect of Westley's and Buttercup's relationship seriously, not seeming to realize it's a PARODY of classic "World's Most Beautiful Woman + Swashbuckler hero" romances, given how appearance-based their love is note Westley only ever lists Buttercup's "beauty and faithfulness" as his reason for liking her, and Buttercup calls him "poor and perfect, with eyes like the sea after a storm", and how little they have in common or get along when they aren't pining over each other from a distance.
A number of real-live couples model their wedding after Buttercup and Humperdinck's royal wedding with the priest with the speech impediment, even though, you know... she's been coerced to marry the Big Bad, who intends to strangle her right after the ceremony.
Vizzini crosses it by mentioning the little detail where the Sicilian Crowd kills Buttercup, and later cements it by holding her at knifepoint when the Man in Black, having beaten both Inigo and Fezzik in succession, gets too close before their battle of wits, just so that his death by iocane—while he's still laughing in perceived triumph, no less!—can be considered all the more satisfying. Indeed, after everything it's not much of a surprise that he's the only one in the Sicilian Crowd to cark it in his showdown with the Man in Black.
Humperdinck: You truly love each other, and so you might have been truly happy. Not one couple in a century has that chance, no matter what the storybooks say... And so I think no man in a century will suffer as greatly as you will.
Fezzik's Mook Horror Show while wearing the holocaust cloak is intended to be played seriously while contrasted against the comical scene with the Impressive Clergyman... but both sides of the scene come across as somewhat comical thanks to the fact that André the Giant actually hasthe exact same speech impediment that the clergyman does, and has to call himself "the Dwead Piwate Woberts".
Humperdinck responds to the first part of Buttercup's "Reason You Suck" Speech with the very cold, quiet, and well-delivered, "I would not say such things if I were you." After she follows up with an even more nasty and contemptuous speech, he repeats his words in a hammy, growling rage, and... proceeds to grab her arm, take her down a hall, and lock her in her room before going to kill Westley. It works for a villain as wimpy as he is, but for those who expected him to do something far worse to her (forgetting he had to wait till no one would blame him for her death, and intended to do it after they were married), it rather falls flat.
The R.O.U.S. can veer into this territory for some people because the effect wasn't particularly good for its time, and has definitely not aged well. In various shots it's either a little person in a mechanical costume, or just Cary Elwes wrestling a dummy.
The old lady. Boooo! And if you can believe it, that was the tamest of Buttercup's nightmares in the book!note Some of the other ones include an imagined future baby daughter failing to breastfeed from her because her betrayal of Westley was so vile it turned her very milk rancid, causing the baby to collapse into ash in her arms.
Billy Crystal and Carol Kane in the Miracle Max scene. Rob Reiner had to recuse himself from personally filming the scene as he could never keep from laughing, and Mandy Patinkin hurt a rib trying to hold in his laughter during this scene.
Parodied in the book, with an explanation that a scene with Fezzik and Inigo going on minor quests to save Westley was 'cut' because it seemed like a ripoff of The Wonderful Wizard of Oz, even though the 'original version' came out first.
Vizzini saying "All aboard!". The book notes that this was before trains, but the saying actually comes from carpenters loading lumber, and this was well after carpenters.
Playing Against Type: Christopher Guest is best known for appearing in his series of mockumentaries and other projects as wacky, weird, or at least subtly funny characters. Here, he plays a straight-up villain without a hint of humor.
"Seinfeld" Is Unfunny: The movie doesn't carry the same bite for people who grew up watching genre movies made afterwards, that don't feature or lampshade elements this one parodied and/or played straight.
When the heroes leave on the four white horses, it is quite clear that Fezzik has a body double. Necessary, though, in that there isn't likely a horse in the world that could carry André the Giant's mass.
The dummy in the holocaust cloak in the far shots of the "Dread Pirate Roberts" is particularly bad.
The R.O.U.S.'s, as mentioned above, have really not aged well.
Strangled by the Red String: As mentioned under Misaimed Fandom, Westley and Buttercup are basically this being done deliberately; their relationship development is completely skimmed over, they don't seem to get along much in the small number of scenes where they are together, and they don't seem to have any reason to be together other than that they both think they're beautiful. Fortunately, what they lack in development, they make up for in passion. (It's addressed in Buttercup's Baby.)
Fezzik. Not so much in the movie, but after learning that in the book his parents forced him into wrestling at the age of nine and threatened to leave him forever if he wouldn't fight, you start to feel sorry for him.
Similarly, Inigo. He grew up dirt-poor, and only had his father. Then, a nobleman kills his father for taking offense when the nobleman refuses to pay more than a tenth of the agreed-upon price for the sword he commissioned. Not only that, but in the book, plenty of villagers knew the nobleman murdered Domingo Montoya. They all just let him leave though, because there was no way they could convict him of his crime.