In 1066 and All That, this is the predominant form of humor. The "Errata" page is illustrative, though it barely scratches the surface:
P. 3. For Middletoe read Mistletoe. P. 9. For looked 4th read looked forth. P. 44. For sausage read hostage. For Pheasant read Peasant, throughout
Archvillain: The mysterious plasma which gave Mike superpowers also scrambled his brains a bit, leading to him using the wrong words a lot.
A recurring gag in I.L. Caragiale's shorts and comedies will involve characters regularly mispronouncing and mixing up words, as a sign of illiteracy. One of the most well known examples is a policeman saying "enumeration" instead of "remuneration" or "capitalist" instead of "capital city".
Lee has a tendency to accidentally say the wrong word when she's nervous. She's annoyed by this trait, feeling it makes her sound like she's stupid; in Cat Caper, she actually thinks to herself that if she's going to have a speech impediment, she'd rather have a lisp, since at least people would recognize that as a problem.
Mikki White, introduced in Pirate Plot, has the same problem.
In the Clémentine books, Clementine sometimes uses words like "astoundished" or "bore-dumb," or says things like that her friend Margaret "went all historical" on her.
Clue: Mr. Boddy suffers from a blow to the head in the chapter "Dressed To Kill" of the 1990s book The Case of the Invisible Cat. The next day, he's having a great deal of trouble using the right words.
The Dark Tower: A character calls sex with the dead "narcophobia".
The slow-thinking trolls are often Malapropers, especially Sergeant Detritus:
Detritus: He are not glad about being in a tent, as they say. William de Worde: Has he ever been a happy camper?
Lots of Discworld characters are given to malapropism, to the point of it being something of a Running Gag. This extends to their writing, in addition to Ye Olde Butcherede Englishe. Carrot in Men at Arms writes in a letter home that there are lots of new faeces in the Watch.
Everyone in the Discworld says that "The leopard can't change his shorts, you know."
Nanny Ogg had one as well with "the worm is on the other foot now!" (mixing 'the worm has turned' with 'the boot is on the other foot now')
"That was a pune, or play on words, you know!" (This comes from everyone, really, including Death, for example.)
The Last Hero, Old Vincent is reported to have "choked to death on a concubine." When questioned as to whether Vincent would have been into that sort of thing in his late 80s, it's revealed he actually choked on a cucumber...
Sergeant Colon has a tendency to do this as well: In Guards! Guards! he attempts to threaten people with "You're geography!" and "You're home economics!"; in Men at Arms he manages to turn "every soldier has a field-marshal's baton in his knapsack" into "a field-marshal's bottom in his napkin"; and in The Fifth Elephant he claims that Überwald is "a misery wrapped in an enema".
In Feet of Clay, a drunk Nobby tells Colon that he's not going to "sell m' birthright for a spot of massage". Colon, also drunk, says that Nobby means "a pot of message". A bystander corrects them both to "a mess of pottage", then wonders what a mess of pottage is anyway.
Even Vetinari has malapropped, for example saying that Moist von Lipwig has "danced the sisal two-step" instead of the existing idiom of "the hemp fandango" or referring to a street fight as a "gang crumble" rather than "rumble". And also in Going Postal, confusing "the glass ceiling" with "the Agatean Wall". Though with Vetinari, it's equally likely he's getting the phrases wrong on purpose, especially the second example. Perhaps most egregiously, when discussing an incident involving a ship with the already-awkward name of the Wonderful Fanny in Snuff, he accidentally refers to it as the Enormous Fanny.
Hercule Poirot: Poirot is prone to this due to the fact that he is a Belgian man unfamiliar with some English idioms. It is also suggested that he plays it up at times in order to lull the suspects of the crime he's investigating into a false sense of security around him.
Genome: In Cripples, before the battle, Veronica brings Demian a shot of vodka, placing it in front of him on the firing controls, and telling him it's "100 grams for the boy". He once again tries to explain to her that the correct Ancient Russian (it's the distant future) saying is "100 grams before a battle". He has no idea how "battle" got turned into "boy" over the centuries, while she tries to explain it through some psychological mumbo-jumbo. In fact, it has to do with the Russian word for "battle" being "boy" (бой), meaning someone transliterated the word into English instead of translating it. And also could she please stop putting liquids on the damn firing control panel before a battle?!
In The Glasstown Game by Catherynne M. Valente, the soldier Gravey often mixes together two synonyms into one word, like "converscuss".
Billy Bunter of Greyfriars is wont to mangle any non-pedestrian word he is forced to repeat, coming out with monstrosities such as 'unparallelogramed' and 'voluntaciously'.
Advised against in How NOT to Write a Novel; "You may think the occasional slip-up won't matter, but the language you choose is the clothing in which your novel is draped, and saying 'incredulous' when you mean 'incredible' is the prose equivalent of walking into a meeting wearing your underwear on the outside."
Leo Rosten's H*Y*M*A*N K*A*P*L*A*N stories depict a malaprop farm: immigrants from all over the world, learning English at night school.
Eve Dallas in the In Death series has an ongoing difficulty with idioms and figures of speech, frequently overthinking them and questioning their meanings, or using them slightly off-kilter; for instance, in Vengeance In Death she accuses Roarke of getting on a "golden horse" instead of a "high horse." Combined with her tendency toward Pop-Cultural Osmosis Failure, this seems to come from Eve being pretty poorly-socialized up until adulthood, as well as having an overly analytical mind and a general preference for blunt, straightforward honesty.
Wobbler from Johnny and the Bomb always gets a joke about fast food and religion wrong. This is actually relevant to the plot at one point.
"Make me one with everything", he said, "because I want to become a Muslim."
Joey from "Joey: A 'Mechanical Boy'" refers to masturbating as "master painting." Naturally, Bettelheim assumes that this is deliberate obfuscation to further separate himself from the world.
A very minor example appears in Johannes Cabal the Detective when Cabal describes someone as being in the middle of "derelicting" their duty. Leonie points out that there's no verb meaning "to derelict" and Cabal simply replies "there is now." A rare slip-up for Cabal, but justified that he's not a native English speaker.
A Running Gag with young Amy March Little Women, whose attempts to be a Proper Lady make her prone to using more sophisticated words than are typical for her age, which she usually mispronounces. ("I know what I mean, and you needn't be 'statirical' about it! It's proper to use good words and improve your 'vocabilary'.") This is usually flanderized in the films to have her mispronounce common words too: for example, in the 1994 version, where she says "expectorating" when she means "expecting".
I said to her, "Well, my dear, tell Peter what you feel, but do remember he's just as vain and foolish as most men and not a chameleon to smell any sweeter for being trodden on." On consideration, think I meant "camomile".
Frau Stöhr in Thomas Mann's The Magic Mountain does this a lot. One notable example being her suggestion to play Ludwig van Beethoven's 'Erotica' at the grave of a recently deceased fellow patient who was a soldier.
Alessia from The Mister, due to her incomplete grasp of the English language, has a tendency to do this, most notably referring to smartphones as "clever phones".
The protagonist of The Muddle-Headed Wombat has a tendency to get his words mixed up, such as reporting that his favorite fairy tale is "Cindergorilla".
In The Mysterious Benedict Society, minor character S.Q. Pedalian often attempts to use a Stock Phrase (such as "A stitch in time saves nine"), but messes it up ("A stitch in time saves time"). This is part of his characterization as "not the brightest bulb in the chandelier," which ends up helping the main characters in their mission more than once. He also has a tendency to combine words to make new ones, such as "astounded" and "astonished" into "astoundished." (It's later revealed that he's as befuddled as he is because he has been continuously brainwashed to forget his dark feelings about the series villain, Ledroptha Curtain.)
Nursery Crime: In The Fourth Bear, cloudcuckoolander Lord Spooncurdle comments that someone reminds him of "a governess who ran off with the handsome young silver and half the family's boot boy."
Angela Caxton in Keith Waterhouse' Our Song well earns her affectionate pet name of "Lady Malaprop"
The Tamuli: The serving girl Alean calls the Interior Ministry the Inferior Ministry and is mortified when an ambassador laughs. Having come to value her Simple-Minded Wisdom much more highly than the Corrupt Bureaucrats of the Ministry, The Emperor quickly reassures her that they're not laughing at her.
Thursday Next: In One of Our Thursdays is Missing, Mrs. Malaprop from The Rivals is one of the Bookworld characters, and her famed characteristic is exaggerated to the point that she must communicate entirely in malapropisms, which often requires that the reader sound out her sentences in order to understand her meaning.
Mrs. Malaprop:[in reference to a particularly annoying dodo] Eggs tincture is too good for the burred.
The Duke de Beaufort in 20 Years After and The Vicomte de Bragelonne is famous for mixing up words like "affliction" and "affection", which nearly forces him into a duel on at least one occasion.
In Dan Abnet's Warhammer 40,000 novels Ravenor and Ravenor Returned, Unwearth. Constantly. A sample: "I would be most ingratuitious if you were kindly permissive and removed your personable from my ship."
Done a fair amount in Winnie the Pooh, imitating children's tendency to get it wrong. For example "Contradiction" instead of "introduction".
In Xanth, the Demoness Metria has a speech impediment where she often uses the wrong words for things (often a synonym of a homonym, such as "shoe" for soul). Then, the person she's talking to will usually say "what?" and she'll start listing synonyms for the word she means, and they'll finally suggest the word and she'll say "whatever". One time, she even gets the wrong word for demon: "We need more dybbuks!"
Emily Thompson, one of the main characters of The Year of Secret Assignments, is the queen of this trope, and gets called out on it often.
