This is done in a lot of mystery and Private Detective fiction. It's especially prevalent in short stories, as it's a quick and easy way to bring in a new character and explain how/why they've come to seek the main character's services without having to waste limited space on unnecessary details.
Noodle Incidents pepper a lot of Simon R. Green's works, usually as gags (e.g. a hand reaches out of a top hat to retrieve a drink from the bartender at Strangefellows, who quips "Boy, that rabbit was mad at him...").
There are several references to Colonel Baslim's rescue of the Hansea from slavers, but the subject is a taboo among the People so we never find out exactly what happened.
When Colonel Brisby, Commanding Officer of a military starship, gets the identification back on his newest recruit, a young ex-slave and ex-Free Trader. Turns out the youngster - our protagonist - went missing after a starship "accident" when he was a baby and is in fact the much-sought-after sole heir to a Galaxy-spanning mercantile empire. After reading the message, Brisby muses "Why do things like this always happen to Hydra?", leaving the reader to wonder just what other adventures the Hegemonic Guard Cruiser Hydra has been involved in.
There is a whole paragraph devoted to this in Double Star:
"He mentioned a couple of details in my past that I would have sworn were buried and forgotten. All right, so I did have a couple of routines useful for stag shows that are not for the family trade — a man has to eat. But that matter about Bebe; that was hardly fair, for I certainly had not known she was under age. As for that hotel bill, while it is true that bilking an 'innkeeper' in Miami Beach carries much the same punishment as armed robbery elsewhere, it is a very provincial attitude — I would have paid it if I had had the money. As for that unfortunate incident in Seattle — well, what I am trying to say is that Dak did know an amazing amount about my background but he had the wrong slant on most of it."
In The Moon Is a Harsh Mistress there is the "Wet Firecracker War", which is mentioned at least twice but never explained. It's implied that it was an attempted nuclear war between the United States and the Soviet Union... which turned out to end quickly with relatively little destruction (Minus the loss of Colorado Springs in the US, probably because of the many military installations). Hence, "wet firecracker".
In The Rolling Stones, after Cas and Pol are arrested on Mars for tax evasion, their father notes that at least it wasn't for experimenting with atomics inside city limits like the last time they were arrested back on Luna.
Starship Troopers has several. At the end of Rico's training, he mentions that he left a lot out, specifically, that he said "...nothing about the time we dropped everything and fought a forest fire for three days, no mention of the practice alert that was a real one, only we didn't know it until it was over, nor about the day the cook tent blew away..."
Pongo Twistleton and his uncle, Fred, who always pulls his nephew into his complicated schemes are recurring characters. In every story that mentions them, they always recall when they were arrested at the Dog Races, but it's never revealed why.
Likewise, in Wodehouse's Blandings Castle series, repeated references are made to the never-actually-recounted "Story of the Prawns" which relates a humiliatingly hilarious incident in the youth of stuffed shirt Sir Gregory Parsloe-Parsloe. At the end of Summer Lightning, Galahad Threepwood starts telling the story, but the book ends before we would find out what it's about.
In Eric Flint's novel 1632, when Jeff and Gretchen's impending marriage stirs up trouble, Frank threatens to tell how exactly he met his Vietnamese wife, Diane, something no-one else wants to know. Given that the trouble is mostly because Gretchen was once a camp follower, and Frank is a Vietnam War veteran, the obvious implication is that Diane used to be in a... similar line of work.
Alien in a Small Town is full of these, making casual references to the setting and its future history. Humanity's first alien contacts included "the Kimsons encounter with the Tsor-Konapites, the Ad Astras violent run-in with the Ploogs, the legendary lost ship Ograe Nyha." Earth has been through at least two big wars since the present, the Genomic War and the Android Uprising, of which we are told nothing more than tidbits. Indira and Tendai's tour of the Solar System has them visiting historical sites like Piratesnest, Webra's Snare, God's Rock, and "the rose greenhouse atExpury."
In Another Note, Mello explains that he's gotten to meet L in person, and heard from him the stories of how L arrived at Wammy's House, and how he bested Eraldo Coil and Denevue...but chooses not to share those stories, or the story of how he got to meet L, with the readers.
Also, we know that A and B were the first generation of would-be L-successors. We also know that Mello and Near were the fourth. We don't know a) who the second and third generations were and b) what happened to them.
In Arabian Nights, a sage about to be executed observes that his fate is like the reward of the crocodile. When asked about the crocodile and his story, the sage says that he is not in the mood to tell a story at the moment.
Arcadia Snips and the Steamwork Consortium: Though the victim of this Noodle incident is later introduced in a somewhat proper fashion, (Jake "The Beak" Montgomery is also, at the same time, the assassin following Snips and the inspiration for the titular character in very bad need of a Mary-Sue edit "Professor Von Grimskull"), and the effects of it are even put to use, The Incident is never explained.
Morgrim was suddenly looking extraordinarily comfortable. At least you dont know about the duck, Snips said. Check the back side. Snips flipped the document over. Oh. I hear Jake The Beak Montgomery still shrieks like a little girl when he hears a quack.
In the first book, Root mentions how Holly screwed up, causing "The Hamburg Affair". One of her perps tried to bargain with the humans for asylum. "Four mindwipes, a time stop, and a retrieval squad" were needed to sort it out. All the comic book adaptation shows us is an elf smiling at the camera as the POLIZEI car he's in the back of pulls away.
As of Book Six, we now know more of the details. One of Holly's fugitives locked himself into a car in Hamburg. She tried to unlock it, but her omnitool had been stolen by Mulch Diggums a few hours before. The target was apprehended by humans, and he tried to bargain with them for political asylum. The rest is easy to piece together. Thinking he's a confused kid, they take him to the police station. A daytime raid on a police headquarters a retrieval squad needing a time stop, with four mind wipes for all the humans involved.
In A Bad Case of Stripes, a whole bevy of people try to cure Camilla's case of changing into whatever is suggested. Some of these attempted cures are unseen, but resulted in Camilla growing branches, a cat's tail, and feathers.
In the third book of The Bartimaeus Trilogy,Ptolemy's Gate, the djinni Bartimaeus mentions twice the Case of the Anarchist and the Oyster that he helped his master Nathaniel solve. Upon bringing it up, Nathaniel winces and tells Bartimaeus to please not talk about it. This is possibly also a Rule of Three situation, since the Anarchist and the Oyster is the third of three such situations, the first two being the plots of the two previous books.
The Belgariad: In Polgara the Sorceress, Polgara describes how she has to go undercover in Gar Og Nadrak, laying the groundwork for an alliance with the West. She gains a reputation as a dancer, eventually getting an audience with the young and notoriously lecherous Nadrak king, Drosta Lek Thun. She dances, pulling out all the stops to impress him, reducing him to a quivering shambles, but refuses to describe her routine "the children, you understand".
In Black Legion, there's a place called Drol Kheir and something happened there to make everyone think that this is where Khayon met his end, but apart from reassuring people that he is, indeed, alive, he says nothing more about it.
In The Caine Mutiny, Keefer and Maryk are discussing the upcoming court-martial of crewman Stillwell:
Keefer: You're whistling in the dark. Ever hear of a captain in his right mind trying to rig a court-martial as crudely as he (Queeg) is doing it? Maryk: It happens every day. What the hell is a summary court-martial but a farce? Nobody on a ship ever knows any law. Hell, what about Devriess with Bellison-and Crowe? Keefer: That was different. DeVriess fixed the court to let them off. He was going through the forms because the Aukland police were so sore about the riot.
Gateway of the Monster: The Black Veil and the Moving Fur;
House Among the Laurels: The Steeple Monster;
The Horse of the Invisible: "The Black Veil case, when young Aster died. You remember, he said it was a piece of silly superstition and stayed outside. Poor devil!"
The Searcher of the End House: The Three Straw Platters, the Dark Light case, and the "trouble of Maaetheson's".
The Whistling Room: the Buzzing Case, the Grey Dog, the Yellow Finger Experiments, the Silent Garden, and the Nodding Door. The "Grunting Man" case was probably the story titled The Hog.
The incident for which Montresor wants revenge against Fortunato in Edgar Allan Poe's The Cask of Amontillado. Just what did Fortunato do that made Montresor feel it was necessary to wall him up in a tomb and leave him to die? He never says. While the full nature of the "insult" may never be known, Poe scholars have narrowed it down to being related to class conflict. Montresor is the scion of an ancient noble family, while Fortunato appears to be "new money." Arrogant, vulgar, and ignorant of the manners of high society, Fortunato inadvertently slighted Montresor's family honor in such a way that could only be redressed through violent retribution. Some have even theorized that Fortunato made his money by fleecing Montresor or one of his fellow ancient noblemen.
In The Caster Chronicles, since Beautiful Creatures, Link and Amma had been quarreling over something Link did in Ethan's basement when he was a kid, and Ethan never knew what it was, because Link refuses to tell anyone what bad thing he had done. In the end, we learn that he had put on a Union soldier's uniform.
The Will of the Empress: Our four protagonists remember the time they got drunk. It ended with a barn being destroyed.
The Cinder Spires: The incident on the Perilous that led to Grimm getting cashiered out of the Albion Navy. It's implied that whatever happened was really Rook's fault, and Grimm was pressured into taking the fall due to Rook's powerful relatives, but no details have been provided.
In Arthur C. Clarke's short story Wacky, the protagonist mentions the "Case of the Elastic-Sided Eggwhisk", adding that he would almost certainly not have survived it had it ever actually occurred.
In Sarah Pennypacker's Clementine and the Family Meeting, part of the clementine series of books, Clementine's paternal grandmother twice begins to relate to Clementine and her mother, via speakerphone, a tale involving Clementine's father, his little brother, and an incident with a garden hose. Both times, however, before she can get properly underway, Clementine's father runs over, grabs the phone, cuts off the speakerphone, and then makes an excuse about it getting late and hangs up.
Not to mention "the wrong my man did your father, Robert Poste's child." Flora finds out some of it, but the reader doesn't, and even Flora never learns if the goat died.
In Christopher Anvil's "Colonization" setting, the narrator of the stories "Bill For Delivery", "Untropy" and "The Low Road" is fond of these. For instance, no-one knows exactly what was done to the crew of the Worst Yet, who thought they were "tougher than any combination of officers".
You know, Sam, when something's only half-bad, a man's eager to talk about it. When it's real bad, you have to wait and let it come out, a sliver at a time. You can take one of the tough crewmen who shipped out on Worst Yet, with Upper Jaw for captain and Lower Jaw for first officer, and go to the most wide-open joint in the easiest-run space center you can find, and pour double shots of super-nova down his throat as long as the place stays open; about 0200, he'll stare into his empty glass and growl, "The bastards." That's it, Sam. That's all he'll say about it.
Within Crime and Punishment (an otherwise very serious novel), Raskolnikov is reading the newspapers trying to find an article on the pawnbroker's murder, Raskolnikov relays the headlines of several terrible happenings recorded within, one of which involving the Spontaneous Human Combustion of a shopkeeper from alcohol. Nothing more is said on the matter.
Dave Barry Slept Here pointedly refuses to explain the Ballinger-Pinchot Affair, a famous incident that occurred during the Taft administration, after acclaiming it as "truly one of the fascinating and bizarre episodes in the nation's history." Apparently, it had a part involving a dwarf goat.
The Demolished Man has one, used several times. Lincoln Powell, police detective and head of the Esper Guild an upstanding citizen, one would think has a prankster side that he calls "Dishonest Abe". He always blushes when asked "Who stole the weather?", apparently referring to one of Dishonest Abe's escapades.
Deltora Quest: Secrets of Deltora is a guidebook, not an encyclopedia (and its in-universe author was famously terse). So it advises travellers against certain specific things like burning green twigs or building campfires on ground that starts shaking after the fire is lit, but it doesn't explain what will happen if these things are done or detail further the sapience that some regions/plants/ponds display. In fact, no one in the series seems to understand the magic that infuses/originates from Deltora.
A semi-example: in the DI Frost books, by RD Wingfield, there is a reference at least once a book to a joke told by Frost involving a man who drank a spittoon as a bet. The rest of the joke is never given, but anyone who has heard it reacts with alarm at any attempt by Frost to tell it again. When, in the final book, he does tell it to an unsuspecting individual, it happens off-screen.
They occasionally refer to the unlucky Mr. Hong, who disappeared in mysterious circumstances after opening The Three Jolly Luck Takeaway Fish Bar on the site of an old fish-god temple on Dagon Street during a full moon (some references also state that said full moon was on the Winter Solstice; thankfully, that's where the chain of unfortunate coincidences end there's no "after a delivery of a rare kind of squid" to make it worse). No one knows quite what happened, but it wasn't pleasant: one of the references mentions that he left behind "one kidney and half an earhole". Note that Dagon is the name of a Philistine fish god, and is also a malevolent deity in the Lovecraft mythos...
A more sinister example is given just enough detail that the reader can figure out the likely story: In The Fifth Elephant Sybil Vimes nee Ramkin is reflecting on how she worries about Sam. There was one case, involving someone who "kept that little girl's shoes", where if Detritus hadn't been in the room the troll was pretty certain only Vimes would have walked out of it...
And on a similar note, the last king of Ankh-Morpork, Lorenzo the Kind, was said to be "very fond of children", and had various "devices" in his dungeons. The fact that he was apparently so bad even the notoriously corrupt and apathetic people of Ankh-Morpork wanted him dead speaks volumes.
In Wyrd Sisters, Granny Weatherwax hasn't been on speaking terms with Sister Rodley, a fellow witch, "ever since that business with the gibbet."
The wizards at Unseen University are eating and it is mentioned that the Bursar has to have wooden utensils instead of metal ones after what they have since referred to as "the Unfortunate Incident at Dinner".
This same "Incident" may also be the one that led to the grounding of the senior faculty's High Table. Prior to this, it had hovered in mid-air during meals.
Night Watch has Vimes threatening a recalcitrant prisoner with the "Ginger Beer Trick", aproximated by a finger popped from the mouth, a hissing noise and a blood curdling scream. note (This one's not really a mystery. You shake a glass bottle of soda or mineral water and spray it straight up the victim's nostrils, leveraging the pressure with the thumb. It hurts like hell and leaves no marks on the victims body. It is commonly done by drugdealers and corrupt cops in Latin America. For an on-screen example, you can watch it happen on an episode of Dexter.) And it doesn't have to be a nostril, either—any mucus membrane will do, but the nostrils are simply convenient. Ginger beer is, however, particularly effective for it, as the ginger adds a burning sensation—even just incautiously drinking the stuff hurts.
Then there's Bloody Stupid Johnson (a Shout-Out to real world Capability Brown), the "unique" designer/architect always mentioned in passing (along with his creations—which work, just not the way you expect them...or are supposed to)...and it is hinted in Jingo that an ancestor of Lady Sybil's had something to do with said passing, as well.
A specific noodle incident occurs in Johnson's custom bathroom 'Typhoon Superior Indoor Ablutorium with Automatic Soap Dish' which was found boarded up hidden behind a bookcase in the University. The Archchancellor used it until there was an unfortunate incident, after which he solemnly ordered it sealed up again, only more thoroughly and with extra warning notices. It's implied that it involved an interaction between the shower and the university's pipe organ, which was also designed by Johnson (it's mentioned that to Johnson, all pipes were the same) and was being played by the Librarian at the time. It's also stated that "they never did find the soap".
This one can be explained with a little logical deduction. When the Librarian activated the organ's afterburner (why Johnson thought an organ needed an afterburner is probably a Noodle Incident in its own right) with the Organ Interlock lever in the shower activated (Ridcully thought it would pipe music in while he washed), nitrous oxide flowed into the shower, creating nitric acid. Ridcully was showering in acid rain. Which would also have reacted with the soap and terminally dissolved it.
A more minor one beforehand involved Ridcully having a bad experience with a tap marked "Old Faithful" (the name of a famous geyser).
And another is Jeremy Clockson's reaction to a fellow clockmaker who deliberately kept his watch fast. All we're told is that people are very understanding when it comes to genius, at least once they've cleaned up the mess and taken the hammer away.
Also, don't ask the Lancre Men's Morris Team (especially Jason Ogg) about the Stick and Bucket dance. Apparently, it's legal, which is surprising.
And nobody in Lancre is prepared to go into exactly how the village of Bad Ass came to get its name.
Supplemental material about Unseen University describes their school holidays and traditions, including one at which it's customary for the wizards to inflict "a plunking" on any red-haired men they come across. Not only is it never stated what "a plunking" actually is, but an unspecified incident is mentioned that led the University to exempt Captain Carrot Ironfounderson from this treatment. (This particular Noodle Incident apparently required a ladder to retrieve three student wizards from the eaves nearby.)
The very last sentence in Making Money mentions one, involving something that the ghost of Professor Flead did to prevent his chair from being removed from the Pink PussyCat Club.
Many a crime had been solved because of things that had fallen on [Colon and Nobby], tried to kill them, tripped one of them, been found floating in their lunch, and in one case had tried to lay its eggs up Nobby's nose.
For a long time, the events of The Amazing Maurice and His Educated Rodents was just something mentioned occasionally as being in the past (and never at the same time, as later mentions tend to be fewer years in the past than earlier ones).
Dragon and Damsel: Apparently, Bernadette ran a cash register once in the bookstore. Edward came by...and whatever happened, Azrael thinks is funny, while she is still slightly embarrassed and annoyed about it.
The book expands the scene where Rose talks to a Conspiracy Theorist with pictures of the Doctor, and she is shown photos of the Ninth Doctor fighting a pterodactyl in around 2000, and the Thirteenth Doctor being chased by a giant frog in front of Buckingham Palace.
The Doctor mentions having to stop the Eiffel Tower, which is apparently a spaceship, from taking off, and also that he once left a pair of boots on Mars that he'd like back.
It mentions that the Doctor disliked UNIT moving its headquarters to the Tower of London because of some incident where he tried and failed to save a woman from being executed there centuries prior. The only details given are that the woman had very pale skin, and that the Doctor rowed her either in or out of the Tower.
UNIT arranged for the Gherkin to be given its nickname because they don't want anyone to suspect what they're building inside it.
Miron's past has some of those. Apparently, he once lived for a few decades with a vampire coven, and became a classical, household-name English poet after publishing his works under a false surname.
It's mentioned once that after Viola's last visit to Thorn, the vampires are still double-checking whether what they're about to drink is actually blood. The reader is left free to imagine just what could be in there.
Vlad Taltos constantly drops references to things he's done in the past, some of which are quite pointedly not explained. Like the guy who could jump an eleven-foot crevasse, but who Vlad survived because he was wearing the wrong kind of boots.
In Five Hundred Years After, Paarfi cites a Dragaeran folktale of the For Want of a Nail variety, "The Tale of the Smudged Letter". It apparently involves a leaky roof, a river boatman, a seer and a wizard, but what actually happened isn't stated.
In Dragon Bones, it is mentioned that an aunt slapped Ciarra, and that the local Friendly Ghost, Oreg, who likes Ciarra, did something to the aunt that caused her to never visit again. The incident is referred to two times, but it is never mentioned what exactly Oreg did, though it seems it was a bit worse than just hide behind her and say "boo!".
The Dresden Files: Jim Butcher peppers his books with these, and this series is no exception.
Harry will periodically refer back to cases that had happened previously. Sometimes, these are the events covered by previous books (his remarks along this vein are a good way to deduce that the books generally happen a year or so apart), but now and again he drops a name, date, location, or supernatural threat that doesn't come up in any of the chronicles we've seen.
One of the few genuine supernatural scoops the Midwestern Arcane had managed to publish involved the Unseelie Incursion of 1993, when Milwaukee, Wisconsin, disappeared for two hours, with satellite images revealing the city's location having reverted to virgin forest. When the city returned, no one inside remembered anything had happened.
At one point, Harry suggests getting "creative" with the potions he can brew. Bob brings up a few incidents that (unfortunately) we never saw anything of.
Bob: What about the time you tried making anti-gravity potions? Harry: We FIXED the floor! It was no big deal!
Fool Moon: Harry had a job in Minnesota because "Somebody saw something in a lake".
Summer Knight: Several wizards are, for various reasons, unable to fill Simon Pietrovich's seat on the White Council after Pietrovich's death. Reasons given include "sleeping off that potion", "pyramid sitting", "living under the polar ice cap" and "got REAL married".
One of these actually becomes a plot point about 3 or 4 books after he drops the reference. "You should have seen the look on the stormchaser's face when he realized the tornado was chasing us" is implied to have the Summer Queen owing Harry a favor. Which he transfers to Charity later in a bit of manoeuvring to let the Summer Knight and Lady help him through her, since they couldn't do so directly.
Harry offhandedly refers to trying to bathe his cat in Skin Game. Apparently it didn't go over very well with Mister.
He's also mentioned having attempted to craft a flying broomstick, with very poor results. Turns out flying isn't as easy as he'd hoped, and it wound up a UFO rumor on the internet.
We never get to hear what Binder was doing in Belize with thirty monkeys, a panda, and a pygmy elephant.
Author Irene Kampen note Best known as the author of Life Without George, the book that became The Lucy Show. returned to college at UW - Madison in 1969. In her book on that experience, Due To Lack of Interest, Tomorrow Has Been Canceled, this is a running gag. When the CSDU ("What's the CSDU?") holds a meeting in her apartment, a member tells her they can't meet on campus anymore after that Timothy Leary thing. Later, a classmate asks her to chaperone a frat party, explaining that they must have an "older person" present because of all that trouble about the goat. "What trouble about the goat?" "Oh, there was just some trouble about a goat." As she leaves at the end of term, she's asked to sign a petition and is told it's "about the protest." She knows by now not to ask.
In Astrid Lindgren's book Emil of Lönneberga, the narrator occasionally mentions that she promised Emil's mom never to tell what Emil did on the 3rd of November, even though she laughs every time she thinks of it.
Emily the Strange: Stranger and Stranger: Some of the things Emily (the one who is writing the diary) didn't know about herself. Especially why the phrase "nothing but a thin broth" is so funny to the Other Emily and her mother.
Ethan Brand by Nathaniel Hawthorne does this with the Unpardonable Sin, the one act God is incapable of forgiving. The title character, who committed the sin makes occasional references to the woman he apparently did it to, but the act itself is never described, presumably because it's best left to the imagination.
Lazlo Woodbine, hard-boiled Private Detective in Robert Rankin's "Far-Fetched Fiction", has a long string of Noodle Incidents in his past, all of which cost him a loved one, a body part, and a valuable artifact.
The Flashman novel Royal Flash begins, "If I had been half the hero everyone thought I was, or even a half-decent soldier, Lee would have won the battle of Gettysburg and probably captured Washington." He only says this to illustrate how history can turn on trifling events, and as a story for another time, never mentions it again.
In Steven Brust and Emma Bull's Freedom and Necessity, the letters between Kitty and Susan are filled with references to "the Vicar's pet dormouse" and with oaths on "the sacred pony saddle". Nor are those between James and Richard exactly empty of them ("Remember the time you threatened David with the nursery toasting-fork?"). We learn why Friedrich Engels calls Susan "Spider", but we never do find out why Engels and James score points against each other in terms of who owes who a cherry.
In between the first two books of The Girl from the Miracles District, Nikita recovers a trinket of some sort from Sawa for Madame Butterfly and ends up having a run-in with a pack of Japanese demons. That's all she ever tells anyone of the matter.
The frame story of Goodbye, Mr. Chips is about an octogenerian retired school teacher reminiscing about his past. In between the more fleshed-out stories that make up most of the book, there are also references to incidents that are not described, such as the "queer business" of Archer's resignation, and the humorous incident of Rushton and the sack of potatoes.
Whatever happened to the third baby in the mix-up with the Antichrist and the Satanic nuns? You don't want to know what they could have done with him. Let's just imagine he was safely placed in a loving home, where he lived happily ever after and raised tropical fish. Turns out that's exactly what happened to him.
In one scene, Adam starts comparing the war between Heaven and Hell to the rivalry between the Them and Greasy Johnson's gang, and Pepper starts to bring up some trouble they got into during "the ole folks' party in the village hall". Adam brushes it off as not really a loss, because both Them and the Johnsonites got into trouble for that one.
"Look at this neat little packet of manuscript; it is paginated, you see, and I have indulged in the civil coquetry of a ribbon of red tape. It has almost a legal air, hasn't it? Run your eye over it, Austin. It is an account of the entertainment Mrs. Beaumont provided for her choicer guests. The man who wrote this escaped with his life, but I do not think he will live many years. The doctors tell him he must have sustained some severe shock to the nerves." Austin took the manuscript, but never read it. Opening the neat pages at haphazard his eye was caught by a word and a phrase that followed it; and, sick at heart, with white lips and a cold sweat pouring like water from his temples, he flung the paper down.
Mostly it's in the form of books mentioned, of which three (Fantastic Beasts and Where to Find Them, Quidditch Through the Ages, and Tales of Beedle the Bard) are actually made — but there are many which aren't, such as Hermione's favorite go-to, Hogwarts: A History. There are goblin uprisings, house-elf history, and all those creatures — even vampires, which judging by HBP are not openly hostile.
Also a nice twist and version with Grindelwald. When in first book we hear that "Dumbledore... is famous for his victory over the dark wizard Grindelwald", we imagine it being a simple story — that no one could beat Grindelwald, until young Dumbledore duelled the Dark wizard to the death and killed him. In the last book it is revealed that the story is muchmorecomplicated...
Aberforth Dumbledore's incident involving using illegal charms on a goat counts as this, seeing as what the charm was is never mentioned, and that Aberforth generally likes goats, enough for one to serve as his patronus, anyway. However, some people have guessed what the "illegal charms" referred to either on their own in their gutter-minded imaginations or they guessed from a conversation between J. K. Rowling and a Harry Potter fan who asked about this particular Noodle Incident. J. K. Rowling started off by asking the fan's age and some people theorize that if the fan was not eight years old, J. K. Rowling would have said explicitly that the "illegal charms" were of the bestiality variety, judging by the fit of hysterical laughter she had when discussing it and the fact that she felt it necessary to ask the age of the fan.
The reason for Hagrid's expulsion from the school is treated like this in the first book and part of the second. It's only brought up about once or twice, but apparently his reaction generally involves [paraphrasing] "clearing his throat loudly and suddenly becoming deaf until the subject is changed." Subverted when we find out the story later in Chamber of Secrets.
Philosopher's Stone mentions two, both related to Quidditch. One is a claim that referees are sometimes known to vanish and turn up in the Sahara Desert months later, which later turned out to be an exaggeration—it only happened once, and it was because his broom had been turned into a Portkey. The other one is the 1473 Quidditch World Cup, where all seven hundred fouls in the game were committed (and several were likely created). Among the things we know happen are that a Chaser was turned into a polecat, some players brought actual weapons onto the field, and that the Transylvanian team released a storm of vampire bats from under their cloaks. One has to wonder how many players survived the 1473 match, and what the hell kind of foul is worse than trying to kill someone with a broadsword.
Some of these fouls are Noodle Incidents in-universe, as, according to Quidditch Through the Ages, the full official list is not even publicly available, due to concerns that people "might get ideas." The book's author was able to get a look at the list for his research, and ended up fully supporting this policy. (Though he does note that the overall ban on using a wand against the opposing team makes about ninety-percent of them impractical in any case.)
Also, in Chamber of Secrets Tom Riddle recalls Hagrid getting in trouble for "trying to raise werewolf cubs under his bed." This might have been untrue, since werewolves have human babies, not cubs, but one wonders what Hagrid really did get up to.
On the Pottermore site, in a recent update for Prisoner of Azkaban, Rowling said that, once, a werewolf couple mated and produced offspring that were physically wolves, but of much higher intelligence; their cubs were in Hogwarts' custody and put into the Forbidden Forest. Could be what that was. Could also be Rowling covering up her mistake in calling children born from werewolves cubs.
There's also a bit from Goblet of Fire, after the second Defense Against the Dark Arts class, where Ron asked Harry "Did you hear [Mad-Eye Moody] telling Seamus what he did to that witch who shouted 'Boo' behind him on April Fools' Day?" with no further explanation made, ever.
And then from The Deathly Hallows, during Harry's infiltration into the Ministry of Magic. A young witch, among a group of employees present to witness the remains of decoys Harry set off, remarks: "I bet it sneaked up here from Experimental Charms, they're so careless, remember that poisonous duck?" The incident regarding the duck has not been mentioned, much less expanded upon, before or since.
May be a sidelong reference to the duckbilled platypus, a venomous animal so ridiculous it just has to be magical.
On a similar note: on the way to Harry's hearing, a man at the Ministry mentions that they had found something that "We thought it was a bog-standard chicken until it started breathing fire..." This is never explained.
"Here comes Gilbert Wimple; he's with the Committee on Experimental Charms; he's had those horns for a while now"
Dumbledore refers to the "thrilling tale" that hoped to tell Harry someday of how his hand was turned black. This one is later explained to the audience, however.
And speaking of Voldemort, it is mentioned that when he was a child, he took two other children from the orphanage into the cave that he later used for one of his Horcruxes and did something in their presence that traumatized them into silence, but the specific details are never divulged in-story.
One of Snape's aspiring Death Eater classmates, Mulciber, did something to Mary MacDonald involving Dark Magic of some kind, which is never explained.
It's mentioned as part of the prophecy that the parents of The Chosen One need to have defied Voldemort three times by the time it was made. Both the Potter parents and the Longbottom parents apparently qualify, but the exact when-and-how of these instances of defiance, presumably during their time in the Order of the Phoenix, is never quite clear.
In Heart In Hand, Darryl's teammate tries to blackmail him about "that thing with the thing three years ago".
Kurtz in Heart of Darkness. What exactly he did after he went mad among the savage tribes of the Congo is only hinted at.
But this must have been before hislet us saynerves, went wrong, and caused him to preside at certain midnight dances ending with unspeakable rites, whichas far as I reluctantly gathered from what I heard at various timeswere offered up to himdo you understand?to Mr. Kurtz himself.
The Heartstrikers: Svena the White Witch is an ancient dragon who mostly thinks of humans as those silly little animals who are fun to watch run while she burns their villages. It's therefore unknown how she earned the title "Guardian of Slovenia," which she considers the most embarrassing of her many titles. In the last book, Amelia mentions that it involved her getting really, really drunk.
Similarly, Agatha Christie often inserted references in her Hercule Poirot novels to other cases solved by the famed detective; occasionally, these are subtle references to other books in the series, but they are usually mere snippets of information. Example: Poirot makes reference, in one of the short stories, to a man he once arrested — a soap manufacturer in Liège who was guilty of the poisoning of his wife. This is all the information we ever hear about that particular case.
Much of Jason's past adventures, especially Charleston, are this.
Hilary Tamar: In Sarah Caudwell's first novel Thus Was Adonis Murdered, one of Julia's letters from Venice mentions in passing that she can't stand spiders, prompting Cantrip to a reminiscence which is quickly cut off. The narrator says (I paraphrase): "I trust it shall not be necessary to recite the revolting details of the spider incident. Suffice it to say that any woman who retires with Cantrip on the night of March 31 ...."
After a missile is transformed into a bowl of petunias, which rapidly falls to the surface of Magrathea, we get this lovely line: "Curiously, the only thing the bowl of petunias was thinking as it fell was 'Oh no, not again.'"
This one is explained in a later book, so it's only temporarily an example. It remains an example in the TV and movie adaptations (which never get far enough to reach the explanation).
Zaphod Beeblebrox refuses to explain why his father is Zaphod Beeblebrox II and his grandfather is Zaphod Beeblebrox III — apparently, it involved a contraceptive and a time machine.
In Life, the Universe and Everything, Wowbagger the Infinitely Prolonged is said to have gained his immortality after "an unfortunate accident with an irrational particle accelerator, a liquid lunch and a pair of rubber bands." The details are apparently unimportant as no-one has managed to duplicate the events, with everyone who has tried ending up looking very silly. Or dead. Or both.
Trapped on prehistoric Earth, Ford tells Arthur that he took up being cruel to animals as a hobby. "I won't disturb you with the details, because they would ... disturb you. But you may be interested to know that I am singlehandedly responsible for the evolved shape of the animal you came to know in later centuries as a giraffe."
Lee Child's Jack Reacher novel 61 Hoursplays with it by showing what happens when someone determined enough to keep trying actually starts to weedle the details out.
There's some crisis in the Dragon's Lair kitchen that involved a spoon, has happened several times, and requires the Number Two mercenary in the guild to resolve each time.
Something happened in Najica between Nolien and Tiza while they were in the country on a mission. They refuse to talk about it but it involved their obvious attraction to each other and some manner of danger. It remains a sore spot months later.
Ginger Hasina is banned from the Mana Mutation Summit because of something she did at the last one.
Another presumably serious example from the Peter F. Hamilton Space OperaJudas Unchained: Whatever universally-infamous atrocity "the Cat" committed which got her sentenced to a thousand years in suspended animation. (She gets let out early because the government has a dire need for stone-cold killers.)
Kipling does this in The Jungle Book too — in the story "Mowgli's Brothers" he says we can only imagine Mowgli's life among the wolves, because if written it would fill so many books ("Kaa's Hunting" and "How Fear Came" fill in some of the blanks); while in "Red Dog" there's mention of Mowgli's adventures up till then, among them encountering a mad elephant, fighting a crocodile, and being caught in a migrating herd of deer and nearly trampled.
From the Just So Stories of Rudyard Kipling: "There are three hundred and fifty-five stories about Suleiman-bin-Daoud; but this is not one of them. It is not the story of the Lapwing who found the Water; or the Hoopoe who shaded Suleimanbin-Daoud from the heat. It is not the story of the Glass Pavement, or the Ruby with the Crooked Hole, or the Gold Bars of Balkis. It is the story of the Butterfly that Stamped."
Keeper of the Lost Cities. A "Great Gulon incident" masterminded by Keefe is referenced routinely, with Sophie never learning exactly what happened.
In Dennis Lehane's Kenzie and Gennaro Series, Patrick is shot and very nearly dies at some point in the eleven years between the fifth and sixth books; the circumstances that led to this are never expounded upon.
This is invoked in Stephen King's short story The Library Policeman when a character calls in a favor from an old friend. "Tell him he still owes me for the baseballs". However, it does get explained.
In the first Kingdom Keepers book, it's mentioned that Maybeck's Aunt Jelly got her nickname in an incident involving a box of jelly donuts. She seems too embarrassed to elaborate.
In the Knight and Rogue Series, during their third burglary in three books, Fisk complains to Michael that he gave up burglary until the two of them met. Michael tries to defend that it's still always Fisk's idea and they haven't done it in over a year, since the previous book. Fisk mentions a time when they broke into a mayor's house during the Time Skip, and Michael argues that if you're returning something someone else stole, it doesn't count.
Landnámabók (Book of Settlement) is an early medieval Icelandic text which describes the first settlers of Iceland, some of their relations and where they took land. A somewhat dry read, if not for the fact that half the people mentioned have such outlandish names that no fantasy writer could come up with them. Some of the people have the stories behind their names fleshed out in the Icelandic sagas but many don't, making us ponder how people like Thorir Giantbuster or Olaf Witchbreaker got their names.
The Laundry Files: Bob makes reference to several OPERATION CODE NAME scenarios which are never elaborated on.
There is also what happened when Bob responded to one of the myriad 419 Scam-emails he gets for a lark. He told the scammers the truth; that he was a mid-level operative in British Counter-Intelligence. Then things happened and it all ended with him getting yelled at by his boss and told to give the scammers their bank back.
A Lion In The Meadow: The boy's mother chastises him for making up stories again, implying that he's made up a story before.
Two examples are with the character Aragorn. When we first meet him in Bree there is a mention of a dark shadow passing over his face at some memory with the Ringwraiths. Later in the book he mentions once going through Moria and having an "evil" memory of his time there. Nothing is said again about either incident. The character's precursor, a hobbit named Trotter, had a backstory concerning being captured in Moria and taken to the Dark Lord in Mordor, but considering Aragorn's and Mordor's final form it is doubtful that this early snippet of the backstory remained canon by the time LOTR was published.
Tolkien had a lot of references in The Lord of the Rings to past events and people that were never explained there, but most of them already "existed" in The Silmarillion. He wrote in a letter that the only things that didn't have some sort of "existence" were the missing two wizards and the cats of Queen Berúthiel — but he eventually wrote stories about them, too. He thought Noodle Incidents added depth to the story.
The Madgie series actually runs on this, explaining why it is titled the way it is, although this is justified in that Word of God states that she like the readers to play with their imaginations on it and neither does she herself know, along with the fact that the stories are usually told from Bunny's POV and she doesn't know, either. Even when Madgie's POV is shown, it's not revealed either.
Malazan Book of the Fallen: In book 8, Toll the Hounds, Spinnock Durav mentions that his Lord Anomander Rake once sent him hunting a dragon, among other tasks. One would think this would be elaborated upon, seeing the importance of dragons in the storyline, but... nope.
In Maniac Magee, the entire 12th year of Maniac's life is completely unaccounted-for in the urban legend's lore.
From MARZENA: During their first meeting, Marian talks about Helena's past as if the reader was already familiar with the Section's MO (or existence) and of who Livia is. "You want to know why you're here? ... It's something you said to Livia..." (Head Scracthing Reader: Who's Livia??) Also We never get to see the first meeting between Livia and Helena and Book 1 makes mention of it in this way.
In Alexei Panshin's Masque World, a character made his reputation in the Imperial Service thanks to his role in solving the Diced Strawberry Affair — which is a code name for something far more sinister, but we do not find out what.
The Mediochre Q Seth Series contains occasional references to exploits of Mediochre which took place before the start of the first book. Most of them go unexplained. Perhaps the oddest is that Mediochre - a Non-Action Guy and Technical Pacifist - once (presumably successfully) fought off a horde of Skeletons with his teeth, because one of his feet was "stuck" and his hands were "full" (it later implies they were full because he was holding The Lancer, Joseph, who was unconscious at the time.).
In Mishaps, there is the 'school camp incident'. What exactly happened wasn't made clear, but Pen says that it involved her spending time in a decontamination chamber.
In Moby-Dick a character mentions "that deadly skrimmage with the Spaniard afore the altar in Santa." This may be why God is pissed at Ahab.
Exactly who named the whale "Moby Dick" and why is also a complete mystery.
Every once in a while, this is completely serious. For instance, the narrator of The Monsters of Morley Manor by Bruce Coville, possesses a Mook at one point, and starts to remember parts of the mook's Training from Hell. He "still can't talk about" when four trainees were locked in a room with only enough water for two of them to survive until they were scheduled to be released.
Myth Adventures likes this. How exactly a game of Dragon Poker led to Aahz's clothes being two floors below him at some point is never explained, although considering that a Trollop was involved, it may not be that tough to understand why he wasn't wearing them.
In the Nero Wolfe novels by Rex Stout, Archie (the narrator) occasionally hints at previous cases that he'll "write about later", or that "can't be revealed for reasons of privacy/security".
There are also references in many novels to an incident early in Archie's association with Wolfe, after which he resolved to never go out of the house on a murder case without carrying a gun.
The Neverending Story is littered with these; you can't go more than a half-dozen pages before the author mentions that such-and-such a character did x, y, or z, then adds, "But that's another story and shall be told another time." It became a plot point later on — Bastian nearly couldn't leave Fantasia because he had to finish all those stories.
Nursery Crime: Played with in The Big Over Easy, in which numerous previous cases are referred to, including the Karma Houdini pigs who 'deserved to fry' for what they did to that wolf, and DCI Jack Spratt constantly having to defend himself against a reputation for being a giant killer ("Technically, only one of them was a giant; the others were just tall."). Whilst no further details are given, any reader who is familiar with fairy tales might spot certain similarities.
Odd Thomas likes to bring up various Noodle Incidents in his memoirs, like the time he got chained to a dead body and thrown in a lake, or the time someone threatened to shove an angry lizard down his throat.
Lennie, the huge but simple-minded protagonist from Of Mice & Men, has a sinister one which is never quite revealed but increasingly implied to be an assault on a small girl.
Origami Yoda: In The Secret of the Fortune Wookiee, Lance asks Sara and her Fortune Wookiee how to save himself from humiliation, because Jen had seen him taking dance lessons the night before. She asks him if Jen even knows who he is, and he says that she hasn't forgiven him due to an incident involving a pudding cup in the 5th grade.
In John Ringo's "Paladin of Shadows" series, there are recurring references to BUDS (SEAL training) Class 201 as a horror story that makes even veteran SEALs shudder. It's never explained, but it's apparently where Mike Harmon and Senior Chief Adams acquired the nicknames "Ass-boy" and "Ass-boy Two".
"Sure," the chief said, standing up. "I've done weirder things." "Really?" the OIC asked, standing up as well as the chief headed for the door. "Yeah," the chief said, pausing in the doorway. "I was in Class 201." "No shit?" Roman asked, his eyes wide. "Jesus, Chief!" "No shit," the chief said, his demeanor suddenly cracking slightly and a shiver shuddered through his body. "After that, being shot out of a B-2 at twice the recommended altitude into a dogfight and a mission with no damned plan or even a damned map . . . well . . . it ain't much."
In The Pale King, it's never revealed what landed Leonard's mother in the hospital, but it apparently involved the kitchen oven malfunctioning. Nor is it revealed what went down at the annual corporate picnic, but it involved mosquitoes, an infectious disease, and some spiked Kool-Aid.
[Peter] would come down laughing over something fearfully funny he had been saying to a star, but he had already forgotten what it was, or he would come up with mermaid scales still sticking to him, and yet not be able to say for certain what had been happening.
The Marquis de Sade's work, Philosophy in the Bedroom, shows this happening after all four of the characters involved have engaged in excessively disturbing acts with each other in various combinations throughout the book. note The LiveJournal user who posted this on the community "Weepingcock" (a celebration of bad pornography) says "We never learn exactly what Dolmance and Augustin do in that room by themselves. Let's just assume that they were engaging in that unspeakable vice of cuddling."
MADAME DE SAINT-ANGE: Is there, do you think, any considerable infamy we are not worthy to hear of and execute? LE CHEVALIER: Wait, sister. I'll tell you. [He whispers to the two women.] EUGENIE:[with a look of revulsion] You are right, 'tis hideous. MADAME DE SAINT-ANGE: I suspected as much.
Mercer, the protagonist of Cordwainer Smith's A Planet Without a Name, has committed a horrendous "crime without a name". We never learn more than that. Well... It's never spelled out explicitly, but if you put the clues given by a few lines together, it's clear that he killed infants belonging to the Imperial family.
Pocket in the Sea specifically uses Noodle Incidents to let the reader's imagination run wild regarding what sailors do on long, boring underway missions. Just enough is described to get a feel for the hijinks.
The Postman: Uses the trope beautifully. There are several mentions of incidents around the lead-up to the Doomwar, such as "Slavic Resurgence", "Cuban problem", "Kenyan front", "Slavic-Turkic War" or "One-Week War", but they are only mentioned and never truly discussed. This is to point out that what happened before, no matter how world-shattering it was, is ultimately meaningless in the post-apocalyptic wasteland.
"The Jenna Thing" in Pretty Little Liars, although it is revealed bit by bit throughout the first half of the second book, after which it turns into a gradual series of Reveals.
Used with great effect in The Princess Bride. Author William Goldman claims to be abridging the original novel by S. Morgenstern, which was really just a literary device that allowed him to write only the "good parts" of the story. In the scene where the mostly-dead Westley is to be revived by Miracle Max, Goldman writes about how Max sent Inigo and Fezzik out to collect different ingredients for the miracle pill — but doesn't actually show the trouble they run into in the process. The author actually does this throughout the book, stopping at various points to put on his italic typeface and explain that when his father read the "original" to him as a child, the man would do the exact same thing, but far more efficiently because instead of stopping to point out he was editing for readability, he'd just make a glossing-over comment and skip a massive whack of the book. He'll usually give a general idea what goes on and then get on with the swashbuckling and ROUSes, but he subverts it at least twice: he'll describe how his father made the shortest glossing-over of all, and then describe in detail what his father skipped; the things that went on in those pages, how many pages were devoted to each, and all that. Then he'll describe how it was all the incredibly boring lengths to which Morgenstern liked to go to in order to satirize the upper crust. The whole thing takes two pages minimum each time.
In L. Jagi Lamplighter's Prospero Lost, Mab ends up hanging in the closet at the Christmas feast. He tells Miranda he doesn't want to explain.
In Pyramid Power, people keep bringing up that a minor character managed to get arrested for pizza. Nobody who wasn't present when it happened has the slightest notion as to how.
Relativity has a few of these, and most of them seem to involve Ravenswood's cousin Dolan. One of them is specifically referred to as "The Spaghetti Incident".
Rivers of London: In the short story "The Home Crowd Advantage", among the reasons mentioned for why Peter isn't part of the police presence at the 2012 Summer Olympics is "the thing that happened in Kew that was totally not my fault". The incident is brought up again in The Hanging Tree, where Peter admits he probably shouldn't have used the word "Krynoid" in his report; and The Furthest Station, where he mentions meeting a ghost under Kew Gardens and says that he'd like to learn more about her if they ever let him back in.
William Faulkner uses this in his short story A Rose for Emily. A clergyman is persuaded to call on the reclusive title character. "He would never divulge what happened during that interview, but he refused to go back again." Considering that Emily was harboring the decomposing corpse of a prospective husband, this is not surprising.
British author/screenwriter John Mortimer perpetrated what was probably the greatest inverted Noodle Incident in the history of the trope. In every script/story from the beginning of his Rumpole of the Bailey series of TV screenplays and short story/novel adaptations, the protagonist, barrister-at-law Horace Rumpole, would invariably make at least one reference to his greatest professional triumph, the case of "The Penge Bungalow Murders" (which Rumpole tried and won as a junior barrister "alone and without a leader"). After nearly three decades of teasing viewers and readers with references to this case, Mortimer, nearing the end of his career, finally wrote a novel (titled, unsurpisingly, Rumpole and the Penge Bungalow Murders), which turned that epic Noodle Incident into an epic series of Continuity Nods.
Septimus Heap has too many examples to count them all, including the so-called Wizard Inquistion, The Great Fire etc.
The Great Fire is explained in Book 7, Fyre. Julius Pike was showing a visiting wizard the alchemie Fyre, and to impress him, threw the Two-Faced Ring into it. The ring settled on the bottom of the cauldron holding the fire and Migrated through it, cracking the cauldron and causing a disaster. Flames reached up through the Fyre venting system and started the Great Fire.
Noodle Incidents are plentiful in A Series of Unfortunate Events. This is partly because, often when the narrator describes a particular kind of situation, the examples are so bizarre and specific that they are obviously references to the experiences of Mister Snicket himself.
In Book the 12th "...'Exit by mutual agreement' is a phrase that means you wanted to quit, and your employer wanted to fire you, and that you ran out of the office, factory, or monastery before anyone could decide who got to go first."
"...Someone who writes twelve or thirteen books in a relatively short time is likely to find themselves hiding under the coffee table of a notorious villain, holding his breath, hoping nobody at the cocktail party will notice the trembling backgammon set, and wondering, as the ink stain spreads across the carpeting, whether certain literary exercises have been entirely worthwhile."
A large part of the reason for so many passing references to unexplained stories has to do with the entire series' themes of secrets and mysteries. Even some of the most plot-important items and pieces of information remain Noodle Incidents and Implements until and after the end of the series.
Even the ending to the last book is a noodle incident.
Older Than Television: As students of the Sherlock Holmes canon know well, Dr. Watson liberally sprinkles various Noodle Incidents in his narratives of Holmes's cases. Sherlockians have long been tantalized by references to such matters as "the shocking affair of the Dutch steamship Friesland, which so nearly cost us both our lives," the case of Wilson the notorious canary-trainer, the repulsive story of the red leech, the story of "the Giant Rat of Sumatra, for which the world is not yet prepared," and the Curious Experience of the Patterson Family on the Island of Uffa; for some reason, Dr. Watson never got around to writing these adventures up for publication. These references have been a fertile ground for amateur Sherlockian Fan Fic and professional Sherlock Holmes pastiches alike for years..
Exploits of Sherlock Holmes, written by Sir Arthur Conan Doyle's son Adrian Conan Doyle, is a short-story collection consisting entirely of cases Watson referenced in the original works.
Likewise, a number of these Sherlock Holmes incidents were transferred almost word-for-word into one of the Star Trek novels, with Spock reminding Kirk of them.
All-Consuming Fire attempts to explain these "missing" Sherlock Holmes adventures by cramming them all into the same adventure, to the point where it's really not funny.
Fred Saberhagen takes on "the giant rat of Sumatra" in his novel The Holmes-Dracula File. Of course, the title of that novel rather implies a few other Noodle Incidents along the way.
In an author's note by Dennis L. McKiernan, McKiernan refers to this Holmesian trait of cases that never were and tells of imagining a reference of the case of the "red slipper" — i.e. a case or item that you'll only be teased about and never get to see what it was all about. McKiernan also has the habit of sprinkling his epilogues heavily with these "red slippers".
The BBC radio series has created episodes from references made by Watson once they ran out of Canon stories to adapt. Example: The Ferrers Documents, from a line in The Priory School.
Moriarty by Anthony Horowitz has Athelney Jones refer to the Abernetty case (the one mentioned in "The Adventure of the Six Napoleons" with the parsley in the butter) and seems to be making it more of a Noodle Incident, with details like three identical figurines and the fold in a former convict's sock. Subverted, however, as the case is included as a short story ("The Adventure of the Three Monarchs") at the back of the book.
In The Spectre's Shadow, an accompanying prequel to the Nyctophobia books, Watson's narration in the first chapter says he and Holmes had just returned home after something called "the Paris case." Word of God confirmed this is intended as a reference to Sherlock's Lawyer-Friendly Cameo in Leblanc's Arsène Lupin stories.
Many of the events of the Great Tourney at Harrenhal before the War of the Usurper take quite a few books to come to light but are referred to from various perspectives fairly frequently before they do, if they are explained at all. And when we finally do get an explanation, it's a second hand account disguised as a fairy tale. And the aspect that has the most relevance on the plot of the books (i.e. the details of Rhaegar and Lyanna's hook up) are left "for next time."
Perhaps equally important is the tragedy of Summerhall. Referred to multiple times by multiple different people. A lot of the things that it caused can be inferred from historical context, but you have to wonder just what it was that was bad enough to kill the king and crown prince, as well as reduce the entire castle to smoking ruins. Summerhall generally comes up when someone mentions wanting to raise dragons; whatever happened at Summerhall is a good indication that it's madness to do so.
Also the Doom of Valyria. The characters all seem to know what happened, and thus, never explain it for the readers' benefit; that being said, hints indicate that Valyria was a thoroughly-settled large volcanic island that erupted.
In Space Hostages by Sophia McDougall, it's the attempted escape of Rasmus Trommler.
In the sixth Spellsinger novel, a dragon who is intending to eat the party states if they can make him laugh he "might" let them live. So Mudge the otter tells this very long joke involving a baker's college, a traveling ladies choir, six chimps and an elephant, of course we don't get to actually hear it. But it was apparently so funny that even Cautious (a raccoon who is rather stoic) was rolling on the ground laughing.
In the Twenty-Eighth Voyage of The Star Diaries, Ijon Tichy admits that his grandfather Jeremiah was unpopular because "not everyone knew how to take his humour"; hence
the affair with the milkman and the two mailmen, who doubtless would have gone insane anyway because of hereditary predisposition; the more as the skeletons were on bicycles and the pit's depth was no more than two and a half meters.
The Selelvian-Tholian War from the Star Trek: New Frontier series (started due to the events of Gods Above and Stone and Anvil) isn't shown, because we skip ahead three years to the next book. All we do know is that the Federation won, Admiral Jellico no longer hates Captain Calhoun, and Soleta was drummed out of Starfleet after her Romulan heritage was revealed after saving Captain Shelby from an Orion raiding party.
Prior to that it was the Grissom incident that Calhoun refused to discuss until The Captain's Table #5: Once Burned.
Similarly, in Star Trek: Destiny the U.S.S. da Vinci makes a planet disappear to save it from the Borg. It's even mentioned in a follow-up book that one of the engineers that used to be assigned to the daVinci won't tell how they did it (mostly because he wasn't there and doesn't know, but also because he's having too much fun keeping people hanging in suspense).
Tales from a Galaxy Far, Far Away Volume 1: Aliens: The climax of "The Crimson Corsair and the Lost Treasure of Count Dooku" involves the reveal that the titular treasure is actually the clone trooper Kix, the 501st Legion's medic. Kix's panicked rambling after he's unfrozen, and what he tells the pirates who freed him later, establishes that he discovered the clones' control chips, the reason for Order 66, and tried to go public with the information, leading to him being hunted down and captured on Coruscant before being frozen to be shipped to Count Dooku for interrogation, only for the ship carrying him to be attacked, leading to it crashing on a remote world and not being discovered for decades. The exact details of Kix's investigation and capture are still unknown, however.
Thrawn: The title character met Anakin Skywalker sometime during the Clone Wars. During that meeting, he first learned of Chancellor Palpatine and deduced that the Chancellor was making Anakin some kind of servant, something the Jedi himself was unaware of at the time. The incident would be resolved in the sequel, Thrawn: Alliances.
The Bacta War has the Rogues leave the New Republic and worry about not having as many support personnel as they had before, meaning they might have to start performing some nonmission-specific duties.
Tycho: I seem to recall the meal you tried to make out of tauntaun meat on Hoth and... Wedge:I get the hologram, Tycho. [...] You know, with the right ambiance, that tauntaun would have tasted just fine. Tycho: Sure, Wedge, believe that if you want.
In the Stephanie Plum series, Stephanie blackmails her cousin Vinnie into giving her a job by threatening to tell his wife about an incident involving a duck. This is never fully explained, but it's implied that you *really* don't want to know what he did to the duck.
The Stormlight Archive: In Edgedancer, Wyndle uses the one time he had to grow a garden for a keenspren as a benchmark for how weird the conversation has gotten. What kind of conversation he had and what keenspren are, for that matter is left unsaid.
In The Strange Case of Dr. Jekyll and Mr. Hyde, Stevenson never goes into great detail about most of the things that Hyde does on his nightly escapades before crossing the Moral Event Horizon by murdering Sir Danvers Carew for no reason; the narrative only states that his activities were of an evil and lustful nature. Given the Victorian England setting and what was considered abhorrent for the time, he may have been engaging with prostitutes and drinking heavily in shady taverns, but we can only surmise.
Sticking with David Eddings, in The Tamuli trilogy, no ones knows exactly why the Delphae became shunned and despised 10,000 years ago to the point that their god turned their Phosphor-Essence into Cruel and Unusual Death, and the only entities in the know — the Gods — refuse to talk about it on the grounds that continuing the debate won't solve anything.
In the Teenage Worrier series, Letty often refers to an unspecified embarrassing incident between her and Brian Bolt, involving a bag of flour and a bicycle tyre.
Roger Zelazny creates a Noodle Incident in a sort-of inverted way in This Immortal, when his narrator gives the story behind a song his wife is humming, about a wrestler who challenged the gods and was shortly thereafter killed by someone we quickly realize was the narrator, centuries before ... and then he adds, "Besides, that's not the way it really happened."
This trope is also present in his Thursday Next series when the characters refer to The Great Samuel Pepys Fiasco, originally intended to be one of the books in the series.
In the Time Warp Trio, the three titular kids sometimes meet up with their granddaughters, who are both kids from the future and Distaff Counterparts of themselves. Their granddaughters explain that they're wealthy because in their near future, the boys experience an accident involving a bowl of cereal that leads to them inadvertently discovering anti-gravity technology. They don't want to explain just how this happens, so as not to cause a paradox. But it causes them to wonder — how the hell could cereal and antigravity possibly be related?!
Tortall Universe: In the first book of The Numair Chronicles, Master Ramasu mentions that he was once judged by the god Mynoss. And after he's briefly possessed by the Graveyard Hag, he mentions that wasn't the first time he's been possessed by a god.
Occurs in Tsukihime, Plus Period Talk. At least two incredibly epic battles are glossed over. One, between the Wind mage Forte and Satsunjiki, and one, the big final battle between a sentient forest, Ciel, and Shiki at full power. The first is only seen when the viewpoint enters. Forte is totally, completely defeated. The second, "What happened after that would be needless to say. Since it would be what anyone would imagine."
Broadbent and Sally reference Honduras several times over the course of Tyrannosaur Canyon. All they'll actually say is that Broadbent's father died and Sally handled a firearm well.
The Underland Chronicles: A conversation between Vikus and Ripred in Gregor and the Curse of the Warmbloods gives us this little gem:
Ripred: Well, you have to at least credit her [Nerissa] with a certain instability. Remember when she told you I was plotting to take over the Fount with an army of lobsters? Vikus: You did try and take over the Fount with an army of lobsters.
Unique has a heated conversation in which an incident in Nevada is referenced. Afterward someone inquires about the incident in question.
Oh, it was before my time. Granny had to go deal with something at Area 51. Oh, isn't that the place where they build the experimental aircraft? It is now.
Throughout the Void City series, Eric makes a number allusions to a very strange week he once had in El Segundo. It apparently involved him killing a demon, preventing the apocalypse, getting shot by a very stupid vampire hunter who had carved crosses on his bullet casings rather than the bullets themselves, and managing to accidentally set himself on fire every single day.
There's the "Incendiary Cat Plot", mentioned at least once in Lois McMaster Bujold's Vorkosigan Saga. This may in fact be a reference to the classic filk song "Never Set the Cat on Fire" by Frank Hayes. The song itself is rules for children living aboard a spaceship including the rule mentioned in the title.
Happens often in War and Peace, to the point that it's left up to modern endnotes to explain what the characters are talking about. The trope is most clearly used when characters refer to Dolokhov's Persian adventures.
The Ciaphas Cain'' books are full of vague references to past events. Some are covered in the short stories, but most go unexplained. Thus far.
There is also the growing list of accidents involving the Guardsmen Penlan, which gained her the nickname Jinxie.
"... or the time I found myself charging a daemon of Khorne with just a rusty bayonet and a vial of holy water..."
The Horus Heresy novella Prince of Crows by Aaron Dembski-Bowden has a rather grim example. During a meeting of the Night Lords Legion commanders one character is introduced as the 13th Captain, Naraka the Bloodless. His name originates from a compliance action where his company took a planet without firing a single shot, earning them the name "The Bloodless", and they have sworn an oath to never speak of it again. Sevatar notes in the narrative that he knows what happened, and he likes the story; and he's a deranged sociopath.
One actually happens during the events of one The Wheel of Time book. While Elayne is attempting to divine the purpose of several ter'angreal, she tries waving Fire into one of them. The next thing she knows is waking up in her bed, with her traveling companions wearing highly amused grins and refusing to tell her what happened after she tried channeling.
In Hugh Cook's The Wishstone and the Wonderworkers, the Originator's manuscript is bowdlerized by members of a sinister organisation, effectively obscuring a number of improbable and unwholesome incidents.
From "Selections from the Allen Notebooks", in Woody Allen's book Without Feathers:
"Good Lord, why am I so guilty? Is it because I hated my father? Probably it was the veal parmigian' incident. Well, what was it doing in his wallet?"
World War Z: Some truly chilling examples, as the novel is told through peoples' own experiences, and they don't necessarily take the time to explain the details.
There was something about "those sick fucks at that weapons research facility at China Lake" that disgusted even a hardened war veteran, and also caused said research staff to suffer one of the highest suicide rates among US personnel.
There are lots of those. "The Great Panic", for example, although that is explained. For example, there was a certain incident on Flight 575. Details are sketchy - in fact, there aren't any - but it is strongly implied that a stow-away infected managed to break out of the cargo hold, with fatal consequences.
The Alpha teams. We know they were US special forces units, one of the first military responses to the zombies, tasked with finding and eliminating very specific zombie threats covertly worldwide, a stopgap while the US and the rest of the world geared up for a major, official offensive. We know that they met with great success, and were only let down by the followup offensive not taking place. We know they were intended to be 100% covert. We also know that their battle record is sealed for 140 years, and that one Canadian infantryman, recounting an early zombie encounter during drug ops in Kyrgyzstan, began his account by disclaiming any connection with them.
The first Wyatt novel Kickback contains brief mentions of some of Wyatt's previous heists, such as knocking over the ticket office at the MCG on Grand Final Day, but provides no detail.