Although Walter Moers has something of a soft spot for the Deus ex Machina, he included at least three of these in The 13 ½ Lives of Captain Bluebear. Rumo the Wolpertinger, Nightingale's darkness from deep space, and - oddly enough - Deus X. Machina himself.
Adventure Hunters: While visiting the Gargoyle Library, one of the gargoyles explains that they don't use magic light spheres for illumination because such things will turn them to stone. Marcus does just this later on to capture someone that knows too much.
Age of Fire: About halfway through Dragon Champion, Auron's dwarf friend Djer gives him a dwarsaw (a chain that can produce razor sharp blades when pulled taught) as a parting gift. At the climax of the story, it's used to free the imprisoned dragonelles on the Isle of Ice, starting the rebellion that brings down the Wrymmaster.
The sun-shard at first just appears to be an oddity of NooMoahk's horde, but it ends up being plot-crucial in the back half of the series.
Similarly to Bond, at the beginning of Anthony Horowitz's Alex Rider books and the film version of Stormbreaker, Alex is given a set of gadgets — all of which will be used. In fact, most spy films involving gadgets do this, as if the equivalent of Q has the ability to see into the future.
Some of the later books avoid this and have Alex given gadgets that don't get used. Snakehead in particular has an unusual way of using a Chekhov's Gun - Alex gets given a belt packed with a jungle survival kit. Although the belt gets confiscated by the Big Bad before it can be used (and Alex does end up stranded in the jungle), the fact that it was proves to be an important plot point when it helps Alex to figure out who The Mole is.
Neal Stephenson's Anathem has a character suggest about one-quarter way in using a sextant as weapon against a heavily armed alien space vehicle. About three-quarters into the book, they use a sextant as part of their plan to invade the vehicle.
In the A to Z Mysteries book The Jaguar's Jewel, Dink looks at the case holding the titular jaguar while Ruth Rose feeds the fish, and Josh notices a letter opener. The kids solve the crime by finding the jewel in the fish tank and examine security footage to note when the letter opener changed directions.
Zack Walker, the narrator of Linwood Barclay's novel Bad Move, is obsessively concerned about potential dangers to his family. He hates when his teenage son and daughter leave their backpacks at the top of the stairs because someone could trip over them. Near the beginning of the book, he describes how he once tried to teach his kids a lesson by lying at the bottom of the stairs, pretending that he'd fallen over a backpack and gotten seriously injured. His son panicked and called 911; the paramedics weren't amused. Near the end of the book, a bad guy has broken into the house and is trying to kill Zack and his wife. Their lives are saved when the bad guy trips over a backpack that was carelessly left at the top of the stairs, causing him to fall down the stairs and accidentally stab himself with his own knife in the process.
The bronze dragon. Festus first appeared in the short story collection The Demigod Files, and later becomes the figurehead of the Argo II.
And now Terminus. Seemingly a one-off gag, but then comes back in the climax to help Percy defeat a Giant.
Gorgon blood, which is used by Percy in a gambit to get rid of Phineas.
Chinese Handcuffs, which are used by Annabeth to trap Arachne, who is guarding the Athena Parthenos.
Iapetus/Bob the Titan, who appears in The Demigod Files, and later helps Percy and Annabeth get through Tartarus.
In Sandra Cisneros's Caramelo, it is played straight with people as well as the reboso, but completely inverted with other objects, to the point that every setting the narrator goes to will have rooms or objects lavishly and poetically described, and then almost none of them will ever come up again in the story.
Pervasive in Catch-22. Many characters or events are briefly mentioned only to become fleshed out in later chapters. E.g. Major Major.
Used in The Chronicles of Prydain, several times. In The Castle of Llyr, Eilonwy gives Taran a horn as a goodbye present. It later turns out that the horn can summon Fair Folk aid once, for any situation and is used to save someone Taran cares about (not that it works out so well). In Taran Wanderer, Kaw keeps insisting on bringing Taran a piece of bone, which handily turns out not too long after to be the one thing keeping an evil sorcerer alive. Doli receives the gift of invisibility at the end of The Book of Three, which is used in virtually every book after when the group needs someone to spy or sneak around. And then there are the two biggest: Dyrnwyn and Eilonwy's ring. Both are introduced in the first book, briefly mentioned in several others, but only given real emphasis in the last book, when it is revealed that Dyrnwyn is the only weapon to kill the Cauldron-Born and Arwan and that the ring has the power to grant Eilonwy any one wish of her choosing (which is used for her to renounce her magic powers and marry Taran).
The moonshine in City of Devils is mentioned in Nick's first trip to the storage room at the Bomb Shelter, and in his second visit is used to knock Sam Haine the pumpkinhead unconscious.
If Tavi learns a new skill at the beginning of a book, expect him to use it on the Big Bad at the end. Some even show up several books later.
At the beginning of the third book, Tavi and Magnus are making a catapult from an old Roman design as an experiment. Max promptly breaks it, and the plot moves on to more important things. At the end of the sixth, Bernard and Amara turn out to have installed hundreds of them along defensive walls in Calderon, and when they're loaded with a ton of little glass spheres full of fire furies that even kids and grannies can make, they deal more damage to the Vord than the Persons of Mass Destruction.
Also in Mistborn: The Original Trilogy, Vin's earring, given to her by her mother, was actually a Hemalurgic Spike, which was the reason why Ruin was in her head and why her bronze was able to pierce copperclouds.
Averted with Kelsier's bones in The Hero of Ages. Ten Soon takes his bones with him in a satchel after using them, but he never ends up using them again.
Warbreaker has what Word Of Adonalsium confirms is a deliberate Inversion of this trope. During his prison break, Vasher Commands his cloak to protect him. However, he doesn't end up needing the protection, so that particular gun just gets put back in the cabinet without ever going off.
Darkness Visible uses the phrase "The Dark Tide shall rise" as a Chekhov's Gun, and also has the more physical examples of a bottle of nitroglycerine and Marsh's cigarette case.
A major subversion in the Darksword trilogy, where in the final book it turns out that the prophecy driving most of the plot was not referring to the titular Darksword after all.
In the Darkwing Duck spinoff book The Silly Canine Caper, Launchpad's headphones and radio appear early in the story when he pays too little attention while dancing to music and upsets a bunch of papers. They show up again more importantly later when it turns out that using them protects the wearer against the silly signal Matronic developed.
The seemingly insignificant teddy bear Sal remembers in Day of the Predator is later revealed to suspiciously be in the costume shop near the team's archway. Later, this is one of the key plot details when the team realise that they're support units.
Used a lot in the Deltora Quest series. If it's mentioned or introduced near the beginning of the book, it'll be relevant later (or way later in the series). Most of which are relevant to puzzles (e.g. the names of the Diamond Guardian's pets), plot twists or eventual reveal.
In Deryni Rising, when Morgan, Duncan and Kelson go to Brion's tomb to retrieve the Eye of Rom, Kelson wants to leave something behind, and Duncan produces a gilded silver crucifix, which Kelson places in his father's hand. The next morning, the guards on duty outside the tomb have had their throats slit, Brion's corpse is taken from his tomb and stripped of its robes and jewels, and the nobleman commanding the guard is found dead as if by enforced suicide with the crucifix in his hand implicating Duncan in the night's events.
Also in Deryni Rising, the flask Brion sees Colin of Fianna drinking from during the hunt at the start of the book. Brion jocularly asks for a share (thinking it's from Colin's famous wine-producing region), and Colin obliges. It turns out Charissa drugged the wine and gave the flask to Colin in hopes Brion would drink it. The flask turns up years later when Bishop Arilan produces it and reveals he took it from Colin on the day of the hunt, giving Kelson, Nigel, Morgan Duncan and Dhugal a major shock.
In Stephen King's Desperation, a shotgun shell becomes a key item in the last few pages of the book. They use it as a blasting cap to detonate explosives that trap a demon / evil god in an abandoned mine.
Used straight in The Colour of Magic where Rincewind rescues a small green frog from the ocean that ends up saving his life.
Another instance in the same book has Rincewind throwing a bottle of wine at someone in an effort to distract him and escape; the man just uses magic to halt the bottle in mid air. About half a scene later, the magic wears off, and the bottle continues its interrupted journey, right into the face of a guard, distracting him and giving Rincewind the opportunity to escape.
Also used straight in The Light Fantastic. Having been established as a pathetic wizard in The Colour of Magic, Rincewind is revealed to have come by this trait after reading a powerful grimoire and getting a single, powerful spell stuck in his brain. It is this exact spell that must be cast at the end of Fantastic to avert complete annihilation of the Disc.
In Small Gods, the opening paragraphs discuss eagles picking up tortoises and dropping them to crack their shells, and says something to the effect of a tortoise possibly taking advantage of this someday. Close to the end of the book Om, a god trapped in turtle form, gets an eagle to drop him on Vorbis' head (by threatening the eagle's sexual organs), killing Vorbis, and causing the crowd that's watching to become believers of Om.
This takes place in a world where you can inherit scars from your parents and powers from your ADOPTED grandfather. All science on Discworld takes a backseat to the Rule of Funny.
Not just the rule of funny, but the fact that the Discworld runs on Narrative Imperative and the story itself displays the power of mass belief. If the people in the area believe eagles have external gonads and the story being played out requires it, eagles (or this eagle anyway) will have them.
Subverted Maskerade. Several characters point out, in increasingly ominous tones, that the enormous crystal chandelier in the Ankh-Morpork Opera House looks like "an accident waiting to happen", but unlike in The Phantom of the Opera (which Maskerade parodies), the chandelier completely fails to be dropped on anyone. Not that the bad guy didn't try.
Subverted in Feet of Clay where the main mystery of the book is how Lord Vetinari is being poisoned despite his food being safe. Repeated references are made to the horrible green wallpaper in his bedroom, and the implication is that it may have something to do with it, emphasized by the popular theory that Napoleon was killed by green wallpaper (arsenic was once commonly used in green paint). The wallpaper has nothing to do with it, and Pratchett has admitted to getting emails that amount to "We were sure it was the wallpaper, you bastard!" When one re-reads the book, one discovers that the clues to the real murder weapon were there all along ...
Used Straight in Thief of Time where Lu-Tze shows his apprentice how yetis "save" their lives and create a sort of premonition ability. He then proceeds to use it later on. One knows he is about to do so when the fact "they cut off his head" is mentioned, because this is how the ability was demonstrated with the yeti.
Another fine example in Interesting Times where an experimental Discworld cannon is used as teleportation counterweight to send Rincewind to the other side of the disc. They send it back the way it was (ready to fire).
Pratchett can place a Chekhov's Gun so smoothly, you barely even notice it's there. In Reaper Man, Miss Flitworth is seen brewing up rat poison in the kitchen, which appears (along with the chicken's demise) to be purely a part of Bill Door's lessons in what death means to mortal creatures, human or animal. Yet this passing reference also provides the basis for the debut of one of Discworld's perennial scene-stealers, the Death of Rats.
Hogfather has at least two examples. An almost literal gun is Susans poker, a deadly weapon. At the beginning of the book, we are told explicitly that it only works on monsters. At the end of the book, the poker—which we have long since forgotten all about—is used to kill a monster. Elsewhere, Archancellor Ridcully offhandedly mentions that someone with access to part of another person's body has the power to control them. Turns out to be a major plot point.
In The Fifth Elephant, Vimes finds a mortar flare and reads the instructions, "Light fuse. Do not place in mouth." He also explains why it is a stupid weapon since it can't be aimed. Both of these come into play at the end of the book.
A pretty subtle one appears in I Shall Wear Midnight, where the Nac Mac Feegles hitch a ride on Tiffany's broomstick, and Daft Wullie, for no apparent reason at all, decides to start a fire while they're up in the air, damaging the broomstick and forcing them to land. Tiffany's reproach hints that this isn't even the first time Wullie has done this exact thing. But towards the very end of the book, Daft Wullie's eagerness for lighting fires comes in handy, when Preston desperately needs to start a fire, but all his matches are damp and useless... and guess who shows up out of nowhere to lend him some dry matches and help get the fire going?
Disney Fairies: In Four Clues for Rani, Rani does some research in preparation for the next day's treasure hunt, and learns that the fairy greeting "Fly with you" was originally a much wordier phrase. The very last clue has a phrase that they have to say in order to win, but the clue got wet and only the first few words are legible. Rani realizes at the very last minute that those words are the same as the phrase she read the day before, and her team wins.
One of the baddies is motivated to get revenge on John Marcone because her daughter was killed in a mafia shootout. Nine books later, we find out that her daughter is the coma patient Marcone is protecting, the one he stole the Shroud of Turin to try and heal (in Book Five) and the guilt over which motivates him to protect innocents and help Harry out sometimes.
Harry sees a ThreeEye junkie who tells him: "I see the things that follow, those who walk before, and He who Walks Behind! They come, they come for you!". Five books later, He Who Walks Behind is revealed to be the demon Justin sent after Harry. In book nine it's revealed to actually be an Outsider then finally, in Cold Daysyou find He Who Walks Before. As well as revealing the Outsiders' main goal.
Fool Moon: Harry's silver pentacle necklace, given to him by his mother, becomes relevant when the loup-garou can only be killed by inherited silver. The pentacle becomes a makeshift bullet.
Harry's literal fairy godmother, Lea, receives a ritual athame from the book's big bad. Later in the same novel, it's used for an attack on Harry, and it's been referenced in multiple books since, constantly emphasizing its importance. 11 books later, it turns out to have infected her with the adversary, which she then spread to Maeve, causing the entire plot of Cold Days. This also ties together a number of Lea's unexplained scenes in several books, such as the one where she's frozen in ice during Book 8.
Death Masks: while Harry and Ebenezer McCoy discuss Harry's astronomy lessons under McCoy, they remember when they discovered "Asteroid Dresden", which turned out to be an old, disused Soviet satellite. At the end of the book McCoy drags the satellite from orbit and drops it on the mansion of a Red Court duke, in retaliation for cheating in a duel against Dresden.
Cowl stated that there was so much that happened at that party that Dresden was either unaware of, or does not comprehend the consequences of. To quote Harry, "What an incredibly fucked-up night that was."
Where he throws a paragraph in describing the awesome nature of Sue, the near-complete Tyrannosaurus rex skeleton at the Field Museum. Seems fairly innocuous, since... well, it is cool. He later raises her from the dead in the single most awesomely overblown moment in the entire series. I doubt anyone saw that coming on their first reading of the book.
At the beginning of the book, we're introduced to Butters' one-man polka suit. Butters, being the Bunny-Ears Lawyer he is, refuses to leave it behind to be mauled by zombies and Harry has to wrestle it into the back of Billy and Georgia's SUV. The polka suit's drum happens to be the only thing around that's the right size to keep a beat going to control the zombie dinosaur.
Small Favor: He even sets one up by the prominent failure to mention something that should have been there. Harry's an Unreliable Narrator, so when Mab messes with his head, we get the effects as well.
Dying Embers: Amadahy Love's canceled trip to Haiti with the charity group Mamma Baby Haiti foreshadows what she will actually be doing for the duration of the books.
In An Exercise in FutilitySpurrig carries around a magic torture knife that bonds a person's soul to his body after death. Another character is a necromancer.
In Father, Forgive Them, a motion-activated talking frog and a magic eight-ball are mentioned to be on the counter near the light switch. When Dr. Brandt is holding Father Wolfe at syringe-point, Father turns on the light, which sets the frog to talking, which distracts Dr. Brandt long enough for Father to smash the syringe with the eight-ball.
In Cold Comfort, the electronic equipment mentioned in the beginning of the book is hooked up to force a public confession from the murderer.
In Zero Tolerance, a student threatens to accuse Father Wolfe of molestation unless given a good grade. The teacher who had fixed the computers in Father Wolfe's classroom back in Cold Comfort had left a small hole for wiring between their two classrooms, through which he videotaped the student's threat.
Early in Ryan Graff's The Fires of Affliction, protagonist Khan Eilon kills a giant wolf-man with the help of Mysterious Waif Lori. Later, when he and his party come across its skeletal remains, he removes two of its fangs and gives one to her. Near the end of the book, when Khan and Lori have been imprisoned and separated, Lori hides the fang from her captors, then uses it to interrogate one of them.
Royal spymaster Elayne Arnheim demonstrates a flesh-colored wristband used for concealing weapons. Sir Roland later uses one to escape from his cell.
A literal example: Early on, Khan finds evidence that Reverse Mole Arikk Tresbitt may have some kind of superweapon. Arikk later reveals the weapon one of the world's first firearms and uses it to gun down the kingdom's greatest swordsman.
Freya has plenty. There's the piece of Ahriman Garen shows Freya, right at the start of the story, which leads to her seeking out the spot it teleported him and ensures she runs into Samantha Drass mid-escape. Finemdi's space-warping magic comes back when Freya realizes she can break it to kill Impulse Station, and their interconnected shortcut doors come back when Freya uses one to dump lava all over Gideon Drass.
The Gauntlet 2017: The Turkish Puzzle Rings Madame Nasirah gave Farrah before the game proper began turn out to be just what Farrah needs to best The Architect and Lady Amari. She challenges them to see which of them can assemble it the fastest. Farrah figured that, since The Architect was a kid when he came to live in the Gauntlet, he never reached the age where he could be introduced to the puzzle. She was right. He doesn't know how to fit the rings together, and just gives up.
Lampshaded and averted in the comic neo-noir Get Blank with the flamethrower in the backseat of the Man In Black's car. Blank quips, "What is that, Chekhov's Flamethrower?" but it never appears again in the book.
In The Go-Between, Ted Burgess shows Leo how to clean his farmer's shotgun. He later uses the shotgun to kill himself.
The M25 London orbital motorway in Good Omens seems like just one more example of how Crowley was able to use a little work to generate a lot of evil. It ends up making it very difficult for several major players to reach the site of the Apocalypse, and perhaps worse destroying his Cool Car.
The first three titles of a six-book children's series called The Great Railway Adventures, revolves around the Holden family wanting to enter the 1939 New York World's Fair for a dome car design, but the kids Tuck and Billie have to stay home as they cannot afford tickets for all of them (three more books, dubbed "Series 2" focuses on different adventure). The rest of the series involves them accidentally getting on a train, and following their parents to the fair. Anyway, books one and three both had one. In the first book, The Daylight Limited their father mentions that two short whistle blasts means go forward, and four short blasts means break. Seems useless until the train car which Tuck and a famous movie star (Loretta La Rue) are in gets disconnected from the train due to a rockslide, and Billie blows the whistle four times as a way to tell Tuck that there is a handbrake in the car. And in The Torpedo Run, Mr. Winterbottom, president of the railroad mentions the steam lever makes things so smoggy you can't see a thing. This comes useful when two newspaper photographers named Scoop Jackson and Tiny stop the train to takes pictures for the paper (one is incredibly strong) and it looks like the won't get in time for the fair, Tuck remembers Winterbottom's advice, and turns on the steam so the photographers can't see and fall off a bridge into the river.
In Helm, on their first meeting in the library, Leland shows Marilyn "the best hide-and-seek place in [Laal] Station": a nook beneath a window-seat. When Marilyn escapes her kidnappers in Laal Station at the end of the novel, she successfully escapes their notice by hiding there.
There's one that gets set up in the second Heralds of Valdemar book that doesn't become important until quite a few books later. In Arrow's Flight, it is heavily implied, though not outright stated, that Gwena, Elspeth's Companion, is Grove-born. In Winds of Fury, it's revealed that, yes, Gwena isGrove-born, and that it is because The Powers That Be felt that Elspeth would need a special Companion, since she's the first Herald-Mage since Vanyel.
In the first book, two missiles are turned into a whale and a bowl of petunias in mid-air. The whale's thought-processes as it falls are described in detail, but the petunias' only thought is 'Oh no not again'. At the very beginning, reference is made to eating oysters, and elsewhere Arthur swats a fly, both trivial events mentioned for background. At one point he kills a rabbit and makes a bag out of its skin. He teleports to Lord's Cricket Ground, where his sudden materialisation causes a man to have heart attack and die. At the same time, the bag is replaced with another in a running gag, and Arthur mentions a bag he lost at an airport once, which contained a bottle of retsina wine.
Later, he is hijacked while teleporting and ends up in a mountain where he is confronted by Agrajag: a monstrous creature with hugely impractical sharp teeth, who reveals that Arthur has killed every reincarnation of it, including the heart-attack victim, the petunias, the rabbit, a fly, and an oyster. Agrajag then accidentally stabs himself through the brain with the teeth, before self-destructing the mountain.
Arthur escapes by learning to fly note a long-running Hitch-Hikers' joke in the books, where the trick to flying is to be distracted when throwing yourself at the ground, causing you to miss after noticing that his bag has again been replaced - with the one he had lost at the airport.
He rejoins his friends at a party which they are prevented from entering for not having a bottle. The bottle of retsina in his bag solves this, and they enter the party to save the universe. Thus reducing Agrajag's character to a device enabling the characters to overcome an unnecessary obstacle to get into the party. Into which masses of background has been invested over the course of the entire series (including the parts still to come).
We're not done yet. Throughout Mostly Harmless, Arthur is convinced he has Contractual Immortality since at least one of Agrajag's deaths hasn't happened yet. Until it does on the last page of the book.
The first book also has a Gun that used a bit earlier in the series: the beginning of the first book shows Honor's ship getting outfitted with a gravity lance, which she has to figure out a way to use in war games. It is repeatedly discussed how impractical the device is for real combat situation. This same ship is the one she used against the Q-ship mentioned above. In the end, the only way Honor can defeat the Q-ship is by using the grav-lance.
The gravity lance was impractical. The reason it's the only way she can destroy the Q-ship is because it's the only effective weapon she really has because of the weapon refit (which stripped her ship of most of its conventional armaments), and she can only use it by getting suicidally close to the Q-ship. It's mentioned by several characters that she could have done a lot more damage to the Q-ship right off the bat if the ship hadn't been refitted at the beginning of the book. The only reason she won was because of overconfidence on the part of the Q-ship captain.
Which she explained, in great and scathing detail, to the weapon's principle advocate in the Manticoran military hierarchy, Admiral Lady Sonja Hemphill, after the battle, earning the long-term hostility of Admiral Hemphill's allies. Hemphill herself had sufficient intelligence and integrity to get over it she and Honor work quite well together in some of the later books, as some of her other innovations, like missile pods and the gravity-based FTL communicator, turn out to be much more effective than the grav-lance and Honor is instrumental in proving the field effectiveness of several of them.
One interesting use of the trope: After the events of the first book, in which Honor and her crew successfully destroy a Q-ship (essentially a warship disguised as a freighter) before it can spark a war, the ship's home nation demands Honor be extradited for murder charges on the grounds that she massacred the crew of an innocent freighter. It's an obvious propaganda ploy, and nobody pays much attention, but later in the series (after war breaks out anyway), Honor is captured and the murder conviction the court handed down without her present is used as a pretext to ignore interstellar treaties dealing with the treatment of prisoners.
There's a much more literal example in Field of Dishonor: early on we see Honor practicing with her "antique" Colt M1911A1. Sure enough, later on she uses another slug thrower to blow away a man who, like everyone else she kills personally, we're assured deserved it.
Yet another literal example occurs in Honor Among Enemies, in which David Weber provides loving detail of Harrington practicing with her Colt "hand cannon" in chapter 3, only to have her, in chapter 32, unexpectedly pull it out of a remote-detonator box to take out Warnecke's bodyguards and bring him to the justice he thought he'd just escaped.
Horatio Hornblower: Done with literal guns in Commodore Hornblower. Lady Barbara gifts Horatio with a pair of well-made, very modern pistols with rifled barrels and waterproof caps. They're described in great detail. A few chapters later, Finnish-born translator Braun steals them in an attempt to assassinate the Czar and Horatio has to stop him.
In one Horrible Harry book, Song Lee brings an origami pig to school and hides it in her desk, along with four improperly made "prototype" pigs. Later, when a substitute teacher takes away the pig and puts it in the "June box", Song Lee waits until she leaves the room to attempt to steal her pig back. The substitute returns, opens the box, and finds the pig still there, leaving the protagonist to assume that Song Lee failed. She actually didn't, but remembered to fool the teacher by replacing the pig in the box with one of her prototype pigs.
The Schlegel family's sword in E. M. Forster's Howards End, which Charles Wilcox will eventually use on Leonard Bast.
The dumpy, mushroom-colored bonnet in Diana Wynne Jones' Howl's Moving Castle. At first it's simply a cleverly-written joke when the story Sophie tells while making the bonnet comes true, but then in the end, it turns out that Sophie is a somewhat powerful witch without even knowing it - she has the ability to dictate the fate of any inanimate object by speaking to it.
The Hunger Games: In Catching Fire, Finnick is mentioned multiple times to count the bread they receive as gifts rather obsessively. It turns out that he was in on District 13's plan to break the tributes out, and bread was a signal. The district the bread came from indicated the day they'd be rescued, the number of rolls the hour.
The Infernal Devices: In The Clockwork Angel, Will talking about Boadicea later on gives Tessa the strength to fool the Magister.
The first book in the Knight and Rogue Series has a scene early on where Fisk gives Michael grief for an old, now illegal pracitce where nobles would get their wives by giving them a drug called Aquilas to make them compliant, and Michael indignantly insists that for generations no one in his family has resorted to such a horrible method. It's actually mentioned even earlier, within the first few pages, while they're breaking a woman out of what they believed to be a cruel lord's tower, and were worried she may have been dosed. Towards the end of the book they use Aquilas on that very woman, who was in fact a imprisoned murder suspect, in able to break out of her own stronghold.
Larry Niven might just have pulled off the longest delay between the appearance of a Chekhov's Gun and its firing in the history of modern literature within the boundaries of his Known Space universe. In his 1966 short story "At the Core", Niven introduces the Quantum II hyperdrive, which is capable of moving a starship a light year in a minute and a quarter (as opposed to the Quantum I hyperdrive, which moves at a mere 3 days to the lightyear). In Niven's 2006 novel Ringworld's Children, the Quantum II hyperdrive is used for its ultimate purpose: to unilaterally end the Fringe War by removing the Ringworld from Known Space entirely. Thirty-eight years from mention to ultimate use just has to be some sort of record...
Rufus shows off a Mini-Mecha he plans to put to work in construction and he uses it to fight mooks in the climax.
Lydia's stuffed bunny from childhood carries the heart of Kthonia.
Legacy of the Drow Series: Drizzt Do'Urden kills a drow warrior in The Legacy, and takes the warrior's hand crossbow with sleeping-poison-tipped arrows. Later on, Drizzt fights Artemis Entreri, with the contest seeming to end when Entreri falls off a cliff. Turns out that Entreri had a magical item that allowed him to fly when activated, however, and he starts attacking Drizzt from the air. Drizzt pulls out the crossbow and shoots him. Ever try to fly while under the influence of drugs? Don't. It never ends well. Just ask Entreri.
The Legends of Ethshar: With a Single Spell opens with an apprentice wizard who, prior to his master's death, learned only a simple fire-starting spell. Near the beginning of the story he discovers by accident that casting it on a fire results in an explosion. Much later in the book, he casts it into the mouth of a fire-breathing dragon, killing it with a single spell.
Light and Dark: The Awakening of the Mage Knight: Early on, Tyramear gives Danny a dagger to protect himself from future shadow attacks and tells him to never ever go anywhere without it. It seems to have served its purpose when his friends can see it as a dagger instead of a pen, thus proving they have the 'gift of sight' but it turns up again later. In the squire duel. Also subverted He still has the dagger in the climax but realizes it is useless against Gran shadows.
Michael Crichton's The Lost World (1995), the sequel to Jurassic Park, subverts this. Early on, a trailer is mentioned as having a bear deterrent in the form of a button that causes thousands of volts of electricity to runnote well, volts don't "run" but you know what we mean across the outside surface of the trailer. Later on, while two T-Rexes are trying to push the trailer off of a cliff, a character accidentally activates it. It deters the Rexes for about five seconds.
A more traditional gun is a candy bar wrapper that gets dropped by a character, an action that is given way more detail then it deserves. Until it attracts the raptors, that is.
A particular kind of toxin is described in the first novel, as part of the process where the modified nuclei are implanted in the ovum. Later in the book, Grant finds himself trapped in the egg nursery by some raptors and several syringes' worth of the toxin...
A subversion when that same toxin is explicitly mentioned in the second movie, and the character describing it makes specific mention of all its properties (such as it being so quick "you'd be dead before you felt the prick [of the needle].") The gun armed with this toxin is completely unable to save the character when it gets its sight stuck in a net, letting the two T. rexes tear him in half. The gun is then lost over a cliff.
Another one from The Lost World: after a very close call with the T. rexes, Levine says that they're good parents. They're such good parents, that they probably teach their offspring how to hunt, by bringing small or weakened creatures to the nest for them to finish off. Well, guess what happens to Dodgson when the T. rex gets him but doesn't eat him outright. (Also a case of Karmic Death, as the infant T. rex who ends up killing him is the one whose leg he had broken earlier.)
The Lost World has plenty of these: Arby's printout of the Isla Sorna facilities (hint: boathouse and river docks;) Eddie's insistence on adding backup systems and safety devices in Thorne's vehicles without telling anyone; the observation cage with its prodigious resistance to impacts; the maia eggs stolen by King; Levine's damn candy bars; also, the rifles armed with neurotoxins are finally put to good use during the raptor chase.
Early in the third Mercy Thompson book, Adam installs a state-of-the-art security system in Mercy's garage without her permission. The footage of Mercy being raped and beating her rapist to death comes into play at the end.
In Victor Hugo's novel Les Misérables, Éponine writes the sentence "The cops are here" to prove to Marius that she is literate. Later on, Marius uses the note to save Leblanc's (aka Jean Valjean's) life.
In City of Bones, Madame Dorothea's tarot cards: the Ace of Cups was the Mortal Cup.
Also, the lake that Clary and Luke fall into during City of Glass turns out to be the Mortal Glass.
Dorothy Gilman's Mrs. Pollifax: In The Unexpected Mrs. Pollifax, the pack of playing cards a secret agent casually tosses to Mrs. Pollifax as she walks out of his bookstore turns out to contain the microfilm she was sent to retrieve.
Terry Pratchett doesn't only do this in Discworld. Nation features its hero Mau undergoing his tribe's manhood ritual, which ends with building a canoe with an axe that is always left stuck in the same tree by the last person who did the ritual. During his climactic duel with Cox, he notices that the tree he's hiding under has some axe marks in it...
William Gibson's Neuromancer averts this: Molly gives Case a shuriken as a souvenir, and he keeps it with him for the entire book, never actually needing to use it in anger (he comments on this toward the end).
It also plays it straight. Early in the book, we find out that the psychotic showman Riviera projects holograms thanks to a projection unit implanted in his chest, where a lung used to be. The Finn notes "He could narrow that into a pulse, fry a retina over easy." Riviera does just that later in the book to 3Jane's ninja assassin, Hideo. This turns out badly for RIVIERA, as Hideo is very experienced in hunting and fighting in the dark, and is now pissed off and immune to deception via hologram.
Nightmare by Willo Davis Roberts. About a third of the way into the story, a side character finds shotgun shells in the back of their RV. These end up saving them from death when the same side character uses them as a diversion making the Big Bad's sidekick drop his shotgun.
An example involving an actual gun: in Isabel Cooper's No Proper Lady, Joan has a "flashgun" powered by her own blood which she uses upon her arrival in the past to save Simon from being killed by cerberi. She's obliged to keep it hidden for most of the rest of the book, but at the climax she uses it again to put down the Big Bad, nearly performing a Heroic Sacrifice in the process by connecting it to a major artery.
Older Than Feudalism: In Homer's epic The Odyssey, the first book/chapter references a number of spears on the wall at Odysseus's home. At the end, Odysseus and his son Telemachus use the spears to kill the suitors, among other weapons.
In Otherland, Serial Killer Dread's Snuff Films that he records for his private amusement come back to bite him, as they prompt his latest "girlfriend" to make a HeelFace Turn. He shoots her for it, but as she lies there dying, she manages to combine the videos with a nasty computer virus and upload them into his system. This fatally distracts him just in time for the heroes to win.
Philip K. Dick's Paycheck is almost entirely composed of this trope. The hero Jennings has just had his memory erased of the top secret project he was working on, only to discover that before it happened he arranged to substitute his paycheck with several seemingly trivial and useless items, including a small piece of wire. Then he's arrested, whereupon it turns out the wire is just the right size to pick the lock of the squad car's back door. It seems the project was a window into the future, which Jennings used to see what was going to happen to him, and so every single one of the items has some purpose to help him stay alive and out of the bad guys' clutches. Half the fun of the story is just seeing what purpose all of them have.
The Power of Five: Richard is given a tumi knife by the Incas at the end of Evil Star, with the warning that he would come to "hate them" when he had to use it. Its purpose is unrevealed until the end of Oblivion, where Richard has to use it to kill Matt to spare him further suffering at the hands of the Old Ones after they were both captured. This allows Matt's past self to be summoned to the present, whose presence is instrumental to the second and final downfall of the Old Ones.
Also, the way the Old Ones were tricked the first time is, with a little variation, also their downfall the second time.
In Rainbow Six, the need established earlier on for cellphone jamming technology pays off majorly when it helps foil the PIRA attack. The pussy-assheartbeat monitors Tim Noonan tries out also come in handy at the end against the Big Bad's crew.
It's brought up in the first half of the novel Eddie Price smokes a pipe, and is seen at the end of each mission lighting up. Popov notices and from this figures out who Rainbow really are well enough to plan an attack with the IRA.
The first chapter of Ravenheart features Jaim Grymauch being lightly mocked for carrying around a huge two-handed claymore, when the plate armour it was designed to counter became obsolete centuries ago. It's not seen used again, and the reader has nearly forgotten that he owns it, until the final act — where he urgently needs to kill four members of a conservative knightly Order who are wearing full ceremonial plate armour...
In Ready Player One, Wade Watts earns a random quarter after playing a perfect game of Pac-Man and, not being a clue to reaching the Easter Egg, he forgets about it. Turns out it's an extra life in the permadeath virtual world of OASIS. He only realizes the truth after an entire sector gets blasted by the Sixers.
Realm of the Elderlings: Liveship Traders: In the first book, while living aboard the beached liveship Paragon, Amber cuts a hatch into the floor of the captain's quarters to better be able to access her supplies. In the third book, when Paragon is set on fire and the crew trapped in the hold, it is their only way to escape.
In Remote Man, Janet buys a car in poor condition, which makes a horrible noise and has a noticeable crack in the exhaust pipe. During the climactic car chase, the exhaust falls off and hits the villain's car, causing him to flip over.
Revelation Space Series: Chasm City manages to feature Chekhov's Brain Surgery. Early on, we hear about an assassin who used Grand Theft Me to kill and replace a loyal retainer. At the climax, we learn that the protagonist used the same process in reverse, to overwrite his own personality with a different one.
In Rot & Ruin by Jonathan Mayberry, when Benny and Tom go out into the Rot and Ruin the second time, Benny accidentally leaves a bottle of Cadaverine open and spills it on Tom's clothes. Later on, it turns out that the amount of Cadaverine saved Tom's life when he was shot by the Hammer by making the zombies think he was also a zombie.
Lampshaded in Ruins by Eleanor Arnason. There's frequent mention of a BFG brought along on the expedition by the Russians, but it never gets used. At the end the author notes tongue-in-cheek that despite what Chekhov said, you can have a gun in your story without firing it.
In Sabina Kane: Red-Headed Stepchild Adam Lazarus summons Giguhl to test whether Sabina is really the long-lost twin sister of the leader of the Hekate Council by having the demon attack her with an apple wood stake. She survives being staked because she's half-mage. It saves her life in the book's final battle when Lavinia tries to kill her.
At the beginning of The Wide Window, the third book in the series, Mr. Poe gives the Baudelaire orphans some peppermints - forgetting that the orphans are allergic to them. Later in the story, they end up coming in handy - as the orphans take advantage of their peppermint allergy to get themselves out of a sticky situation.
Reading The Bad Beginning the first time, a reader might be confused as to why Snicket is so specific in which hand Violet uses to hold her spoon, or throw the grappling hook. Snicket makes sure the reader knows Violet is right-handed. At the end, Violet foils Olaf's plot by signing her name with her left hand, thus not fulfilling the marriage requirement that a bride sign her name "in her own hand" Also, in book the eleventh, Sunny finds some wasabi in the underwater room. This turns out to be vital in curing Sunny from a near-death infection.
Quite literal use in 'Silver Skull' in The Shadow series of pulps, when a gun The Shadow gives to a companion gets smuggled past captors and across the USA, only to be handed back to the Shadow at the climax when his own brace runs empty.
In Sharpe's Tiger, early on Sharpe mentions his lockpick. Never mentioned again until he and Lawford are thrown in prison and need to get themselves and Colonel McCandless out.
Arthur Conan Doyle uses this trope in a lot of his Sherlock Holmes works. A good example would be The Hound of the Baskervilles. In this novella, Mr. Jack Stapleton is perceived as a quiet and 'nice' person by the account of his profession (at the time when Holmes and Watson meet him for the first time). The profession being a botanist, this means that he should not be considered a suspect for the death of Sir Charles Baskerville. As we read on, it was indeed Mr. Stapleton that caused the death of Sir Charles.
Sisterhood Series by Fern Michaels: Early on in the series, there is a teddy bear called Willie, who belonged to Barbara Rutledge. Barbara's ghost mentions Willie more than once, then eventually it is forgotten. However, at the end of Home Free, Barbara's ghost tells her mother that she is going to give Willie to Jack Emery and Nikki Quinn's child! Gold shields, which give anyone (usually hand-picked FBI agents) who possesses them carte blanche and s/he can answer only to the president, are brought up a lot early on. Later on, they are not even mentioned. However, the book Home Free has president Martine Connor set up an organization that will be composed of the Vigilantes, and there are 14 gold shields, one given out to each member of the organization! Hide And Seek has Mitch Riley, assistant director in the FBI and a J Edgar Hoover wannabe, keeping loads of files on supposedly everyone. Between his wife and the Vigilantes, his files get snatched from him and put somewhere where they'll never see the light of day. However, Deja Vu has the Vigilantes needing to look through those files on Henry "Hank" Jellicoe. It turns out that Mitch not only has files on Henry, but there are at least 6 boxes worth of files on Jellicoe!
In the first Skulduggery Pleasant novel, the main character's (a skeleton) head is a fake: his real skull was stolen by goblins. This is mentioned as trivia at the time, but becomes important when they need a part of him to bring him back from another dimension at the end of the third book.
Smaller & Smaller Circles: The victims' free dental records turn out to be useful for identifying the victims. Also, the shiny object Joanna notices in her crime scene footage which turns out to be a dental instrument.
Lampshaded in Smoke and Ashes: "Raise your hand everyone who's surprised by this." "According to Chekhov, you should never hang a coffee shop on the wall unless you intend to use it."
Y.T.'s scary futuristic anti-rape condom, the"dentata" (which is a real product, by the way). It is mentioned a couple of times offhandedly whenever Y.T. thinks she might get into a compromising situation. Then, when she decides to willingly have sex with Raven, she forgets to take it off.
Also, Y.T.'s skateboard includes a sonic blast device that shatters glass. While this gets used effectively halfway through the novel, it becomes important at the end, when Uncle Enzo gets her a replacement, and is fighting Raven. He uses the sonic blast device to shatter all of Raven's glass knives.
In Someone Else's War, Matteo's mother lends him her necklace for good luck. The necklace winds up saving his life in various and surprising ways, like the time he uses it to trick an enemy into thinking he's holding the ring to a hand grenade.
Needle, the sword given to Arya Stark by Jon Snow to train swordsmanship, ends up as being crucial: It saves Arya at least once and being transformed in the most significant token of her past.
The dragon eggs, a wedding present from Ilyrio to Daenerys Targaryen, simple dead rocks even if they are expensive. Only Mostly Dead: They hatch, giving Daenerys living dragons for the first time after centuries.
The Space Odyssey Series by Arthur C. Clarke: Subverted in 2010: Odyssey Two: In the beginning of the book a mechanism is constructed to deactivate the to-be-repaired HAL 9000 instantly in case it malfunctions again like it did in the first book. The remote control for this mechanism, a rigged pocket calculator, is given to one of the characters. Surprisingly, it is never used and the end of the book reveals that it wouldn't have worked anyway because it had been disabled at some point.
Throughout The Sparrow, the author Mary D. Russell drops hints about subtle changes being introduced or taking place in the alien environment. The protagonists observe these things without understanding their significance. When they lead to catastrophic conclusions, it is quite a shock, even though each is traceable to an earlier chapter and though the story opens by telling you the mission was a disaster.
In Eaten Alive, Tash and Zak play simulated starfighter combat against each other. Zak tries to escape his sister by slingshotting his simulated fighter around a simulated moon using its gravity, and Tash retorts that that is the oldest trick in the manual. Later, riding in the Millennium Falcon, Han Solo helps them escape with that same technique, smugly saying that same line.
In The Nightmare Machine, Lando Calrissian teaches Zak about playing cards and knowing when someone is bluffing him, and then lets him handle a deck-shuffling doohickey, only to have to duck when Zak handles it too roughly and accidentally fires the deck at him at high speed. Lando doesn't take offense, but lets Zak keep it. Later, Zak fires the deck at someone at close range, startling them enough to let him and his sister escape.
The Thrawn Trilogy has an interesting example. Early in the first book, Grand Admiral Thrawn has found a storehouse of the Emperor's goodies. Inside, he hopes to find a functioning, practical cloaking shield, and one other "small, almost trivial, bit of technology." The cloaking shield gets a lot of play in the first and second books, but only at the end of the second book of the trilogy are we told that the "small, almost trivial bit of technology" was a cavern full of Spaarti cloning cylinders, which Thrawn has been using to augment the Empire's manpower. The second book is chalk-full of hints, though.
An ironic version in Camus's The Stranger: Meursault and Raymond get into a fight with some men, including the brother of Raymond's ex-girlfriend. Meursault takes away Raymond's gun so that Raymond doesn't do anything rash. Later on, Meursault encounters the brother, and shoots him for no reason.
The Sword of Truth series features what is perhaps the most long-term genuine Gun. In the seventh book, Naked Empire, Prelate Annalina is arrested in the People's Palace by Nathan Rahl and thrown into its most secure dungeon cell, specifically designed to hold in magic-users. When she is eventually released, she leaves behind her Rada'Han, a collar meant to suppress the magical ability of whomever wears it, which she had meant to use on Nathan. When the final book of the series, Confessor, rolls around, Nicci is placed into custody to be delivered to Emperor Jagang in exchange for him and his Sisters of the Dark not destroying the world through the Boxes of Orden. Eventually, Richard manages to inflict Jagang with dreams of longing for Nicci, such that he leaves the Orden preparations to collect her. Once he arrives, Nicci wastes no time snapping the Rada'Han in that very cell around his neck.
This is to say nothing of the Magic of Orden itself, which was introduced in book one, all but forgotten in book two, and then isn't so much as mentioned again until the final trilogy...at which point it becomes the key to victory on both sides.
You call that a long-term Gun? Shar died at the beginning of book 1, and said that should richard need help of the night wisps, to say her name. He did it near the end of book 10.
The Sword of Truth itself, given to Richard in the first book, turns out to be the real key to unlocking the Magic of Orden in the last book, instead of all those magic prophecy books.
The character Ellysetta has suffered intermittent 'fits' all her life- bursts of excruciating pain with no discernible cause. They later turn out to be a side effect of Ellysetta's empathic link with her birth father, who is being tortured by the Big Bad.
It's casually established that many Fey have been exiled from their homeland. Enough Fae, apparently, to form a village- which shelters the protagonists in book 5.
An oft-repeated maxim is that "Tairen defend the pride." Even when doing so sparks war with the Fey, as it does in book 3.
In the first book, one of the cow facts ends up as this. The fact that cows can walk up stairs, but not down them, is used to fill the school with cows.
The concept is discussed in book 2. The dissolving thread and Barry Barkin's sewing projects both become relevant by the end. And thus, the concept of a Chekhov's gun is in itself a Chekhov's gun.
Those That Wake has the Global Dynamic conversation, which is mentioned early on and elaborated later in the book.
The Thursday Next series is a truly fascinating juggling act of various plot threads that feature all kinds of little moments that pay off down the road, either in the book they appear in or several books later. Amazingly, judging by some statements Jasper Fforde has made it seems he really doesn't do that much planning ahead for the series; instead he just has an amazing memory for everything that has happened so far and can come up with ways to refer back to it all that all make perfect sense.
THE ONE RING. It is just this random magical ring that Bilbo wins from Gollum in The Hobbit, but in The Lord of the Rings it's revealed that it's the most dangerous artifact in existence, and crucial for the return of Sauron, driving the entire plot.
The One Ring is actually a retroactive Chekhov's Gun. The Hobbit was not written with The Lord of the Rings in mind, and the meeting between Bilbo and Gollum was rewritten for later editions after the release of Lord of the Rings in order to retcon the ring to fit this trope.
Almost all of the items given to the Fellowship by Galadriel. Whether it's characters not being spotted from afar due to their elvish cloaks, a supernatural flashlight, magic dirt, or even a belt that only serves to identify a dead character for sure.
Well, except for Gimli's Galadriel-hair. He just made a necklace out of it...
As examples of this trope turning into an Ass Pull, the scene where Galadriel gives these items to the Fellowship was edited out of the theatrical release, yet most of them are specifically referenced during the remainder of the trilogy.
In addition to Galadriel's gifts, there's the random coils of rope that the elves of Lórien stow in the Fellowship's boats and which Sam takes a fancy to, having moaned about not having any rope since they set off. It takes him a while to remember it when Frodo is stuck halfway down a cliff.
The hobbits first acquire enchanted daggersnote of Númenórean make, not Elven; that's why they're grave-goods in the lost North-Kingdom in the Barrow-Downs during Book 1, a relatively unimportant plot point until Book 5 when Merry stabs the Lord of the Nazgûl behind the knee, weakening him for the final kill by Éowyn. This is made possible only by the fact that Merry's blade was specifically designed for combat against the enemies in Angmar, under the rule of this very foe, the Witch-King of Angmar.
On the subject of J. R. R. Tolkien, The Silmarillion introduces a Chekhov's gun in the chapter concerning the creation of dwarves by Aulë, where the Sheperds of the Trees (ents) are created by Aulë's spouse Yavanna to counter their harmful axes. Ents are never mentioned again throughout the book until following the slaying of Thingol in Doriath by the dwarves of Belegost, the dwarves flee eastward to the mountains with the prized necklace of Thingol only to meet the Shepherds of the Trees who rise up and defeat them.
A possible attempt at this is shown in "Unfinished Tales", where in Tuor and his Coming to Gondolin the Elf Ecthelion is described as having a diamond-tipped spike on his helmet. In the only full account of the Fall of Gondolin, written long before, even if it contradicts a lot of Tolkien's later writings, it is mentioned Ecthelion performs a Mutual Kill on Gothmog, Lord of Balrogs, by impaling him in the chest with his helmet and causing them to both fall into a fountain. This is still possible to have happened as in the published Silmarillion it is mentioned Ecthelion and Gothmog kill each other.
When Turgon is commanded by Ulmo of where to build Gondolin he is told to leave armour in a cave. Centuries later the human hero Tuor finds it and is used by Ulmo as a messenger to Gondolin, he is recognised as this by his armour.
Arguably, the One Ring is an example of this trope in The Hobbit itself. It starts out as an ordinary ring that Bilbo happens to find on the ground, only for him to discover that it happens to be a magic ring that can turn him invisible and pave the way for him to get into adventuring. Not bad!
Aragorn is potentially this, he starts out being mentioned when Gandalf gives his history of the ring but by the time he actually appears to Frodo, most readers have forgotten he ever existed, then Gandalf mentions that he had mentioned Aragorn.
In the short story "The Toymaker's Workshop", Mr. Silver takes some supplies from a whimpering box while working on the doll. As it turns out, the box contains the girl he kidnapped and is creating a replacement for.
Early in the real world plotline of The Traitor Game, Francis tells Michael about lightlead glass, which he thought up for their shared imaginary world, Evgard. It's a type of glass that slows down light, meaning what you see through a window made of lightlead glass would be what happened 30 minutes ago. In the climax of the Evgard plotline, Argent uses the view of the half an hour late sunset through a lightlead window to trick some soldiers into thinking there is a fire.
In Universe 3's story The Death of Doctor Island, the monkey that Nicholas kills turns out to be the reason that Doctor Island lets Ignacio leave but not Nicholas. Nicholas killed the monkey out of anger, but Ignacio killed Diane out of curiosity, and because she wanted to die anyway.
A minor, subtle, and almost literal one in Vampirocracy. Leon makes mention about not letting anyone see the bullets in his gun when he voluntarily disarms, and a few mentions are made of the vampires keeping some big secret. It's later revealed vampires have a vulnerability to gold, and Leon's carrying gold bullets.
Elizabeth Moon's Vatta's War: Trading in Danger has two: the model kit and the fruitcakes given to the main character near the beginning both turn out to be very useful by the end, though neither in the way that's hinted at during the various times they are mentioned. The model kit contains the makings of a communications beacon and the largest of the three fruitcakes holds a small fortune in diamonds and a letter.
Videssos: Towards the beginning of The Legion of Videssos, the fact that the Khamorth have a superstitious fear of frogs comes up in the context of a prank (a handful of frogs dropped in every well in town). Later on, it's demonstrated that this fear can be exploited for more than just pranks...
Vorkosigan Saga: Early in Barrayar, Cordelia purchases a swordstick for Lt. Koudelka. This helps to further establish Cordelia's Fish out of Water status on Barrayar (not knowing that non-Vor are not permitted to own personal weapons — a loophole permitting Koudelka's superior officer to issue the weapon gets around this), and keeps the early plot going through a couple of character-driven sequences. The swordstick becomes a vital part of the climactic "shopping trip" to the capital, where both the powerful spring-loaded sheath and the superb blade — as demonstrated when she bought it — come in very handy indeed.
Waltharius: When Walther sets out from Attila's hall, he arms himself with a two-edged sword at his left side and an additional one-edged Hunnic sword at his right (v. 336-338). In the final combat, Walther's sword breaks on Hagen's helmet and a moment later, Hagen cuts off Walther's right hand. Walther at once changes his shield to his maimed arm, pulls out the Hunnic sword with his left and strikes. Hagen, who is taken by surprise, is cut across the face, rendering him unable to continue the fight and saving Walther from defeat.
In Dan Abnett's Gaunt's Ghosts novel Honour Guard, there is a very brief, off-handed scene at the beginning of the novel where Captain Daur is handed a small, insignificant trinket by an old woman who seems insistent that he keep it. At the end of the novel, a psychic vision reveals to Daur and other Ghosts that this trinket is the firing key for Saint Sabbat's massive Chaos-frying psychic weapons system buried underneath her tomb.
Dan Abnett's Brothers of the Snake opens with a Space Marine dealing with a planet invaded by Dark Eldar. Much later, the Marines realize that their purpose there has had repercussions.
As a more minor example, the same Marine is bitten by a snake halfway through the story, his superhuman body absorbing the venom and storing it for his own use. This saves his life in the very end of the book.
In the Warrior Cats novel Sign of the Moon, a reflection of the moon Half Moon sees actually indicates that she must become leader of The Ancients.
The Power Of Three, Jayfeather's stick saves his life twice during the climactic scene.
In Welcome to Night Vale a couple of plastic pink flamingos are sold at Jackie's Pawn Shop to show how routine Jackie's life is. Later on it's discovered that the flamingos can open up the way to other dimensions.
Wet Desert: Tracking Down a Terrorist on the Colorado River: The spillways at Hoover Dam are described in detail, as well as the flood of 1983 when they were put in use. Fred wonders whether he'll see them in use again; sure enough, they are later used to drain the lake when the floodwaters from Lake Powell reach the dam.
The Wheel of Time: Aes Sedai Cannot Tell a Lie. In one of the earlier books, Verin claims to have been sent by Moiraine, who denies having sent Verin. This seems like an authorial error, until many, many books later, when Verin says to a woman wearing a white dress:
"By the way, that dress you are wearing is green."
While decorating a Christmas tree, Coleman carves a candy-cane shiv in Tim Dorsey's When Elves Attack. It comes in handy...
In The Witchlands, it's established early on that Mathew owns a chain of cafés across the continent that serve as hubs for his information gathering network. While the protagonists in the know spend much of the first two books out in the wilderness, the moment one of them returns to a proper city, they bump into such a café pretty much instantly.
In The Witch Watch, London's new electric lights come up frequently, as Simon has never seen them before. It turns out the entire electric streetlight system is a massive magical circle.
Despite P. G. Wodehouse being a writer of light comedy, these turn up regularly in his work. If some seemingly irrelevant object or person gets mentioned in a Wodehouse story, no matter how briefly and casually, there's a very good chance it/they will become a plot-point before the story's over.