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.
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.
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.
Hitchhiker's does this with everything but towels. Towels are clearly stated as important from the beginning.
The character of Agrajag is a mix of Karmic Retribution and Chekhov's Arsenal. In the first book, approaching Magrathea, it is referenced that two missiles were turned by the infinite improbability drive into a whale and a bowl of petunias. The whale's thought-processes as it falls to Earth are described in detail, and it is stated that the bowl of petunias' only thought was 'Oh no not again'. At the very beginning, reference is made to 'eating oysters' to provide background to an event. Elsewhere, Arthur talks about 'this damn fly', before remarking 'Got it!' At the beginning of the third book/Tertiary Phase, it references that Arthur kills a rabbit and makes a bag out of its skin. He teleports to Lord's Cricket Ground - where his sudden materialisation gives a man with a heart condition such a shock he has a heart attack and dies, and Arthur's aforementioned love of cricket is reiterated - and the bag is replaced for some strange reason with another one. He expresses his love for the lost bag, and talks about a bag he lost at an airport once coming back from holiday, which had a bottle of retsina in it. Later on, he teleports somewhere, but is hijacked and ends up in some form of mountain, where he is confronted by a monstrous creature - with hugely impractical sharp teeth - who reveals that all of the creatures Arthur has ever killed in his life were various reincarnations of itself, including the heart-attack man, the bowl of petunias ('cruelly dragged back into life after I had given up'), and the rabbit, 'whose skin-bag', Agrajag noticed, 'he had lost'. Agrajag then accidentally stabs himself through the brain with the teeth, before self-destructing the mountain. Arthur escapes by learning to fly - a long-running Hitch-Hikers' joke in the books, which states that the trick to flying is to be distracted when throwing yourself at the ground, causing you to miss - after noticing the bag he thought he had lost at the airport a long time before. He then rejoins his friends at a party in a flying house, which they are prevented from entering for not having a bottle. He gets the bottle of retsina out of the bag, 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 two previous books and the rest of the book, along with the further two books.
The entire plot sequence above then turns into yet another Chekhov's Gun in Mostly Harmless, in which Arthur is convinced he has Contractual Immortality since at least one of Agrajag's "deaths" hasn't happened yet. Until Agrajag dies on the last page of the book.
At which point, the "hugely impractical teeth" come back into play, as Arthur ended up in the place he was trying to avoid because he had mistaken Stavro Mueller's—Beta, the location that said death was supposed to take place, for a planet named Stavromula Beta due to Agrajag's monstrous form being a tad hard to understand.
One of the baddies in the first Dresden Files book, Storm Front, 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.
In the beginning of The Dresden Files book Death Masks, while Harry and Ebeneezer 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.
Another example which has yet to be resolved. In the 3rd book Grave Peril, 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. 8 books later we still have no bloody idea what the thing is!
In Dead Beat, 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."
As of Cold Days, we know that the athame infected her with the adversary, and then she spread it to Maeve, which caused the entire plot of Cold Days.
Also in Dead Beat: 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. Said 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.
In 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.
Harry's silver pentacle necklace, given to him by his mother, and first mentioned in Storm Front. In Fool Moon, it became relevant when the loup garou could only be killed by inherited silver. Guess what became a makeshift bullet.
Not just in The Dresden Files— Butcher's Codex Alera series is full of them. 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.
Special mention: 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.
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.
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 run 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.
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 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.
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.
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.
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.
The dumpy, mushroom-colored bonnet in Diana Wynne Jones's 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.
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.
In 1408 (just the story, not the film) the lucky Hawaiian shirt becomes a textbook example of the trope.
In Rage, written under the psuedonym Richard Bachman, the lock pocketed by the lead character.
Elizabeth Moon's 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.
Larry Niven might just have pulled off the longest delay between the appearance of Chekhov's Gun and it's 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 it's 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...
Gary Paulsen's The Rifle is a story told from the point of view of a Chekhov's Gun.
In Peter Straub's "Ghost Story", Stella Hawthorne makes use of a Chekhov's Hatpin. Oddly, despite being a somewhat obvious example of the trope, it doesn't really affect the overall story very much.
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.
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.
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's head (by threatening said 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 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.
There's another in Hogfather: 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 hint 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?
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.
Y.T.'s scary futuristic anti-rape condom ("dentata") in Neal Stephenson's Snow Crash (which is a real product, by the way). 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.
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 said heavily armed alien space vehicle.
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 Asspull, 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 daggers *
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 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.
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.
Happens often enough in the Harry Potter series that fans used to obsess over seemingly every little detail in the books in an often fruitless attempt to figure out what would happen in future installments... but only a few picked up on Dumbledore's put-outer, i.e. Deluminator, introduced at the very beginning of book one, which became important in the seventh and final book (a long-term Chekhov's Gun that was apparently too subtle and too weirdly-used for the fandom to easily notice). It's pointedly reintroduced towards the beginning of the book, making it suddenly a whole lot less subtle and a more traditional Chekhov's Gun, but veiling its importance for that long, in hindsight, is impressive given we're talking about roughly a few million obsessive fans here. The Harry Potter series has its own page of Chekhov's Guns, but here are a few notable examples.
One object and action regarding said object is also mentioned in the first book, and built upon in significance in the subsequent books: the Vanishing Cabinet.
And in contrast, the "chess game" scene in the climax of the first book was expected, quite firmly and very widely, to be of help in predicting one of the people who was going to die in the final book. It wasn't.
In the second book, Dumbledore introduces Fawkes the Phoenix and recounts the various abilities of the Phoenix species - heavy lifting, loyalty, healing tears - all of which are used in the final scene, to the extent that Harry might as well have replied "Thanks a lot, Q - sorry, Headmaster..."
For most of the third book, Sirius Black is presented as the main villain. In the very first chapter of the very first book, it was noted that Sirius helped get Harry to safety. (He lent Hagrid his flying motorbike.) This is mentioned in the tavern scene in the third book. Hagrid had an upset rant about how he should've suspected something was wrong when that happened, and believed Sirius gave it away so he wouldn't be noticed while on the run - a flying motorcycle stands out pretty well.
The motorcycle itself fits this trope, as it vanishes after the mention in the third book, only to show up again in Deathly Hallows, when Hagrid uses it to ride Harry to safety.
Several times for Peter Pettigrew. He posed for the first two and a half books as Ron's harmless rat, and turned out to be responsible for betraying Harry's parents to Voldemort. Both Pettigrew's severed finger and the rat's missing one are mentioned in the third book. Then at the end of that book Harry spares his life: now Peter owes him a life debt. In book 4 Peter receives a silver hand to replace the one he severed as a sacrifice to resurrect Voldemort. Finally, in book 7, he hesitated in killing Harry because of the life debt, and his silver hand choked him to death.
In the 4th book there are several times when bad things happen and a bug just happens to be there. It is later revealed that a nosy reporter can turn into that bug and had been spying on Harry.
Within Order of the Phoenix, the mirror is a subversion: Sirius gives it to Harry as a secure way to get in touch in the event of an emergency. Harry never opens the gift: he has no intent to ever use whatever was inside it, not wanting to risk getting Sirius arrested. At the climax of the book, the mirror would have come in very handy, but by then Harry has forgotten about it. Harry only discovers the mirror as he's packing at the end. (It then becomes a Chekhov's Boomerang in Deathly Hallows.)
Another subversion: in book 4 Sirius gives Harry a penknife that can unlock any door and untie any knot. While in the bottom of the lake during the Second Task Harry notices that he could've used it, had he remembered to bring it. Then in book 5 he takes the penknife with him to the Ministry, only to end up ruined the only time he tries to use it to open the only locked door they find. The existence of the penknife also explains why Sirius slashed the Fat Lady's portrait in book 3: he was trying to "unlock" the entrance.
The various Horcruxes tended to be Chekhov's Guns more often than not. Figuring out who "RAB" was before the last book came out was easy, but remembering that there was a locket in the house of Black, not so much. And who would've remembered about the diadem, first mentioned in Half-Blood Prince, hidden in the Room of Requirement? There was also the diary (destroyed in the same book it debuted, Chamber of Secrets, but became important again when the concept of Horcruxes was first introduced four books later), Nagini (debuted in Goblet of Fire, and became a Horcrux shortly after her debut as Voldemort was going to make his last Horcrux when he killed the Potters, but never got around to it (or so he thought), Harry (we've known about the lightning-bolt scar since the very beginning, and his connection to Voldemort established when he first gets his wand, but the big hint that he is inherently "connected" to Voldemort comes in Order of the Phoenix, and the ring, which was already a destroyed Horcrux when it debuted in Half-Blood Prince, but turned out to also be one of the three Deathly Hallows. Only the goblet of Hufflepuff appeared solely as a Horcrux.
In Prisoner of Azkaban, Harry tells Dumbledore that Trelawney went all odd and recited some verse to him. Dumbledore offhandedly comments that it appears to be her second real prophecy ever and he should give her a pay rise. Her first real prophecy turns out to be a crucial MacGuffin in Order of the Phoenix.
There are so many important Chekhov's Guns in the series, in fact, that they can often cause Continuity Lockout in the movies. Conversely, some of the movies set up Chekhov's Guns from the books and then don't fire them. An example is the Chocolate frog card of Dumbledore in the first one. In the book, Harry reads the card on the train but only later notices the reference to Nicholas Flamel, which leads Hermione to the book she had borrowed. In the movie, a scene that was deleted has Harry reading the card to Ron and Hermione, again leading to Hermione finding Flamel in her book.
Half-Blood Price has the bezoar, mentioned in passing at the start of the first book, which Harry uses to save Ron's life.
Deathly Hallows brings forth the Golden Snitch from the first Quidditch match that Harry ever played, which is revealed to contain the Resurrection Stone.
In the first book, when Harry asks Hagrid why someone would be mad to try and rob Gringotts, he mentions some of the security measures: spells, enchantments, dragons possibly guarding the high security vaults. Six books later...
Half the items in the series fall under this. The tent the Weasley's had in the fourth book at the Quidditch Cup, that reappears in the seventh. And, the basilisk fangs. And the Room of Requirement, mentioned by Dumbledore, important in Order of the Phoenix, and then vital in Deathly Hallows. We also have Dumbledore's glass that sends Dobby to help him.
In Chamber of Secrets, Malfoy is seen looking at a cursed opal necklace in Borgin and Burkes. Four books later, he uses it in an attempt to murder Dumbledore.
When Luna Lovegood is introduced at the beginning of Order of the Phoenix, she is seen reading The Quibbler, the Wizarding World's equivalent of the Weekly World News. After Hermione calls it rubbish, Luna angrily says that her father is the editor. This seems to just be a nice character-establishing moment until nearly 400 pages later, when Hermione and Luna team up to get an interview with Harry published in The Quibbler.
Honor Harrington gets this one in an interesting manner. 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 said 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.
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 Gravity 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 gravlance — and Honor is instrumental in proving the field effectiveness of several of them.
There's a much more literal example in Honor Among Enemies: 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.
Said character himself falls into this category: he first appears as an apparently minor scumbag who does manage to escape justice in the first book of the series by being smart enough to know when to get out of Dodge. Honor Among Enemies is the fourth book.
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.
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.
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.
At the beginning of The Wide Window, the third book in the A Series of Unfortunate Events 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.
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 the short story "The Toymakers 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.
An ironic version in Camus' 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.
In the 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.
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.
Although Walter Moers has something of a soft spot for the Deus ex Machina, he included at least three of these in The 13 1/2 Lives of Captain Bluebear. Rumo the Wolpertinger, Nightingale's darkness from deep space, and - oddly enough - Deus X. Machina himself.
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 Heel Face 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.
Used in 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).
Used in almost many of Steven Brust's Dragaera books, but most blatantly in Taltos. Kiera gives Vlad a vial of a goddess's blood for no clear reason at the time, which later in the book, and years later in the story, he uses to resolve a problem years later in the storyline.
Lampshaded in Five Hundred Years After, when Lemony Narrator Paarfi describes the phenomenon in stage plays in terms virtually identical to Chekhov's, except with flashstones rather than guns.
Used a lot in 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.
Sometimes used, sometimes not in the Vorkosigan Saga. Vorlopolous's Law was used as a straightforward Checkhov's Gun in Warrior's Apprentice, likewise Imperial Auditors in Memory. But in Barrayar the story behind the Emperor's Birthday Present is introduced in exactly the same way and then isn't used at all.
Pervasive in Catch Twenty Two. Many characters or events are briefly mentioned only to become fleshed out in later chapters. E.g. Major Major.
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."
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.
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.
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.
Early in Lois McMaster Bujold's Barrayar, Cordelia purchases a swordstick for Lt. Koudelka. This helps to further establishes 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 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.
In Sandra Cisneros' 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.
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...
The Schlegel family's sword in E. M. Forster's Howards End, which Charles Wilcox will eventually use on Leonard Bast.
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.
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.
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.
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 the character Mr. Jack Stapleton is perceived as a quiet and 'nice' person by the account of his profession (at the time when Holmes and Watston 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.
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.
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.
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.
Discussed, lampshaded and subverted at length before being utterly averted in Haruki Murakami's 1Q84.
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.
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 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.
Sisterhood series by Fern Michaels: Early on in the series, there is an 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 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.
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.
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 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
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.
In The Go Between, Ted Burgess shows Leo how to clean his farmer's shotgun. He later uses the shotgun to kill himself.
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.
While decorating a Christmas tree, Coleman carves a candy-cane shiv in Tim Dorsey's When Elves Attack. It comes in handy...
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.
Light And Dark The Awakening Of The Mageknight: 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.
In Galaxy of Fear: 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.
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.
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.
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.