Thief is stated to have Ninja Lawyers. They inevitably prove useless however, since when he finally calls them, they turn out to have been dead for a long time.
Also, Black Mage's ability to absorb ambient evil, introduced at Ordeal Castle, turns out to be a vital part of the endgame, as he uses it against everyone, apparently gaining god-like powers.
The Datasphere, in conjunction with that, proves to be crucial in defeating Sarda and bringing about Chaos.
The Onion Kid, having been continually abused in the strip, is revealed to become Sarda in a ridiculously complex Stable Time Loop.
In episode 7, Black Mage, reading a Nintendo Power strategy guide, says "Four White Mages? It'll never work!" One thousand, two hundred and fourteen episodes later, Chaos, the ultimate god of evil and stuff is defeated by - you guessed it - four White Mages. In the words of Black Mage when he takes out the Nintendo Power again: "Oh, goddammit." The episode in question is entitled 'Longest Set Up In Webcomic History'.
A humorous one when Dan is training Gordito and Dark Smoke Puncher to defend against all the attacks a ghost wizard might use. Last of all, he comes up with "knife eye attack," but it's pretty clear it's a joke. Dan certainly seems to think so. Only, guess what the wizard does when they actually fight him?
While the titular Doctor is visiting Count Dracula's moon base, he learns that Bruce Lee didn't die; he simply completed his career as the greatest martial artist ever by jumping to the moon. Later, when the Doctor must fight Dracula without any weapons suited for killing vampires, he slips off and gives Bruce a visit, and uses his knowledge well.
And then there was the time when he became very excited about the particular model of plane he was riding based on its toilet facilities. He was just in it for a cheap shot at pirates, but you guessed it, it becomes important. In the next strip, in fact.
A totally "don't even need to blink to miss it" detail in "Futures Trading": When the Doctor is researching how to attack the dinosaur base, to free a captive held behind an alpha shield, he's been looking at a chart of the phases of the Moon. In the Final Battle, he calls on Dracula to fire his moon laser, the only thing powerful enough to break the alpha shield.
Orestes' Remains from Aisopos, a Sacred Necklace, whose owner receives supremacy over Greece. It appears as something that Lady Sappho has asked Bri to get her. Then, we learn about its power and that it has lead to a war between Tegea and Sparta.
Annyseed Is dashed with these Just what does Professor Tripadiculous do with his guests? What is all this about Tarkwin's daughter, Juliet? Who was she? Are Perry's words to be taken seriously? And just what is Winston going to do, now that he ownes the love potion?
Early on, Zexion puts a fire cracker inside Riku. A few episodes later, Riku tries to use a fire elemental attack which backfires and causes said firecracker to ignite which splits Riku in half.
In Season 6, Axel is seen fighting a shark. The shark appears for only one panel and its only purpose at the time was to show that the characters actually know that they're world is made of pure insanity. Later on, it's revealed that sharks are the only natural predators against werepires (were-wolf vampires...yeah). Lampshaded in that the shark is actually named Checkhov.
And if you thought that Jesus' miracle hang-over cure was just a stand alone joke, well guess who the team decides to call when time-travelling using alcohol consumption.
Blood Is Mine: In an Interactive Comic, never underestimate the readers' ingenuity at invoking these, as even the author himself has expressed suprise at the most irrelevant elements of character backstories coming into actual play, such as dr Finch having access to lab rats when the party needed to turn one into an Animal Eye Spy.
The rabbit statue depicted on page 3 of Blue Moon Blossom hints towards many things later in the story. Since the comic is mainly just pretty art of pretty, unspoiled landscapes before and a good while after this, the simple depiction of a man-made (rabbit-made?) object against yet more pretty landscapes is subtle enough that a first-time reader might not think anything of it, let alone realize it's a Chekhov's Gun except in hindsight. It turns out to be a depiction of the dino's true form, unseen by everyone but the reader.
Crusader Amulet completely chucks out the Kings at the start. The Red King, who switches his names with his enemy, Jon, later invades Trakf, home of the Guild Registry.
Debugging Destiny has the Crypt Sword. After first meeting it, its status as a Chekhov's Gun is lampshaded by the Narrators. True to form, the Crypt Sword is later revealed to be Tamwen, a sword-shaped spirit of earth and fire, that chooses / is chosen by Elizabeth.
Dominic Deegan: Luna's tusks are revealed to be a consequence of a curse placed on humanity by an orc, and her overcoming the stigma of having them enables her to become the savior of their homeland, a full seven years after she and her tusks are originally introduced.
Chapter 13 of The Dragon Doctors fires one introduced in the B plot of chapter 4 - the egg received in payment from the gorgon-turned-human is a phoenix egg, and is crucial in saving Tanica's life after she sacrifices her lifeforce to save Goro.
Dregs: The missing Blastol from the store becomes relevant later.
El Goonish Shive is somewhat notorious for its use of Chekhov's Guns, many of which have yet to go off. Perhaps the most infamous Chekhov's Gun is Lord Tedd, who was first mentioned way back in the Sister Arc (and who's effects have been around since the Goo Arc, the first official arc of the series) but who's never been officially confronted.
The author of newly launched webcomic Era of Errors has said that many plot elements will be foreshadowed, yet not fully revealed for well over 6+ years.
Roughly halfway through the final battle of Errant Story, Jon loses one of his pistols. The climax of the battle has Meji turn off the magic during her fight with Ian, at which point the fact that he's bigger and stronger than her rears its head. She grabs the gun off the ground and shoots him.
Early in Evil Plan, Kinesis mentions "Digital Personality Profiles" in a list of previous inventions. When Computer's identity is finally revealed, we see that that technology was actually used, with unsettling results.
Commonly referenced in Full Frontal Nerdity, along the lines of "If the DM describes it, it must be stolen, because it is important".
The gold brooch that Garanos wears for the first five chapters of the comic goes unnoticed and unmentioned, but several chapters later was revealed to be the key to restoring peace in her homeland.
In an early chapter, Gil shows Agatha a real Heterodyne device that he's trying to figure out what it does. Shortly thereafter, they have to use it to fight a swarm when a Hive is activated. During the battle, other people notice a weird effect going on. Years later, Gil pulls it out and uses the 'weird effect' brilliantly.
That's nothing. Phil Foglio seems to be a master of this trope; if there's a detail mentioned somewhere in the story, whether a visual cue, off-hand comment or subtle hint, you can bet it's going to be brought back up later to make for either a major twist, minor gag or even both. Perfect example: when Agatha joins up with the traveling performers, she gets hooked onto the idea of equipping them with the ability to defend themselves. However, after showing Krosp plans for "a merry-go-round that can level a small town", it's never really mentioned again......until about three volumes later, when Agatha and her performer friends are about to be executed by Baron Wulfenbach's army, and Agatha gives a special signal. Cue the wagons and circus props suddenly becoming clanks Transformers-style and utterly destroying the Baron's forces. Then, finally, as another character is recounting an event at the end of the battle, he mentions that Captain DuPree was found wounded, and she claimed that her injuries were the results of destroying (wait for it) a merry-go-round.
The golden brooch Agatha wears at the start of the series turns out to be a Chekhov's Armoury all on its own. Among other things, it is the device suppressing her incredible Spark; and much later on it is the only thing that keeps the mind of The Other from possessing her.
Then there's the poison pellet Gilgamesh gave Von Zinzer to use as a suicide pill if he chose. While Von Zinzer has shown up again (alive), the poison hasn't shown up again... yet.
The unearthly powers of the spring from which the River Dyne flows. The Castle gives exposition about how the first Heterodyne found it as a shrine to a local battle goddess, and how it would usually kill any who bathed in it but occasionally granted immense curative powers. Not being one for superstition, the first Heterodyne did the unthinkable: he drank the water. Sure enough, in the course of curing the case of Hogfarb's Resplendent Immolation, Agatha ends up drinking Dyne water herself, finishing the revivification and also proving to the Castle (again) that she is one of the family. And then after exclaiming that she's got to try that again, the Castle is doubly assured.
Another, literal Chekov's Gun: see that weapon Tarvek is holding? The one that looks like the bastard offspring of a blunderbuss and a Kalashnikov? As it turns out, about a month later we find that it pulls to the left.
The Torchmen. Seen before activation here, although we didn't know what they did at the time.
In a particularly long example, an early chapter (in December, 2003) introduces Doctor Vapnoople, who has a penchant for making bears, and is distraught due to all the others being "taken away." In March 2004 it is revealed that, having created Krosp, he was skilled in creating intelligent animal constructs. In hindsight, the appearance, over a decade later, of an army of intelligent bears that were waiting for Krosp to show up and lead themwas probably inevitable.
In the "Ivo Sharktooth, Private Jager" side-story, it's mentioned that the police chief of Mechanicsburg is the only known survivor of an experiment by one of the old Heterodynes to create Secret Police with the power of invisibility. At first this seems like another bit of random local color, but then it turns out that there's another survivor working for the villain of the story.
Forgath's Anymug (magically able to fill itself with any liquid). Turns out there's a highly-flammable liquid Forgath is familiar with, and a cup full of that is very handy when fighting a higher-level enemy with a wooden body.
In the earliest days of the comic, it was mentioned that Minmax knew 38 ways to kill somebody with his thumbs. This came in handy when Psionic-Minmax was made vulnerable only to something based on the number 38.
The handles on Kore's shield, which allows him to get up if he falls on top of it, have been there since his first appearance.
Gunnerkrigg Court fans speculate wildly on every background object and character in the comic because of the number of Chekhov's Guns that have already popped up.
Homestuck has several, though due to how quickly it updates, said guns usually don't exist for long before going off.
Early in the series, PM tries to contact Jade using her terminal, however, due to interference, the terminal blows up. About a year later, it's revealed that the interference was the newly prototyped Becsprite.
Also, in this update, Karkat snarks that Superman is the humans subconsciously admitting to the troll's superiority. Four months later, we discover that every completed session of Sburb creates a new universe. Guess who's responsible for ours.
Ingress Adventuring Company features a few of these, notably in Chapter 3 and 4. Played for laughs in Chapter 3 by showing way too many in the ghost room that are later used to fight off the big bad ghost.
James Stud was given a literal Chekhov's Gun from Ü. Consider that in every James Bond film, every gadgets ends up being useful in some way, this probably is the most useful thing ever. And yes, there's a link to this page (and Red Herring, which the strip also talks about).
From the same author, in Darths & Droidsepisode 240, Morgan-Mar mentions that the would-be assassin of Padme Amidala in Episode 2 possesses shapechanging, and while it is introduced, it is never developed afterwards (in an averted Chekhov's Gun).
In this 2009 Kevin & Kell strip, it's established that R.L. employs a homing pigeon as a living GPS. Four years later, it's shown that Richard Rodent secretly illegally uses a homing pigeon to navigate the Mouscar race maze.
A Magical Roommate is peppered with very subtle plot points, which often don't come into play until hundreds of strips have passed. Even the Running Gag of Aylia transforming her sister became a gun when Alassa decided to make transformation her focus... and later tried to kill a boy by turning him into a rat.
Misfile had glimpses of the Monster XR in Books 1 and 2 before it was fully revealed in Book 3. The liner notes for Book 3 show that invoking Chekhov's Gun was intentional.
The MS Paint Adventures, series Problem Sleuth has both candy corn and 'Sepulchritude', which are introduced early in the comic. The main character has to abstain from their use several times before using them to kill the final boss.
Iris describes her fragile valuables and wonders why. Dave explains that he causes foreshadowing.
One of the many bits of mad science is the Tinasky Case. We see Helen B. Narbon invited to the Mad Science Symposium to lecture on it. We see Dr. Lupin Madblood getting upset about it. It isn't until the last couple arcs that we realize it is the Chekhov's Gun of the entire seven years of the comic... and why.
The Belt of Gender-Changing. This one is debatable, though, as the author himself stated in the books commentary that he decided to use it after "remembering he has introduced it". Therefore, it wasn't meant to be a Chekhov's Gun, although it could still be said that it became one.
In fact, almost all of the items the party looted from Xykon's dungeon apply. Haley's gotten plenty of use out of that Bag of Holding, and Vaarsuvius's Ring of Wizardry was mentioned in passing. The only exception is Durkon's Amulet of Natural Armor, though to be fair, the item has a passive, "always on" type of ability.
Subverted in at least one instance: the comic's forums were wildly speculating about what had happened to a poisoned arrow that was misfired. The next comic featured the arrow, in a highly unlikely trajectory, narrowly missing all the most popular potential targets only to bounce off V's protection from arrows... but said spell is a Chekhov's Gun itself.
Even the cast page has one: Haley's panel used to contain a giant diamond. The cast needed 5000gp worth of diamonds to resurrect Roy, so Haley just took the diamond and replaced it with "IO Me: one big-ass diamond."
The silver dragon shown dead in this strip is likely the one he reanimated and rode into the battle at Azure City.
In Comic 131 we learn that Haley's father has been imprisoned for ransom. More than six real-life years and 627 strips later it is revealed that Elan's father's adventuring party is behind it.
Celia mentioned that Dorukan left a gap in the cloister for summoning spells. Redcloak uses it to wipe out the resistance.
The spell Tsukiko is working on in thesestrips, is eventually confirmed to be part of the Snarl-controlling ritual. More to the point, her studying of it allows her to realize that it doesn't do what Redcloak's been saying it does. Which is followed shortly by the reveal that the spell Redcloak is casting here is Command Undead.
In comic 842, it is revealed that when V cast Familicide on the black dragon, it concerned the Draketooth clan, protecting one of the gates.
In 1105, Hilgya comes back... with Durkon's child. The last time we saw her was in 84, back in 2004. This might be the longest setup in webcomic history.
Outsider: The projector for the Historian's avatar is taken off the Tempest's bridge just before the battle begins. Later on, Jardin finds it among the cargo in the shuttle. Shortly thereafter, a "pocket Historian" downloads itself onto a data pad, telling Jardin that help is coming and warning him that he is in grave danger and to not trust the Loroi.
Penny and Aggie: In the first mini-arc, Penny, looking for a way under Aggie's skin, learns she's sensitive about her late mother, something she imagines using against her until having an attack of conscience. She finally gives in to temptation seven (real-time) years later, or about two in-comic, the two having been close friends several (in-comic) months, when Aggie's admission, to an FBI agent pressuring her to divulge more and more information, that she witnessed Brandi commit a violent act, gets Brandi detained for further questioning in an unrelated case.
ThisRock, Paper, Cynic comic subverts the concept with a play about pacifists in a gun shop entitled "Chekhov Was a Filthy Liar".
Lampshaded with a stealth Pun in Schlock Mercenary. When Tagon's new ship, the Cynthetic Certainty, is under fire at Urtheep Industries, Tagon orders the ship to fire her as-yet-untested main gun. He suggests it can be something on her bucket list she can "check off"; the next frame shows the huge letters "CHEKHOV!" behind the gun's firing.
One Shortpacked! strip has a shot of Mike's closet, in which a McAwesome's uniform is slightly visible (it doesn't take a keen eye to see it, but it's not out in the open) about a year before the plot involving it.
Sluggy Freelance does this constantly. Pete Abrams is also very, very good at disguising the Guns, to the point that in June 2009 he was able to reveal that a character has had the often-used ability to create huge fireballs with her mind all along for about a decade, which no reader had noticed, even though it had been shown several times.
The mask used to trap a thief in Snow By Night later turns out to be a key means of finding out information about manitou. Also, Jassart puts it on and becomes the spirit dancer of a mad carnivorous manitou.
Spy6teen: In the first story, the heroine has to do a boring paper on Schrodinger's Cat for the annoying science teacher, which then lets her meet one of the supervillains in a way that doesn't tip him off. Years later (real world time), the heroine defeats a multidimensional supervillain because she remembers that paper. Even better, the supervillain gets a karmic death because he is the science teacher who assigned her that paper.
Tuuri's family portrait. On his first morning with the crew, Reynir gets a good look at it before getting sent back to bed by Mikkel, who suspects he didn't sleep much during the night. While dreaming, Reynir walks into Lalli's protected mage area, gets thrown out of it, and gets lost trying to get back to his own area, accidentally walking into the area of a third mage in the process. Having seen the portrait is what makes Reynir realize that the third mage in question is Tuuri's older brother.
Reynir's paper-drawn anti-ghost runes. He gives them to everyone right before the visit to the shop in Chapter 11. It's not until Chapter 12 that anyone gets close enough to a ghost to see if they actually work.
In Thunderstruck, there are several Chekhov's Skills and other elements that are introduced early on and then used later. In a mild variation, there's usually a link below the comic sending readers back to the previous use of the Chekhov's Gun.
As he's been established to use anything on hand as a weapon, the soda can gifted to Gray by Rowan was inevitably going to see use. Sure enough, seven chapters later, he pulls it out of his bag and wraps it up in cloth to use as a bludgeon.
Jared wins a test in Episode 75, with second place bemoaning that, no matter what he does, there's one question that he can't answer in the same way that Jared did. This offhand comment becomes relevant in Episode 88, where it's revealed that said question was printed wrong. This means that only people with the answer key would answer it the 'right' way, exposing Jared as a cheat.
In Episode 101, there's a brief panel of Timothy ditching Jake to instead tail Gerard for information. A few chapters later this is resolved when Alex and Gray spot the tail, confront Timothy, and Jake defuses the situation. However, Jake knowing Gerard's workplace becomes relevant again later when he goes there to beat up Gerard and provoke Ben into their big showdown.
A background detail throughout Season 2 was Donald submitting a custom-made equation to a math book contest, followed by Gray providing the solution to said problem. This only becomes relevant at the end of the season, when Donald learns that the Gray Yeon from the textbook and the White Mamba that's been destroying his gang are one and the same.
In Yokoka's Quest, the ribbons that Vivi gave to Yokoka and Yfa and Mao are used by Vivi to magically keep track of them outside of Betel's Forest, which is only revealed 7 chapters later.