The Adventures of Brisco County, Jr.: In one episode, the The Dragon escaped from a jail cell and leaves the keys to the cell on the cell's cot. Naturally, the fact that the keys to the cell are inside the cell becomes important later when someone needs to escape from the jail.
Pretty Little Liars: In 'Esc Ape from New York" our heroines have found refuge in the middle of the night at an empty theatre. On the theatre set is a gun that a character remarks is real. Later the gun is used to kill the bad guy...via blunt force trauma.
The Adventures of Lano and Woodley : At the start of "The Two Men" Frank and Colin trip over a signpost, causing them to drop a woman's television several stories off the top of a car park. Later on the sign saves them Humanities building is ideal — otherwise please make sure it's a building that's easy to find and that will be unlocked and accessible at the specified time.
Alias: Happens with the Bond-like gadgets that Sydney gets, particularly in early episodes, though most of them have a specific and outlined use within missions.
Also, Chekhov's Earrings: the pair of earrings Irina brings with her to the CIA, leaves for Sydney in "A Dark Turn", and transmit a message to her in "TruthTakesTime".
Andromeda: In a late-season episode, three crewmembers receive prophecies from an oracle that is "never wrong". By the end of the series (it took two seasons), all of the prophecies have come true. None were disproven or broken.
Another Period: The "Pageant" episode begins with sisters Lillian and Beatrice Bellacourt making plans to appear in a local beauty pageant, while their mother practices her dart-blowgun technique on passing American eagles. During the actual pageant, Lillian realizes that Beatrice may actually win, and uses the blowgun to sedate her sister mid-singing performance.
Arrested Development: Chock full of Chekhov's guns. Nearly every episode has at least one, and there are a few that don't go off until several episodes (or seasons—remember Buster's hand chair?)—have passed.
The Avengers: Emma Peel has a Chekhov's Wardrobe in the original series. Her clothing style either involved wearing a skirt or a skin-tight Spy Catsuit. Proper British ladies cannot fight in skirts, so she was always wearing her catsuit whenever she became involved in a fight. This may suggest otherwise unmentioned psychic powers she possessed, as her unerring ability to recognize hours before a fight that she would later be involved with one, sometimes requiring her to go home and change clothes before taking other actions. Likewise, if she is seen infiltrating enemy territory in a dress or skirt, it's clear that she will not be caught or otherwise need to pound on said enemies. Either this or we must assume that catsuits cause fights and skirts create peace.
The one exception to this otherwise hard and fast rule occurs in the episode Return of the Cybernauts, where fashion sense (Emma was going to a formal party) and the plot (she will later attack Steed after being mind controlled) could not be meshed, resulting in an oddly surreal scene where the villain of the piece pulls off her skirt after mind-zapping her so that she can perform the subsequent, oddly stilted, fight scene.
Calling it a "fight scene" is a stretch; she robo-marches up to an unsuspecting Steed and lays him out a single karate chop.
"Grey 17 Is Missing" referenced this by having Garibaldi discuss an antique gun extensively in Act I, which was then not used in the rest of the episode. This was a bit of an in-joke for the people who hung out in rec.arts.sf.tv.babylon5; series creator/producer J. Michael Straczynski frequented the newsgroup and often cited "Chekhov's Gun" when talking about TV writing.
This becomes a double-subversion (partially) when Garibaldi uses the bullets for the gun, which he conveniently put in his pocket, later in the show to defeat the Monster of the Week.
The alien healing device is used in one episode in season 1, brought out for an episode in season two, and then never mentioned again until the end of season 4.
JMS is arguably the king of this trope. He quotes it frequently on his commentaries for various episodes, along with the little-known corollary: "If you shoot someone in Act III, there better be a gun on the mantle in Act I."
Then there's Tory's murder of Cally, which looked for all the world like it would never be brought up again before becoming a key element to the war's resolution or lack thereof.
Another one is early on in the show, when Baltar asks for a nuclear bomb (As per Head Six's order) from Adama, claiming that it's to help his research. The bomb is then detonated and the fallout becomes the most important tool for the Cylons to track down the location of the humans, who settled in a cold but habitable planet.
This is a pretty subtle and minor one, but early in Season Two when Chief Tyrol, while attempting to prove he isn't a Cylon (lol), is listing the battlestars he's served on he mentions the Pegasus, which showed up a few episodes later.
Big Love: Subverted when Lois hints that Wanda should kill the DA. We see Wanda packing Lois's gun to take to the courthouse, presumably to shoot the DA. She actually just hands it over to Bill and tells him that Lois was going to shoot the DA.
Also when Bill insists on giving his three wives guns for Christmas. Many fans speculated that before the season was over, someone would be shot with one of them. Indeed, Bill was shot, but not by anyone in the house and not with any of the aforementioned guns.
Blue Heelers: In an episode of the Australian cop drama, early in the episode a police officer loses their pen due to it rolling off and falling behind a filing cabinet, to which another officer offhandedly remarks that the cabinet doesn't sit straight. Later on a major plot point develops with another police officer suspected of stealing a vial (containing a blood sample of a suspect) which had mysteriously disappeared - in the end it's revealed the vial had been left on the aforementioned filing cabinet sometime during the episode and had rolled off and fallen down behind it.
Breaking Bad: In all honesty the show should be renamed "Chekhov's Gun: The Series".
The earliest example is in the episode "Cancer Man" Brandon Mayhew aka Badger brings a crossbow when he and Jesse go out to the desert to cook meth claiming they can use it to hung javelinas. After the two get in a fight he ends up firing it at the RV as Jesse drives away.
There's also the hollow-point bullet in the episode One Minute.
The season 4 opener "Box Cutter" introduces, well, the eponymous box cutter. Used in an innocent fashion in the cold opener, it finds a much more macabre use before the end of the episode.
The character of Hector/Tio Salamanca. Played a minor role in the first season, only to show up in season 4 as Gus Fring's mortal enemy, and Walter's weapon against him.
How about in season 4, where Jesse carries around a dummy cigarette filled with ricin, intended to poison Gus. Instead, Jesse's girlfriend's son ends up with a mysterious illness...
Connected to that is the plant that Walt eyed in his garden the episode before.
Season 5 opens with Walt purchasing a very literal example in the form of a high power machine gun..
The ricin was first mentioned in season 2 and attempted its use on Tuco and Gus, which both ended in failure. The Ricin was ultimately used successfully on Lydia, the final death caused by Walter in the final episode of season 5.
Buffy the Vampire Slayer: In particular, Season 5, when they faced an unstoppable god. Almost every single episode in that season, including the ones that looked like filler (Warren's BuffyBot, the angry troll who had once been married to Anya) turned out to have a Chekhov's Gun that got used in the big finale.
Everything in Season 5, including one of Spike's insults. Spike calls Xander all kinds of things, including a 'glorified bricklayer' for working construction, and Xander defends himself with his bowling skills. In the final battle against Glory, Xander demonstrates his skill with construction equipment by hitting Glory with a demolition ball and shouting, "And the glorified bricklayer picks up a spare!"
In the season 3 episode "Band Candy" Giles, under the influence of the childishness-inducing candy, steals a gun from a cop. It becomes a literal Chekhov's Gun when he uses it later on in the episode.
Not to mention that, shortly after stealing the cop's gun, he begins making out with Joyce "Buffy's Mom" Summers. In the Season 4 episode Earshot, this little gem crops back up.
Buffy: You had sex with Giles? On the hood of a cop car? TWICE!?
In Season 2, when Jenny purchases an Orb of Thesulah, the shopkeeper mentions he had previously sold two as "new-age paperweights". After Angelus smashes Jenny's orb and kills her, we find out that Giles was one of the people who had bought these "paperweights".
Burn Notice: It's easy to miss and the gun gets fired right away, but at the very opening, 4x14, Mike and Sam are sitting in a car and Mike is fiddling with a pair of sunglasses. A few minutes later Narrator!Michael says that getting caught with a lockpick set in a police station is a bad thing, but a pair of cheap sunglasses will do the trick. If you've prepared them beforehand, it's a moment's work to get your lockpicks ready and if you get caught, you're just a guy with broken sunglasses.
In the final episode, packing their emergency on-the-run bags, Sam grabs a bunch of duct tape, "just in case". Later, pinned down in a gunfight, he throws the duct tape to distract the guard pinning him down long enough to get a shot off.
Casualty: When the show was still a medical drama (before it became a soap) whatever you first saw after the credits was either going to cause horrific injuries or end up being removed from some unlucky extra in surgery.
Chuck: Pilot has a scene where Chuck and other employees are talking about a new virus making the rounds, which infects via porn website. With said knowledge, Chuck later disables a laptop and a bomb along with it, replete with a This Is No Time for Knitting (in this case, Looking for Porn) moment.
This becomes a Chekhov's Boomerang in the series finale, when Sarah whose memories have been erased remembers the virus, enabling Chuck to disarm another bomb, powered by the same type of laptop.
In a later episode, Chuck and Morgan talk about a guy that sometimes sells them fireworks. Later on Chuck needs to create a distraction in the same general area that the fireworks are being sold. You probably have a vague idea about what happens next.
Subverted in another episode. Morgan accidentally gets stuck in the Buy More's storage cage and calls Chuck to get him out. Later, a pair of thugs come by looking for Chuck. He leads one of them to the storage cage and closes the door, presumably locking it. However, the thug opens it easily and Casey subdues him. Immediately afterward, Chuck runs into Morgan who tells him the lock was fixed.
The season 3 premiere of Chuck has Chekhov's Minigun. Casey remarks that he never got to fire a certain minigun while clearing out their secret base. When Chuck and Sarah are rescued at the end of the episode by helicopter, Casey is (quite gleefully) using the minigun, now mounted to the helicopter's door.
In episode 3x06 Chuck sees a pair of sunglasses in Manoosh's briefcase. He passes over them, believing them unimportant. They turn out to be a new Intersect, the weapon in question throughout the entire episode.
A later season 3 episode has Chuck give Casey a handgun stolen from Castle as a thank-you gift for helping him survive to the point where he has a chance become full-fledged spy. Casey winds up using it to both save Chuck and help him cheat on his final spy test.
The season 4 premiere has a Chuck-ov's EMP device that Casey and Sarah recover in the opening sequence that is eventually used to disable the automated defenses in the climax.
Cold Case: Often during the flashbacks, sometimes from the very first once, something was said, seen, or done that would prove relevant as to why the victim was killed and/or to the identity of the murderer. In the present day scenes as well—in the episode "Sandhogs", the unique cigarette lighter seen in the possession of one of the suspects is revealed to have been given as a present to the victim, thus revealing the man to be the killer they've been looking for.
Additionally, Annie's Boobs. No, not Annie's actual bosom. The monkey. He first appears in the chicken episode, is released and then vanishes entirely. In the Bottle Episode, he's the culprit who stole Annie's pen. Amusingly, the reveal at the end of that episode is itself a Chekhov's Gun when it becomes the basis for a later Clip Show.
In "Epidemiology", Troy shows up for the Halloween party dressed as Ripley in the power loader mech. When this fails to get him the attention of the ladies, he ditches the costume for another one. After the other partygoers start turning into zombies, he puts the power loader costume back on and reenters the infested library to initiate a cure. The zombies gang up on him and easily rip off his costume.
Troy: Okay, I don't know why I thought this would work!
Cosmos: A Spacetime Odyssey uses this in "The Electric Boy". Michael Faraday keeps a lump of malformed glass as a souvenir of when Humphry Davy forced him to work in optics out of professional jealousy. Years later, Faraday struggles to find a medium through which polarized light would be affected by a magnet and tries all sorts of gases and liquids. In desperation, he picks up the glass, and hey presto.note It's not explained in the show, but the metal impurities of the glass itself is what did the trick.
Criminal Minds: In the episode "L.D.S.K", Hotch's second gun. Mentioned casually in the first act, it comes back with a bang in the third.
Degrassi Junior High: Done very nicely with a malfunctioning boiler room and some barrels marked "flammable"
A very subtle one occurs in the pilot: during the Cold Open showing a montage of the chores Mary Alice did on the day of her suicide, the very last one is fetching the mail. We learn later that she was Driven to Suicide by an anonymous note she received.
Dexter: In the season 4 episode "Remains to be Seen", Dexter forgets where he hid a body after suffering a head injury in a car crash. While reviewing the kill site for clues, he spots a drop of blood that he missed. He laments being off his game, cleans it up, and forgets about it. Later, still frantically looking for the body, he comes back to the kill site and sees the drop of blood again. Confused, he looks up and realizes that the body had been in a bag strung up to the ceiling the whole time.
Doc Martin: If someone coughs, scratches an itch, or sneezes in the beginning, they're probably the victim of this week's medical mystery. It happens at leastOnce an Episode.
"Warriors of the Deep" features intelligent reptiles as the Monster of the Week. Early in the story, a character identifies bottles of 'hexachromite gas' as lethal to all reptile life, making the climax rather predictable.
"Parting of the Ways" features two such instances. In the opening sequence, the TARDIS flies towards several missiles launched by the Dalek Emperor's ship, and it looks as though it's destroyed by the volley. However, the missile impact allowed the TARDIS to power the macro-kinetic extrapolator (obtained two episodes prior, in "Boomtown") and generate a force-field that protects the TARDIS. When the TARDIS lands inside the Emperor's ship, the lone Dalek who transports inside is destroyed by Jack using the gun he improvised in the previous episode.
In "Planet of the Ood", the villain, Mr Halpern, is constantly drinking hair tonic given to him by an Ood slave. Later, we find out that the Ood have been feeding him a biological compound... which turns him into one of his own slaves.
In "Journey's End", the previous episode introduced the Osterhagen key, established as a rather obvious Chekhov's Gun; the finale also introduced two further devices with the potential to end Davros' plans, and characters threaten to use all three at the same time. The whole thing is subverted when the Daleks casually separate the characters from their respective doomsday devices. All seems lost until the real Chekhov's Gun goes off when Donna's Time Lord consciousness is awakened from the human-Time Lord metacrisis.
And let's not forget that the Doctor lost his hand in "The Christmas Invasion" which, after showing up a number of times in other episodes (including spinoff Torchwood) became the saving grace three seasons later in "Journey's End".
It's subverted in "The Sontaran Strategem/The Poison Sky." Part one goes to some trouble to point out Martha's engagement ring and her reluctance to use guns, leaving the audience to surmise that the absence of one or both of these will tip the Doctor off when she's replaced by an evil clone at the cliffhanger ending. Turns out it's actually neither; instead, the clone just smells wrong. Though, he mentions that this is one of MANY things...
In "The Two Doctors", it's established early on that Oscar Botcheby collects moths, and to kill them he uses cyanide rather than ammonia. At the end of the story, the Doctor comes across the cyanide and butterfly net, and uses them to finish off the otherwise far stronger and deadly Shockeye.
Subverted in "Last of the Time Lords". Early on Martha explicitly introduces a gun that is believed to be the only thing that can kill a Time Lord. Later on the Master easily destroys the gun and it seems like all is lost - until Martha lampshades the ridiculousness of a plot hinging upon "a gun in four parts", then reveals her real plan.
Also done straight with the Master's ring.Russell T Davies planted the gun intending for a later producer to fire it - and ended up firing it himself in "The End of Time".
End of Time: the Nuclear Bolt cabinet. Originally used by Joshua Naismith to power the Immortality Gate, it always requires one person to be inside it. Towards the end of the second episode, Wilf gets inside the cabinet to save one of Naismith's employees, but in the ensuing chaos the Nuclear Bolt overloads with radiation and the only way for the Doctor to save Wilf, and, presumably, everyone else is to take his place in the cabinet and absorb a massive amount of radiation, leading to his death and regeneration.
In "Flesh and Stone" the Doctor walks away from Amy , having lost his coat to a Weeping Angel. He then apparently returns, now wearing a jacket, warning Amy to keep her eyes closed, lest she allow the Angels access to the visual centres of her mind, and also telling her to remember what she told him when she was seven. In fact, the Doctor who talks to Amy in the forest is actually the Doctor from the future, rewinding his timeline due to the events of "The Pandorica Opens". The thing he told her when she was seven was a story about him and the TARDIS, meant to make her remember him at her wedding after he was erased, so he could be brought back into existence.
A Good Man Goes to War has shown about a dozen of 'em.
River Song's lipstick offers a slight variation on this trope. Initially, her hallucinogenic lipstick is used by her to escape from jail in "The Pandorica Opens", it returns in "Let's Kill Hitler", this time as poison from the Judas Tree, which she has worn as part of her plan to kill the Doctor.
The Teselecta from "Let's Kill Hitler" is revealed in "The Wedding of River Song" to have taken the place of the Doctor at Lake Silencio, allowing him to survive.
The Chameleon Arch introduced in "Human Nature" shows up again in "Utopia", this time being used by the Doctor's old Arch-Enemy The Master, who used it to hide as a human after surviving the Time War.
"The Five Doctors" introduces a gun that takes 30 years to go off. The High Council of Time Lords offers the Master a new regeneration cycle, showing they can do so. In "The Time of the Doctor", by which time the Doctor has run out of regenerations, he is given another cycle.
A minor gun introduced and used in the same episodes. The Third Doctor takes the Seal of the High Council from the Master. Later the Eleventh Doctor uses the seal to decode the signal on Trenzalore.
Another gun with a very long time from set-up to payoff is the plot of "Genesis of the Daleks", in which the Fourth Doctor is forced by the Time Lords to return to the moment of the Daleks' creation to destroy them before they are created - but when he actually gets the opportunity to do so, he decides not to on the grounds that 1) genocide is wrong, 2) the wars they eventually start will unite more races against them than otherwise, and 3) that without them some other race of space Nazis would rise up. All of these decisions come right back to bite him in the arse thirty (real life) years later, when the Daleks' retaliation against the Doctor's failed time erasure led to the Time War between the Time Lords and the Daleks, which created various obstructive alliances attempting to deal with the massive devastation this war caused (such as the Shadow Proclamation), and forced the Doctor to commit genocide against his own species as well as against the Daleks. It got Cosmic Retconned into him merely sealing Gallifrey away in its own dimension later on.
The first victim of The Robots of Death was a meteorologist who was attacked when he was preparing to launch a meteorological helium balloon. It later turns out that the main villain had been reprogramming the robots to kill the humans on board the sandminer but was himself immune because the robots were programmed to recognise his voice and leave him alone. The solution, then, was to somehow change his voice so the robots would turn on him. Helium is good for that, and, as luck would have it, there was a ready supply on board.
Dollhouse: In Season 2, many of the imprints from Season 1 begin reappearing as necessary to the plot.
In season 1 Ms.Lonely-Hearts who is joked about much early on eventually is revealed to be Adelle.
Elementary: In the opening of "The Rat Race", Watson complains that she can't read Holmes' texts because he uses too many abbreviations. After the killer of the week kidnaps Holmes, she sends a text from his phone so Watson wouldn't worry. However, Watson realizes it wasn't from Holmes because it didn't read "like a teenager on a sugar high."
In "The Deductionist", Holmes points out to Watson several continuity errors in the porno that her sub-letter made, which was part of the reason she was being evicted from her apartment. One of these errors would help Watson realize that her landlord was in on it.
Also, Sherlock's single stick practice would save his life later on in the same episode.
Emmerdale: From Nancy Banks-Smith's review of a November 1998 episode (reprinted 04.02.10): "When someone points to a box of fireworks and says, 'They should be in the cellar', you know the whole place is about to go up in a dazzling racket of rockets. Trust me. I'm a critic. No one in the history of drama has ever pointed to fireworks and said, 'They should be in the cellar', and next day put them in the cellar."
ER: In one episode, an African woman gives a necklace of the cross to one of the doctors tending her daughter. He claims that he doesn't deserve it, but she calls him a "man of God" for being here, helping them when no one else would. Later on, when captured by the rebels and as they brutally murdered each of their hostages, they were about to execute the doctor, when they realize he was praying and was wearing the cross, thus believing he was a priest. The woman who gave him the necklace quickly said that even the rebels wouldn't dare harm a "man of God". And so, the rebels let him go.
Eureka: In the beginning of one episode, someone shows off a superpower portable bass amp they just built, but Carter doesn't have time to stick around to hear about it, suspicious things are happening down at the lab. The end of the episode, guess what Carter needs to disrupt the monster of the week?
Eureka did this a LOT. Usually within the first fifteen minutes of a given episode, two experiments would be shown and explained. One of them causes this weeks catastrophe, the other (using Carter's inimitable stupid logic) is the solution.
Farscape: In the episode "Bone to be Wild", M'lee notes that Zhaan "smells like out there" (out there being the jungle), causing a shocked Crichton to exclaim, "You're a VEGETABLE!?" Turns out this is kinds of important, when the botanist Br'nee tries to frame M'lee for Zhaan's disappearance, even though M'lee only has interest in animals. Crichton calls him on it.
Farscape had a bit of a thing for these, on both an episodic and series-wide scale; the best and most notable example is probably the chrysthereum blossoms, which are visible in the season three episode "Incubator" but do not pay off until very late season four. Going back even further, the place that Stark mentioned having seen in season one was probably the chrysthereum chamber, only seen in season four.
Father Ted: Lampshaded in one episode where Ted criticises a fellow priest for buying useless objects, in particular a pair of false arms and a remote controlled wheelchair. "What sort of situation would require the use of a pair of fake arms and a remote controlled wheelchair? Only a completely ludicrous one". Later on in the episode, however...
Again, lampshaded in the plane episode, when Ted complains to Dougal that he bought a squeaky phone for a dog, and a tape dispenser which tells you how much you use. ("You have used three inches of sticky tape, god bless you.") The former is used twice for comedic effect, the latter comes in handy when Ted has to repair a vital fuel line to stop them from crashing.
Firefly: In the episode "Our Mrs. Reynolds", Jayne offers up his very favorite gun, Vera, for the new blushing bride. The Captain refuses, and when the bride betrays them, Jayne happily uses Vera to shut down the electric "net" that would kill them all. Another example in Firefly is Kaylee repeatedly referencing the need for a new part for the engine so they don't get stranded in space. Lo and behold, guess what happens in a later episode.
Frasier: A seemingly insignificant comment or action by one of the characters will often inspire the plot resolution (or at least drive it forward) later on. The show was always very subtle about the way it handled such things.
Also, the more literal example of Maris borrowing the antique crossbow at the beginning of Maris Returns.
Fringe: Humourously subverted in one episode, where Peter decides he needs some protection and buys himself a shotgun at the end of the first act. It never appears again.. all the more effective because this show has quite a few Chekhov Guns usually.
Also subverted when Walter implants a tracking device into his neck after getting lost. When he's kidnapped an episode or two later, Peter and Olivia can't use the tracking device to find Walter because his kidnappers tore it out.
Full House: In the episode "Knock Yourself Out," Stephanie gives Danny a colourful tie tack as a present. Later that night, while on TV as a sportscaster, he interviews a boxer called "The Sandman" and asks about his wife leaving him. Apparently, the boxer never knew this and gets knocked out in the next round. Danny later apologizes to him on the air, and the boxer forgives him, but then fires his trainer for keeping it from him. Infuriated, the trainer punches Danny in the abdomen, but Danny is still standing and unhurt while the trainer holds his fist and moans in pain as he walks off. Looking in the camera, Danny opens his suit jacket and reveals the tie tack pinned to his tie, and thanks Stephanie on the air.
Game of Thrones: In a first season episode, Ros casually mentions that Tyrion gave her a necklace with a Lannister lion on it as a tip. Late in the second season, Cersei kidnaps Ros because of the necklace, believing it to be proof that Tyrion is in love with her.
The Moon Door in the Eyrie, on multiple occasions. Whenever they show it, you know someone's going through it within 1 episode.
Olly, on season 5. Because his parents were killed and eaten by a wildling clan called Thenns, he hates wildlings. When the Lord Commander tries to make peace with wildlings against a great threat, he obviously gets pissed off and in the end he stabs Jon Snow, the Lord Commander. Despite not being a character from the books, every book reader guessed he was going to play a part in the act. He has been given the names Olly Chekhov, Chekhov's Shotgun due to it's plain obviousness.
Grimm: "Sweet Dreams" by the Eurythmics was playing continuously in the IPod of the first victim in one episode. When the killer begins to absently hum the song in front of Nick, it tips him off.
In one episode, Juliette asks Nick to boil some water she had set on the stove. Later, when an orge attacks Nick, Juliette runs into the kitchen and the first thing she grabs is the boiling water and uses it to defend her and Nick.
Rosalee's box cutter. Just before her brother's murderers return to the apothecary she was in, she was sifting through boxes to find the herbs that the murderers wanted. When one of them tries to grab her, she frees herself by stabbing him with the box cutter.
Also in "Leave it to Beaver", Nick was seen practicing with a crossbow. He uses it to kill a Reaper.
Heroes: Does this numerous times. One particular example is the train wreck in the first episode. For the first two and a half seasons, we just know it as the train wreck where Claire tests her power by walking through fire and saving a man. However, in Volume Three's flashback episode "Villains", we discover that the train wreck was actually caused by Meredith trying to escape Thompson and the Company.
This also commonly is used with Sylar's stolen abilities. Whenever he takes an ability, it will play a part in a future episode (or in some cases, the graphic novels), often after people tend to forget he got the power. One example is his cryokinesis, which is shown once in the second episode, then doesn't appear again until two of the last four episodes of the season.
Another example is his ability to know an object's entire history by a single touch early on in Volume 3. That power then becomes the most important element into his transformation as Nathan at the end of Volume 4.
Prior to Isaac's death, he gave his sketchbook to a seemingly random comic book geek. After going the rest of the season, all of season 2, and most of season three without it, it seemed like a dropped plot line. However, in episode 10 of Volume 3, we find out that this sketchbook is what Matt, Daphne, and Ando need to find out what will happen to Hiro when he goes 16 years into the past.
A relatively minor one, but in Volume 2, Claire had gotten a car while she and her family lived under a false name while in hiding from the Company. Eventually, her car ends up stolen. When Maya breaks Alejandro and another guy out of prison, the other guy offers to help by using his car to get to New York (long story short, they wanted to see Surresh Sr.). It then pans in much focus to the license plate and a bumper sticker with "Go Conquistadors!", revealing that "his" car actually belongs to Claire Bennet, and that he was the one who stole the car earlier.
Home Improvement: A holiday episode started with Tim and Al practically blinding the Tool Time audience with some sort of halogen setup. It seemed like a basic opening gag and so I was surprised when Tim's sons activation of the house's Christmas lights (itself a subplot) allowed the airliner he was on to land in previously paralyzing fog.
House: Happens roughly Once an Episode. House sends the young guns to investigate the Patient of the Week's home, where they find some detail which is either the cause of the disease or evidence that leads House to figure out what's wrong.
He once solves a case based on the fact that the patient had Tic-Tacs. It's not so much Chekhov's Gun as it is Chekhov's Secret Satellite Beam Weapon, in that it can really come out of nowhere.
I Carly: Whenever the main characters seek help in searching something, the webshow itself is their main form of problem solver.
Spencer's episode-acquired possessions usually join the main plot like the fishing rod, the Proton Cruiser and the mechanical bull.
In iEnrage Gibby, Freddie sets up another camera to examine how two pieces of bread rot in a period of time for the iCarly webcast. At the end of the episode, this camera also provided proof that the incident between Tasha and Freddie was an accident.
In iHave a Lovesick Teacher, the titular teacher mentions in the first half of the episode that "Well, the Pear Pod wasn't cheap, but I just got the songs off one of those music-sharing websites." It is not spoken about until the end, when she gets arrested for downloading music illegally.
Used lazily with the mood app in iOMG to reveal Sam is 'in love'.
In the episode "The Freezer", for example, in the first few minutes Fred tells Lucy and Ethel that the furnace is off as he just replaced the fire brick in it and the mortar needs to set. The deactivated furnace then gets used later by Ethel to eavesdrop on Ricky and Fred and again by Lucy to hide seven hundred pounds of beef in. And finally at the end the furnace gets relit, cooking all the hidden beef.
Also, the infamous "Vitameatavegamin" episode. The guys are talking about the Vitameatavegamin, and one picks it up and notes all the ingredients, one of which is "Alcohol 23%" (at which he does a double take.) As Lucy rehearses the commercial (unaware of how potent the stuff is), she becomes very, very drunk.
I Spy: The pendant given to the wife of a not-so-late traitor actually contains the secret photos that supposedly had burned up.
JAG: In the episode "Brig Break", Lt. Kate Pike and Lt. Meg Austin have a minor disagreement whether the proper title of the visitor's registration form for the brig is -77401 or -77501. At the climax, gunnery sergeant in charge of the brig has activated a nuclear weapon and it can only be deactivated by entering a five digit code. Since the Gunny had perversely tormented Meg by telling her the 5-digit code he picked "has a seven in it", Meg, Harm and Kate desperately try to choose a string of numbers with a seven in it that Gunny might have used on a regular basis.
Knight Rider: If KITT has a new gadget installed, you know Michael will be activating it by the end of the episode. In fact, it'll probably get used twice.
Subversion: In the fourth season premiere, KITT gets a new button marked "C". You might think it was going to be some new weapon or defensive mechanism, but at the very end of the episode it was revealed to stand for convertible.
Lawand Order Criminal Intent: Near the beginning of one episode, Bobby Goren "buys" a brick from a homeless man (in order to take it before said homeless man hurts someone). Halfway through the episode, he uses the same brick to destroy the window of a car shop belonging to a small-town judge in order to get himself arrested so he can go undercover in the local prison.
Leverage: Loves playing with this in various ways, but "The 10 Li'l Grifters Job" features a very obvious example. At the beginning of the episode, a character remarks: "Do you know how long it took me to hide that pipe wrench in the library?" At the end of the episode, guess where Ford ends up needing an improvised weapon?
Used while travelling back in time to 1954, Daniel Faraday is called upon to disarm an undetonated H-bomb, but instead suggests it be sealed with lead and buried under the logic that, fifty years in the future, it hadn't gone off and destroyed the island, so why worry? Anyone who doesn't think it'll come back into play by the end of the season doesn't read this wiki.
There are countless examples, here is one of the more subtle ones. In Season 3, the Others task Sawyer and Kate with clearing rocks from a dirt region for no discernable purpose. It turns out that they were clearing a runway, which a plane uses to land on during Season 5.
In a more rapid-fire example, Fake Locke tells the Losties they can't use that same plane to take off because Widmore loaded it up with C-4. Take a guess what's in Jack's backpack when he boards the sub.
In the season 1 episode "Confidence Man", it is revealed that Shannon has asthma and her inhaler has run out of medicine. This becomes problematic since the refills were in Boone's bag, which he lost in the crash. Exactly 100 episodes later, Jack and Hurley comes across it on their way to the lighthouse.
Mad Men: Of all shows, had a Chekhov's Tractor. Ken Cosgrove brings a John Deere riding lawnmower into the office (how was he able to fit it in the elevator?)note Service elevator? Assembled from parts? It's a small lawnmower? and goofs around with it. At the end of the fairly lighthearted episode a clumsy secretary riding it hacks through the foot of a suave British redshirt, covering everyone's Gorgeous Period Dress with tons of blood. This one event sets into motion the events that conclude the season.
See also the list on the trope page; there are a lot of cases where things that wouldn't exactly be Chekhov's Guns in any other setting (e.g. the date of a wedding) become massive Chekhov's Guns thanks to the setting being the early to mid 1960s (e.g. the date of said wedding being 23 November 1963).
The Mentalist: In the third season, it takes on the form of a literal gun. Earlier in the season Jane is given a gift of a pistol. He has never owned a pistol and is not the kind to own one, but he holds it in his hand pensively. It's seen again once more briefly but is more or less forgotten about. That is until the very end of the season finale, when he keeps it hidden in his suit pocket and actually kills Red John with it, finally fulfilling the objective he'd been obsessed with since the beginning of the show.
Midsomer Murders: Subverted in one episode. We see a character unpacking a backpack and pulling a pistol out and setting it on the table. Later on, we see the killer looking in his window as he has a revelation and rushes off to call the cops. As he leaves the room the camera zooms in on the gun laying on the table. Once in the phone booth, the man is attacked by the killer wielding ... a hammer. The gun never appears again.
Mistborn Chronicles: Vin's earring.
Monday Mornings: When Dr. Jorge Villanueva ("El Gato") goes to the first M&M meeting in the pilot, he brings a cup of coffee and a newspaper. It seems like he's going to read through the meeting, casually drinking his coffee, not paying too much attention to the meeting itself. M&M stands for morbidity and mortality, and the purpose of these weekly meetings is for the doctors to explain to the Chief of Staff why exactly they killed their patients or how they screwed up. But remember "Gato"'s newspaper! While Dr. Martin ("Double Oh Seven") is getting fired for a misdiagnosis, "Gato" takes the newspaper and reads him an obituary of his patient, a lovely woman adored by her family. When "Gato" learns that Dr. Martin's fired from the hospital, but his medical license will stick, he angrily tosses the newspaper in "Double Oh Seven"'s general direction, and leaves in disgust.
Monk: Has these in several episodes. Minor details are pointed out that serve to help the audience solve the case along with Monk. A massive one is Trudy's Christmas present, which is pointed out in several episodes, but doesn't get fully explained until the finale when it proves who killed Trudy.
In general, if Monk notices anything that's mentioned in a seeming throwaway line, it's essential to the case.
There are also instances where Chekov's guns are shown where Monk isn't the one who noticed it. A notable example is in the episode Mr. Monk and the Captain's Wife: During the scene where Captain Stottlemeyer is going berserk with rage in trying to find the person who shot the tow truck driver and thus hospitalized and nearly killed his wife at the police department, a detective mentions a bank job in which a 22-year old clerk was killed, that Stottlemeyer dismisses. Turns out, that culprit hid the gun he used in his car, which was being repossessed by the tow truck driver on the morning of the shooting.
In Mr. Monk and the Big Reward, Monk makes a point of reminding the police station's cleaning lady to clean under the tables. As it turns out, the lost million-dollar diamond is hidden under a table in an interrogation room with a wad of gum, and the cleaning lady finds the diamond by accident after Monk has figured out where it is.
In "Mr. Monk Goes to a Rock Concert," they spend plenty of time showing Monk being hit by a blue beach ball. Which turns out to have the evidence that incriminates Kris Kedder for killing Stork Murray.
In "Mr. Monk and the Bully," Marilyn Brody mentioning to Monk and Natalie that she was adopted and had an aunt in Texas. The word "aunt" is important, since she mispronounces it later, allowing Monk to deduce that she has a twin.
In "Mr. Monk and the Leper," they do the job of showing you a security panel in the house Monk and Natalie are paid to sneak into, which clearly says is from a security company founded in 2003. Since the person who is paying Monk has been presumed dead for seven years, after Monk overhears Julie reading off a ketchup bottle from a producer that has been making it since 1840, Monk realizes he's been a Detective Patsy.
Murder, She Wrote: Apparently the main employer of Cabot Cove is a factory that makes Chekhov's Guns.
The Musketeers: Aramis leaves his mistress's house in a hurry and leaves his pistol behind. It later leads to her death as ordered by her other lover, the jealous Cardinal Richelieu.
MythBusters: Since a single episode can only showcase a certain number of myths, some of the equipment created for certain myths may appear in the background of certain episodes aired before the episode where it is used is aired. For example, the Faraday Cage used for a myth in the seventh episode of the first season appeared in the background of the same season's first episode.
NCIS: In one episode, Tony steal's McGee's apple, munches on it, and tosses the core away in Abby's trash bin. Just yet another example of Tony treating McGee like the Butt Monkey, right? Yes, except two episodes later we find out that Chip stole the discarded apple in order to get a copy of Tony's teeth marks, and used them to frame Tony for murder.
Actually, Chip uses a few Checkhov's Guns from earlier episodes. In one episode he drives Tony's car around, enabling him to get the fibers from the carpeting. Tony gets a bloody nose at one point that gives Chip access to his DNA. And those silent shoes that spooked Abby in his introduction surely came in handy while he was setting all this up.
Nikita: In a flashback to Nikita's "first kill", her target mentions to Nikita (who is posing as a prospective nanny for his newborn) that he worries his daughter will inherit his peanut allergy. This comes into play later when it turns out her victim faked his death and is now the head of a gang specializing in human slavery. When Nikita is captured, he proves his dominance by forcing a kiss on her, unaware that she was wearing lipstick laced with peanut oil.
Orphan Black: The pilot shows security footage of Beth's suicide, in addition to showing it from Sarah's perspective. In the penultimate episode of Season 1, Art uses it to figure out that Sarah took over Beth's identity.
Party Down: Somewhat lampshaded in the episode "Investor's Deal". A prop gun is brought out and assumed to be real, a scuffle occurs so they decide to hide the prop gun. Casey delivers the line "Well, you know what they say about a gun in the first act". Later a real gun is pulled on party guests and assumed to be the fake prop gun. This turns out to be a real gun while the prop gun remained in the bag.
Power Rangers Time Force: The rangers came from the year 3000 which was later revealed to be a razed earth with cities few and very, very far between. Fast forward eight years later to Power Rangers RPM where we are shown as to how it happens.
Word Of God states that RPM is set in a different continuity, but that was said previously about Lost Galaxy before it was RetConed into the established timeline, so this could still work.
In an episode of Quantum Leap Sam leaps into a member of a fraternity that is seen using old bicycle tubes to launch water balloons. That same catapult method is used to dispose of a bomb planted in the chemistry building.
Robin Hood: In an early episode of the 2006 show, the outlaws come across a ledger that details how to experiment with Greek Fire (that is, explosives). Robin throws it into the campfire, but the episode ends with Djaq discreetly saving it from the flames. It isn't seen or referenced again until the end of Season 2, where it turns out she was going to give Robin the gift of a pig's head stuffed full of black powder for his birthday. She uses it to scare an army of mercenaries into delaying their attack, buying the gang enough time for help to arrive.
The Rockford Files: In the episode "Profit and Loss," there is an ongoing side plot involving Jim's broken garbage disposal that has nothing to do with the case he is investigating. Several objects are theorized to have fallen in, but it never seems particularly important. However, when the main villain takes Jim's gun, he misses five times before having a clear shot with the sixth and final bullet. Luckily there is no sixth bullet. It fell into the garbage disposal when Jim was cleaning the gun.
The Sarah Connor Chronicles: The first season has several conspicuous scenes where electricity is used to disable Terminators, and Cameron shows the Connors exactly how to remove the processor chip from a Terminator by removing it from Vic. In the premiere for the second season, when Cameron is damaged in the car bombing and goes berserk, the Connors end up using both of these methods against her.
The Shadow Line: Has the Briefcase Full of Money in Gabriel's wardrobe. While it's revealed in the very first episode, it only becomes important in episode 6 when it's revealed that it is marked and was used to buy drugs, implicating police officers in drug trafficking.
The Shield: Subverted with the "MAD Document", a notebook/dossier written by Shane Vendrell during season six that contained EVERY single dirty deed that the Strike Team ever engaged in up until that point in time. Conceived as a means to keep Detectives Vic Mackey and Ronnie Gardocki from retaliating against him after the two discovered that Shane murdered their fellow Strike Team member Curtis "Lem" Lemansky, the notebook is ultimately given to Vic in season seven, when Shane and Vic are forced to work together to save each other. As the alliance fell apart and Shane dragged Vic's estranged ex-wife into their war, Vic ultimately made the decision to beat both Shane and Ronnie to the punch and narced on both subordinates, via a cliffnotes confession to the first three seasons worth of crimes the Strike Team engaged in. After doing so, Shane contacts Vic and informs him that he's going to narc to the police on everything the Strike Team did, oblivious to the fact that Vic beat him to the punch for the immunity card. Knowing that Shane could find holes in Vic's confession via revealing new crimes that Vic didn't confess to (which would violate the terms of Vic's immunity deal, as far as loopholes go), Vic mockingly told him that not only had he already gotten immunity for his crimes, but added the lie that Vic had used Shane's own MAD Document as the basis for his massive laundry list of confessed crimes, which Vic then promptly badmouth by way of pointing out that it wasn't even as comprehensive as Shane bragged it to be. Shane then promptly went home and murdered his family, then himself after finding himself checked and checkmated by his own plot device.
In season 4, when Shane enlists the help of the Strike Team to help him clear his name (due to his involvement with Antoine Mitchell), Lemansky ends up beating up a Russian mobster in an attempt to ascertain the whereabouts of the body of an informant he was looking out for. During the raid, Lem takes a brick of heroin and stashes it in his car (which leaves the Russian's female accomplice, who "cut" the heroin, on the hook for it). As the season progresses, Vic and Lem make amends with the Russians by returning the heroin to a Russian higher-up (in exchange for information) and put the female accomplice in protective custody, wherein they're both forgotten about. Once Mitchell is arrested, it's assumed everything is tied up...until Monica Rawling learns in the final minutes of the season that the female accomplice was really a DEA informant, and they have Lem dead-to-rights because he stole the heroin. This motivates the plot of the rest of the series, as it sets off the Internal Affairs investigation and Lem's death in season 5 and the fallout between Vic and Shane in the final seasons.
Aceveda is sexually assaulted in season three by two Latino gangbangers who take a cell phone photo during the incident. After Aceveda kills one of the two assailants during a botched convenience store robbery, the surviving thug (who is arrested) tries to blackmail him, wherein Aceveda pulls his own blackmail stunt and sends the man to jail. It's assumed that the plotline has been tied up...until the end of season 4, where the thug, Juan, returns and tries to blackmail Aceveda again by threatening to release the cell phone photo unless he's paroled. Aceveda then uses Antoine Mitchell to take care of Juan by killing him before his scheduled court date. Everything's settled...until two seasons later, when it's revealed the cell phone photo was passed to another prisoner. Aceveda just can't catch a break.
Stargate Atlantis: In the pilot episode, John Sheppard is drawn to a necklace lying in the dirt of a tunnel. Teyla tells him she lost it as a child while playing and her father gave it to her. In a later episode, there is much suspicion that a spy in Atlantis has been given away the details of team missions, Teyla is the prime suspect. It turns out the necklace has a Wraith beacon in it, which has been giving away their position whenever they step through the gate, and Teyla has indeed been the spy - though all unaware.
Later, in season 3 when Rodney is hit by a beam from an Ancient machine that makes him have super powers and also force him to either Ascend or die, which he manages to avoid by using a sample of his DNA to fix the changes the machine made he becomes smarter than usual and he mentions to Elizibeth that he's designing a hyperdrive for the Jumpers. Later in the first of the fourth season, the expirimental drive becomes very important when they fly Atlantis to another planet and get stranded in space when they have too little power to finish the trip. They use the Jumper to steal a ZPM from the Asurans before the sheild fails and they all suffocate.
A conspicuous non-firing of a Gun occurs in another episode: A new species of cactus is discovered, and conspicuously given to (and named after) Rodney, with the warning "Careful: the needles can break the skin". A bacteria of unknown origin is affecting the entire base, and nobody can figure out where it's coming from. The cactus, however, isn't brought up again, despite it being set up as the explanation. They never do explain the delivery vector or infection method (only that it was brought to the planet a long time ago and that the 'soil samples' didn't have it).
Atlantis' parent series Stargate SG-1 is an example of Chekhov's Armory, but its episode "Emancipation" features a literal Chekhov's Gun, namely O'Neill's Beretta M9 sidearm. Early in the episode he fires it into the air to frighten off some wild dogs, then to startle some Space Mongols. He later trades it to a chieftain named Turghan in exchange for him letting Carter go.
The punchline being that it was almost out of bullets.
Star Trek: The Next Generation: In the episode "Reunion", in which Worf's son expresses interest in an ancient weapon called a bat'leth which is hanging on the wall in Worf's quarters. By the end of the show, the bat'leth has been taken off the wall and has been used to kill Duras.
Star Trek: Deep Space Nine: In the episode "The Darkness and the Light", Kira is taking herbs for her pregnancy that render sedatives ineffective, which later allows her to turn the tables on her kidnapper.
Supernatural: Dean has an amulet that he wears at all times. In the third season episode "A Very Supernatural Christmas," we find out that Sam gave it to him as a Christmas gift years ago. For many fans it represents the (sometimes disturbingly) closerelationship between the two brothers. This was highlighted when Sam was shown to have worn the amulet while Dean was dead (Sam returned it when they were reunited at the beginning of the fourth season). Fast-forward to the second episode of the fifth season, when Castiel reveals that he needs to borrow the amulet, because God is missing, and it can be used to find Him, since it glows hot in His presence.
The rings of the Four Horsemen turn out to be the keys to Lucifer's cage, and are used to seal him away again.
Taggart: Played straight in one episode. A suspect's brother has a conviction for modifying replica guns into working firearms, and Burke mentions that one of his guns recently blew up in the face of the user. At the end, the Criminal of the Week points one of these guns at Burke, pulls the trigger... and it blows up in the crim's hand.
That '70s Show: In the episode "Black Dog" (S5E9), Kelso's BB Gun is a literal example of this trope; it is discussed early on that it went off previously and shot Eric's hamster in fourth grade, and sure enough, it accidentally goes off again and wings Hyde.
That Mitchell and Webb Look: Parodied in the 'Get Me Hennimore' sketches, which parody old timey sitcoms. A preposterous back story (i.e. a giant jam jar for an Eastern European president, a giant wasp, Hennimore's boss' wife going to a fancy dress party as a wasp) results in a Gilligan Cut to the fallout of a mix-up (Hennimore hitting his boss' wife with a bat).
Tracker: Mel finds a strange artifact among some things of her grandmother's that initially is regarded as unusual but not terribly important. Later, it turns out to be the key to the vault beneath the bar containing the alien weapon. The diary they find also counts, later proving to be a big clue to the key and the vault.
Tracy Beaker Returns: We have S2E5 Money concering a bag of stolen money. Frank keeps some of it for his grandad. In S2E12 Grandad Frank tries to use this money to get a gravestone when Grandad dies.
The Twilight Zone: TOS episode "Hocus-Pocus and Frisby". Near the beginning of the episode Frisby is playing his harmonica, and someone asks him to stop because of its poor sound. Near the end of the episode he plays it while he's being held prisoner by aliens, and the music acts as a "death sound" on them.
At the beginning of the first-season episode "Guts", when Rick Grimes prepares to escape the tank, he offhandedly picks up an Army-issue grenade off a ledge and stuffs it in his pocket. The grenade is seemingly forgotten about (as Rick gets to the survivors' camp and has his clothes washed) until the first-season finale "TS-19", when the survivors are trying to escape the Center for Disease Control and Prevention. When the attempts to break the bulletproof glass fail, Carol suggests that Rick use the grenade, which she's been holding in her bag. Rick then uses the grenade to blow the window, allowing the survivors to escape in time.
In "Guts", during the scene when Andrea, T-Dog and Jacqui flee the department store roof, Merle Dixon is still chained to a radiator. As T-Dog flees, he drops a pack of tools (in a split-second shot) in his haste to flee the roof. In the opening of the next episode, "Tell It To The Frogs", Merle manages to use his belt to reach a hacksaw that fallen out of the same bag of tools and escape by sawing off his hand.
In "Tell It To The Frogs", Dale tells Rick that he's looking for radiator tubing for his RV, and to bring any he finds in Atlanta back with him. Rick and the rescue team fail to find any, and when the convoy sets out in "Wildfire", they're forced to stop and find tubing because the radiator has overheated. In the second season, the radiator overheats again while the convoy is traveling down a highway, forcing the survivors to stop, which leads to an encounter with a horde of walkers.
The passing helicopter that Rick follows into the Atlanta city core in the pilot episode is seen once again in the second-season finale, "Beside The Dying Fire", where it's seen by a group of walkers. The group follows the noise and sound of the helicopter, growing in size along the way, and eventually break through a thick fence onto Herschel's farm, where they end up attacking and forcing the survivors to abandon it.
The rendezvous point the survivors set up for Sophia in "Cherokee Rose" ends up being used in "Beside The Dying Fire" as a way for the scattered group to find each other.
In "Seed", Daryl finds and kills an owl in the opening sequence, in order to feed the group. Later on (during the prison assault sequence), he is notably seen firing arrows that are using the owl's feathers as fletchlings, the first of which he fires into the head of a walker to stop it from sneaking up and attacking Rick as he makes the run to the guardtower.
Early on in the third season, the group discovers a cache of flashbang grenades and tear gas in the prison, and mention how it won't be very effective against walkers. They use them several episodes later in "Made To Suffer", during the raid to rescue Glenn and Maggie. In the next episode, "The Suicide King", Rick and Maggie use the rest of the tear gas and grenades to rescue Daryl and Merle from the Governor's arena.
In "Killer Within", Andrew activates the prison's generators, which sets off a number of alarms that draw in walkers from the surrounding area. In the third-season finale, "Welcome to the Tombs", the group employs this same strategy to lure the Governor's men into the prison and draw in walkers to ambush them.
In "Internment", Rick and Carl pull out a pair of rifles from a trolley of guns that is located near the outer fence, and use them to dispatch the horde that has broken through into the courtyard. Three episodes later, in "Too Far Gone", Daryl hands out weapons from the same trolley to the prison group while Rick is talking to The Governor, and they use the same weapons during the ensuing battle.
The bottle of alcohol that Bob Stookey takes from the animal hospital in "Indifference" is used six episodes later, in "Inmates", by Glenn as a makeshift Molotov cocktail which he uses to escape the destroyed prison.
Warehouse 13: Loves this trope. Very frequently, comments from past episodes will turn out to be significant, if not downright crucial to a future episode's plot; that episode will always have a flashback to the moment in the Previously On part of the show, never mind how many artifacts they have in the Warehouse that are fated to become Red Herrings.
One of the more dominant ones that has yet to be fired (that is, used in a major part of a plot) is that each Warehouse agent is allowed to have a Secret Keeper for the true nature of their work, but only one.
Wayne and Shuster: This trope was played for laughs on a parody of the siege of Troy. When Shuster's character suggests the Trojan Horse trick by hiding troops in a giant wooden horse, Wayne's character keeps complaining multiple times as a running gag that he preferred his idea of using a giant cake. At the end of the story, the narrator appears to finish his tale and make a cheap joke about it, only to be suddenly hit in the face with cake. Wayne and Shuster's characters suddenly appear in an inset window with Wayne triumphantly noting, "I told you that cake would come in handy!"
The Wire: Has several instances of this, often taking place over multiple seasons:
Season 4 opens with a humorous scene where one of the characters purchases a nail gun. Several episodes later it becomes integral to the plot.
After testifying in a criminal case in the second season, a district attorney gives Omar a "get out of jail free" card for his help. In the fourth season, Omar calls in this favor after he is wrongly imprisoned for shooting a woman at a convenience store.
In season 4, Chris and Snoop beat Michael's father to death after realizing that Michael was abused by him. Chris spits on him in rage after he's finished, and the spit eventually becomes DNA evidence used to convict Chris at the end of the fifth season.
Daniel's past corruption investigation, which is constantly hinted at throughout the series, but never explained until the fifth season (and then, there isn't much more revealed), when it's used as a way to force him out of his job.
The most important example takes place over two seasons. In season three, Detective Bunk Moreland sees a number of kids pretending to be stickup artist Omar and his crew. In particular, one young boy is playing with what appears to be a fake gun and repeatedly insists that he plays Omar. In season five, Omar passes the same boy, who stops what he's doing (lighting a cat on fire) and recognizes him. Omar goes into a grocery store to buy cigarettes, and is shot and killed by the young boy.
Refrigerator and dead naked girl in Season One, anyone?
Wizards of Waverly Place: A future Harper is introduced in Season 2. She wrote books based on Alex and her adventures. Some of the things she says are used as plot points later on and guess what Harper's doing in Season 4. She's writing the books.
The fact that Mason likes to paint dogs takes on a whole new meaning when it turns out he's a werewolf.
The X-Files: Used countless times in the series. For instance, in Season 5 Episode 4 "Detour", Mulder and Scully are on a trip to a teambuilding conference with two agents (which foreshadows the general theme of the entire episode). The other agents enthusiastically talk about how they built a tower from furniture and how awesome that felt. Mulder is bored and then beyond pleased when an off-side project pops up, which he immediately connects with another X-File case. Scully is worried about the conference, so Mulder promises her they can build a tower from furniture later in their motel rooms. They end up stranded in the woods and fall into a pit. They find there a lot of human bodies, both dead and barely alive. There is no other option than to utilize the bodies and build... a tower from the bodies so that they could get out of the pit. Other examples Chekhov's Gun will be posted at the recap sub-page (Recap.The X Files).
The Young Ones: Humourously subverted (and possibly lampshaded) in one episode. At the end of one scene, the camera zooms in on an innocuous-looking matchbox...who then proceeds to say "Don't look at me. I'm irrelevant." And sure enough, it's never mentioned again.
Zoey 101: In one episode, Zoey's key (which she always wears on a chain around her neck) turns out to be way useful - it is used to stop the reactors in the school's new electric generators from blowing up the entire school. This is a more extreme example, though, as it was something that she did in one of the first few episodes, and this episode wasn't until season 3.
A straighter example in Dance Contest: at the beginning of the episode, Michael is sentenced for taking the maintenance golf cart for a joyride. Later in the episode that golf cart runs over Zoey's dance partner.
Near the beginning of one episode of Castle, Castle is unsuccessfully attempting to befriend a gaggle of 2nd graders. Among the mean things they do to him: take a polaroid picture of him that looks like he peed his pants, and throw a bowl of marbles to make him trip. The murderer of the week is looking for a picture hidden in the camera that took that picture, and Castle throws the same bowl of marbles at the end of the episode to trip the guy as he's getting away with it.
The season 8 opener has a literal gun used this way: at the beginning of the first episode, Castle shows off all the neat PI toys he built in his new office, including a hidden spring-loaded gun in his desk. At the end of the second episode, during a tense standoff, Castle gets to use it. (Neat toy!)