24 did this constantly, due at least in part to the fact that it was tough to resolve every problem over the course of a single day. Then again, it's partly just that the writers don't really seem to care about their secondary characters (and most fans reasoned that they would reappear later in the series, given that the producers had a tendency for doing this with other characters). Major examples include:
Martin Belkin, the assassin hired to kill David Palmer at an early-morning rally in the first season. Belkin is apparently so hardcore that he had plastic surgery to make himself look like a recently-deceased amateur photographer. Belkin is stopped by Jack Bauer, who uses a diversion to get Palmer out of the building. Belkin, who is later seen identified by CTU, is never found again. There was nothing stopping him from getting plastic surgery and going after Palmer again anytime thereafter, in any of the seasons that followed.
Lynne Kresge, Presidential aide, falls several stories down a stairwell when she tries to get incriminating evidence on Mike Novick to David Palmer in the second season. After she's wheeled into an ambulance, she's never seen again, and no one comments on her disappearance.
Alexander Trepkos and his henchman, Max, are introduced (bizarrely) at the end of the second season as the masterminds behind the second day's operation. Jack Bauer kills Max in the official video game, but Trepkos is never mentioned again.
Jack battles heroin withdrawal for the first half of Season 3. Then Ramon Salazar gives him some aspirin and he's fine for the rest of the day.
In the original television airings of the fourth season, Behrooz Araz disappears after being taken hostage by Habib Marwan's thugs. It's resolved, but only in the DVD boxset: Behrooz is saved by Curtis Manning. This has given rise to the term "Behroozed", which is applied to several of the characters mentioned here who were last seen in mortal danger and were never mentioned again.
After Air Force One was shot down by terrorists in Season 4, we see the recovery team scourging the wreckage and finding President John Keeler. It is remarked that he is alive but in a coma and is taken to an Army Hospital. The 25th Amendment is then quickly enacted and his VP, Charles Logan becomes President. Did Keeler live? Die? Part of this has to do with Executive Meddling as what the writers wanted to do and what Fox executives insisted on were very different. Most fans assume he died.
In season 5, Wayne Palmer and Jack discover that a key piece of evidence is kept in a bank's safety deposit box. Since there is a curfew, the banks are closed and time is of the essence, they decide to pretend to be bank robbers and kidnap the manager. They get to his house, take him hostage and tie up and gag his wife. Later, the manager recognizes Wayne, prompting them to reveal the actual plan, and then get his support. Once they get the evidence, a bunch of Mooks working for The Dragon storm the bank and the manager is killed in the crossfire. The question is: what the hell happened to his wife?
Another notable unanswered season 5 question involves Evelyn Martin, Martha Logan's aide. Jack and Wayne manage to rescue her daughter, but she is shot in the process. In the same episode as the one with the bank manager, Evelyn's daughter calls 911 to get help for her mother, but Christopher Henderson intercepts the call and coerces her into revealing what Jack and Wayne are doing. After a commercial break, Henderson can be seen calling Charles Logan and telling him everything Evelyn has revealed, but it's never stated if Henderson killed the two of them or not. Not only that, but prior to kidnapping the manager, Jack and Wayne promised they'd returned to get Evelyn help, and Jack never brings this up again while Wayne is Put on a Bus for the remainder of the season.
Wayne Palmer is hurt by a semi-successful assassination attempt in Season 6 and eventually falls into a coma. He is wheeled to the hospital and never heard from again. The TV movie Redemption does make reference to his death in a newspaper (which was bought by a fan after the film finished shooting), but it's not visible on-screen, and thus isn't confirmed canon.
Bill's wife Karen Hayes disappears between the sixth and seventh seasons, and isn't mentioned by Bill after her last appearance.
She reluctantly agreed to go along with the decision to throw Bill under the bus in order to shield the White House from blame over the day's events, which strongly points to Bill having divorced her acrimoniously. His joining forces with Chloe and sacrificing himself in Season 7 make more sense in that light.
Word of God states that they were still married even during the seventh season's events. Bill can also notably still be seen wearing his wedding band. Plus, even though he was rightfully angry at the circumstances leading to his getting fired from CTU, you can tell by the end of season six that she pretty much redeemed herself in his eyes after she threw her own career away in order to save Jack's nephew.
In Doctor Who Adam Mitchell's plotline abruptly ended with him kicked out of the TARDIS, taken back home, and having involuntarily revealed the microchip in his head to his mother. It is still unknown what happened to him next, or what impact that impossibly far-future technology had on 2012 Earth. One would be forgiven for expecting that at the end of the season, Adam had become the new progenitor of the revived Dalek race. Or at least somehow to blame for it. But, no.
Jack's missing memories... remember those? The two years of his life that the Time agency stole? They were mentioned once and never again.
They were actually mentioned a few times on Torchwood. Most notably, Adam held the memories hostage.
In "The Lodger", The Doctor uses a cat in an attempt to safely find out what's going on in an upstairs flat. Many fans have wondered about the fate of that cat after the first floor imploded.
In "Let's Kill Hitler," Hitler himself is shoved into a cupboard and promptly forgotten.
Presumably after everyone leaves he exits the cupboard and gets on with his life.
At the end of Season 26 Ace leaves with the Doctor, to have many more adventures...but isn't there when he appears for Doctor Who TVM "The TV Movie". When or how she left, or what she is doing now, is unknown.
She is briefly mentioned in an episode of the spin-off The Sarah Jane Adventures.
Jenny stole a ship and went out to see the universe. We never see these adventures, and the doctor doesn't know she's alive.
In the 50th anniversary special, the humans are impersonated by the Zygons to fuel a few rounds of spot the imposter and a subsequent peace conference when their memories are modified so that no one is sure which is which. The scientist gives her inhaler to the duplicate, setting in stone which is which. This is never discussed again and has no relevance to the plot. The peace conference is dropped just as swiftly and we don't hear another word of it for the remainder of the episode.
Characters (or entire races/factions) appearing like they are the next big thing and vanishing again is something that happened so much, the audience has grown accustomed to this. Entire Wiki pages could be devoted to listing examples as this tends to happen more often than the opposite (where the new race or character introduced really turns out to be relevant):
The Goa'uld named Lord Zippacna, a minor noble working for the System Lords, shows up in two episodes. In his first appearance (remembered for the atrocious costume he was wearing consisting of a hawaiian shirt and straw hat), he works for Apophis and his son Klorel. He tries to destroy the Tollan and fails. He's seen a few seasons later, working for Anubis, where he recruits Osiris to be Anubis' Dragon and kills 90% of the Tok'ra, and after that, he's never seen again. A poison is unleashed on his troops on the planet, but as Zippacna was aboard his ship in orbit, even inferring his death seems to be unwarranted.
The Nox, who are shown to have been at one point one of the 4 most advanced races in the universe, appear in few episodes (their introductory one, and two others where, incidentally, the Tollan and Zippacna are respectively introduced) and are never seen or heard from again. Evidently, the Nox are so pacifistic they would not lift a finger to fight against their own extinction if they had to, and they are really good at hiding.
Charlie, a little boy created by a race of aliens (the Re'tu, themselves an example of this trope) to act as envoy to the humans. He's sent to the Tokra to become a host as he's very sick and would not have survived otherwise. It's never shown if he survived the many massacres the Tokra endured.
Strangely, though the Re'tu are never seen again, they are mentioned practically every instance when someone is attacked by something invisible or when there is a threat of unknown origin.
The Tok'ra endure the same thing once Selmak dies and the Ori arrive. They are only briefly seen in Stargate Continuum, and no explanation is given on what they were up to while the Ori were tearing the galaxy a new one.
The Tok'ra were mentioned as playing an intelligence role in dealing with the Ori, and in one episode successfully removed a clone of the Ba'al symbiote from Adria. Not quite gone, but not showing up much either.
Nem and the Oannes. A race of creature that look like the Creature from the Black Lagoon. Very advanced, enemies of the Goa'uld, who agree they could help Earthlings fight the Goa'uld. They are never seen again.
The RPG states the System Lords had already basically wiped out or enslaved the rest of his race.
Given that the rest of his race are presumed long-dead and that Daniel regretfully informed him that his mate was dead, which seems to have been the only thing that had kept him going for thousands of years... it's entirely possible he might have been Driven to Suicide after the team left.
The Hebridan, a race so advanced they defeated the Goa'uld in their sector of space. Being capitalist, they agree to trade some technology with Earth. They are never seen again, except for a brief mention that their planet was conquered by the Ori in season 10. Nothing is seen of their technology either.
The Outcast Asgard of the Pegasus Galaxy, leading to many Epileptic Trees as they seemed like too cool a concept to go unused. Especially egregious what with them being the last Asgard in the universe at that point.
The Travelers eventually return when the Outcast Asgard show up, their settlement getting blown to hell by said Asgard and ultimately helping the heroes out. The series coming to an end, however, pretty much limits their chances of appearing in the near-future.
Athar a.k.a. Chaya Sar, an Ascended Ancient whom the Atlantis expedition meet. She was forced by the other ancients to forever protect a planet. She and Sheppard had the ascended equivalent of sex, and she's never heard from again.
The Foothold Aliens from the episode of the same name in SG-1. Their tech shows up again a couple times, but they themselves don't.
Implied during the episode that the aliens were looking for a new home, inviting the possibility that their homeworld might have been dying or wiped out.
There are a few random spaceships clearly controlled by powerful and unfamiliar alien races that influence the plot conveniently and then disappear, never mentioned again. "Grace" and "Enemies Part Two" are notable examples.
Possibly the most glaring example originates from the episode "New Ground". Nyan returns with SG-1 to Earth in the episode's climax and becomes Daniel's new research assistant at the SGC. He is never seen or mentioned even once after this.
Though the Furlings were never shown at all, which is Lampshaded a couple times by O'Neill, and culminating with an implied explanation/self parody in "200" where they are basically shown to be Ewoks. The point it seemed to make was that the name Furlings was so ridiculous, nothing the show could come up with would work.
The Oranians, of whom you basically only see two specific bounty hunters.
The mysterious aliens from "The Daedalus Variations" of Stargate Atlantis, who appear in an alternate reality as being at war with Atlantis. Presumably they would have been an enemy in the 6th season, but the show was canceled before they could be explored any further.
Jonas Quinn and his planet Langara are hardly ever brought up after he leaves. It's name checked as falling to the Ori in season 10, and eventually shows up in a late episode of Stargate Universe, but Jonas himself is conspicuously absent in an episode that features two prior main characters. Given what they did to his planet in that episode, it's probably a good thing.
Sarah Gardner, an old love interest of Daniel who became the unwilling host to Osiris. She shows up in a handful of episodes, and they catch her and get the symbiote removed. The last seen is of her waking up traumatized and Daniel hugging her. She isn't even mentioned after that.
Happens in Multiple episodes. A random but supposedly powerful McGuffin is introduced, and told to possess magnificent power. But after the plot of the episode is resolved, the McGuffin is never seen of mentioned again. Examples include the Power Eggs and the Sword of Power.
Scorpina after the first season. She seemed to have just left Rita's employ without any explanation, and although she did work once for Lord Zedd after he took over, she was never seen after that. (There was a logical explanation, just not an in-universe one: Saban had intended to keep her as a recurring character for Season 2, potrayed by actress Sabrina Lu - having run out of the original footage of the character - but were unable to retain Lu for more than one episode and was unable to find a replacement in time to film the character's further appearances, so the character was just dropped.)
After leaving for Eltar with Zordon in Power Rangers, Alpha 5 is never so much as alluded to ever again, even when Eltar is taken over and Zordon captured. Fanon holds he died during the attack, and granted the second gen Turbo Rangers never met him, but still...you'd figure a character like him would get some sort of closure.
Also happens with Dimitria (from Power Rangers Turbo) and Auric the Conqueror (from Power Rangers Zeo). Auric is driven off by Rita's Impursonator monster and then is never seen or mentioned again. Dimitria leaves Earth along with the Blue Senturion to go save Zordon (whose planet is under attack), but while the Blue Senturion is later shown fighting the Machine Empire during Power Rangers in Space, Dimitria is never seen again. Which is weird, considering that the last episode of In Space heavily implies that she and Divatox are in fact sisters.
The ultimate fates of Goldar, Scorpina, Rito Revolto, Finster, Squatt, and Baboo are unknown. Were they among those destroyed, or among those purified? If they were purified, where are they now? For that matter, while we eventually see that Rita became the Mystic Mother, we have yet to see what happened to Zedd post-purification. (Naturally, some speculate that he became a rabbi)
what happened is that some of the monster suits became unusable after being used rito and prince gasket's costumes are among those that just fell apart.
The episode "Miri" begins with the Enterprise crew making the astonishing discovery of a planet identical to Earth. They beam down to investigate, and get caught up in a plot about a plague that kills adults and leaves children alive. This presents a mystery and danger that is duly solved. The episode ends without any further mention of the fact that the planet is identical to Earth.
"Identical to Earth" seems to apply to a lot of planets in TOS, down to flyby scenes where the planet model is very similar to earth. The only two that were ever really made a big deal of was a planet that had fought World War 3 in the 1960s rather than the 90s (post-retcon 21st century,) and in another episode there's even a human on an "identical to earth" planet, and it's revealed "his" earth (he even calls it HIS) exists mostly in another dimension. It's enough to make one wonder if this was just a plot point the writers liked to throw around or if it was actually just a term for "earth-like."
The episode "The Big Goodbye", a minor character is shot while trapped on the holodeck with the main cast, and Dr. Crusher says that they have to get him to sick bay by a certain time or he'll die. We never learn what happens to the wounded man. An especially frustrating incidence of this trope, because one sentence of dialogue could have resolved everything. The character may show up alive and well in one of the novels.
TNG also has Silva LaForge, Geordi's mom. Her ship mysteriously vanished, and Geordi nearly loses his mind to the mysterious gadget of the week in his attempts to find out what happened to her. After the incident, he accepts her loss, in a manner that makes the episode seem like a Know When to Fold 'Em plot. The actual fate of the Hera is not touched upon in any way - no clues, no evidence for them being dead or alive, and it's never addressed again.
And what happened after the destruction of the Enterprise-D in Star Trek: Generations? Was Moriarty destroyed in the Stardrive Section, or was he in the Saucer Section and later recovered? Or was he simply left behind and doomed to oblivion, as Voyager revealed that without regular maintenance, holodeck programs left continuously running will eventually develop fatal system errors?
What became of Wesley after he took off with the Traveler?
Star Trek: Nemesis he is at Riker and Troi's wedding in full dress uniform. Meaning that he either relinquished his powers and was forgiven by Starfleet or he was pulling a Q and was simply dressed as an officer to blend in. Which is the most likely? that depends how likely you think it is that an explorer would willingly give up the ability to travel near instantaneously to any point in the universe.
This happened many times on Star Trek: Voyager, likely due to the production team simply forgetting about prior plot threads that were set up in earlier episodes:
The Caretaker's "mate" was a plotline that was screaming for a follow-up - at the end of the episode, the mate, Suspiria, vows revenge on Voyager and disappears. She never appears again in the rest of the series.
Neelix's sensor data from "The Voyager Conspiracy" is never elaborated or followed-up on (which concerns a fully-armed Cardassian warship gallivanting around in the Delta Quadrant, and the tractor beam which pulled the reactor room out of the Caretaker's array before it exploded). That information was chalked up to Seven simply going crazy from trying to continuously process data.
The Borg Baby, picked up (along with several other Borgified children) midway through the series, and promptly forgotten about after a scene where the Doctor runs a diagnostic scan on it. According to DVD commentary, the show's writers apparently thought they had delivered the baby back to its parents right afterwards, even though this was never stated in the episode (or the later-season episode where Icheb stays and the two other children are reunited with their parents).
Ensign Samantha Wildman played a major supporting role in the early seasons, as she gave birth to a daughter named Naomi (who gained prominence in the later seasons as Seven of Nine's protege). Samantha, however, up and disappeared after an episode where she was afflicted with a sickness (but recovered). Like the Borg Baby example above, the writers thought she had died in the episode, leading to her never appearing in the series again.
An interesting subversion occurred with Joe Carey, the former chief engineer who was replaced by B'Elanna Torres in the third episode of the series. Carey disappeared soon after Torres is put in charge, and (with the exception of appearances in time-travel episodes focusing on Voyager's past) many fans assumed he had just died or disappeared without explanation... that is, until season seven's "Friendship One", where he is revealed to have been onboard the ship the whole time just before he's killed to establish the threat of the week.
As noted by SF Debris, it's entirely possible the writers put him into episodes set in the past thinking they'd already killed him, only to realise in Season 7 that he was still alive. Thus they brought him back, since they reasoned "Well, now we have to kill him!"
Fairhaven, a holodeck program that Voyager ran without stop for one episode. After the characters in the program became aware of their true nature and of Voyager's crew, it was promised that the program would be kept running and that they would still all be friends. It is never seen or heard from again.
One could infer that in-universe, Neelix was left wondering this after Voyager returned to the Alpha Quadrant and Seven never contacted him for their game of Kadis-kot.
In Equinox, a two-part episode, Voyager encounters the Starfleet ship Equinox, the crew of which ends up screwing Voyager over, using a sentient alien race as a power source to get home faster, and in general acts disgracefully. At the end, Janeway strips the surviving crew members of all rank, tells them their privileges will be restricted, they'll be closely supervised, and they'll have to earn trust from then on. They apparently disappear forever, and no mention is made of them or whether they eventually redeem themselves or get pushed out an airlock or what.
In the Star Trek: Enterprise episode "Damage", Archer is forced by circumstances to steal a warp coil from an Illyrian ship, leaving them stranded 3 years from home. That the Illyrians' fate is unresolved in that episode is dramatically necessary. Oddly however, once diplomatic relations with the Xindi improve in future episodes, Archer apparently doesn't bother to ask them to assist the Illyrians.
There are plenty of these throughout Merlin. The most egregious examples:
Hunith - when Agravaine's men attack Ealdor, the heroes rush off and she abruptly disappears from the story.
Lady Vivian - is she still under that Love Potion spell?
Alvarr and Enmyria - the former escapes Camelot and the latter evades the knights in the forest, but neither are ever seen again.
Tristan - disappears completely after Isolde dies.
Sefa - where'd she go after escaping into the forest?
Aithusa, the white dragon. It would appear that the writers introduced this character without having the slightest idea of what they planned to do with it. It disappeared for long stretches of time without explanation, we never get any clear answer on why it allied itself with Morgana, it never does anything particularly important or interesting, and in the Grand Finale, Merlin banishes it within the first minute of the episode to never be seen or mentioned again. It is, however, useful in that it helped Morgana create the sword used to kill Arthur.
There's a literal example in Valiant. Merlin watches as Valiant gleefully feeds a live mouse to the snakes that inhabit his shield. Valiant realises he's being watched, draws his sword and leaves the room. The mouse just disappears. This is pointed out on the audio commentary, where it's suggested that Valiant ate it himself.
In The Once And Future Queen, King Odin sends an assassin to Camelot in order to kill Prince Arthur. The assassin is helped in his search for Arthur by an informant who lives in the castle (or at least has easy access to its interior) and feeds him information. Who he was and where he went afterwards is never elaborated upon.
The secret room that Merlin discovers in Goblin's Gold, which is filled with magical books and paraphenalia, but is never revisited or explained.
In The Coming of Arthur Part I, Arthur is hit in the leg with a poisoned dart. Throughout the rest of the episode he struggles with it until finally he gets Gaius the physician to give him a tonic to numb the pain. Gaius warns him that it'll only last for a short while. However, the episode ends before the tonic wears out and the second part of the episode opens a week later. The injury is never addressed again. Presumably Gaius healed it in the interim, but it was still odd that such a big deal was made of it only for nothing to ever come of it.
In The Hunter's Heart Agravaine plants a fake letter on the dead body of a scribe he's just killed. It's given to Gaius who openly states that there's something "very wrong" with it, but this is never addressed.
In Arthur's Bane Part I the tower of Ismere is guarded by several wolves that are apparently controlled by Morgana. One of them somehow manages to render Gwaine unconscious (off-screen), but their presence and abilities are never explained and they disappear halfway through the episode.
In The Death Song of Uther Pendragon, Arthur prevents an accused witch from being burnt at the stake. She dies soon after, but her status as a witch and the issues raised by Arthur's actions are never questioned.
In the third episode of Dirk Gently, Gently injects a microchip into McDuff's hand that allows him to control a university campus' computers and other electrical equipment, but carries side effects including diarrhea, nausea and vomiting. McDuff has one attack of diarrhea almost immediately, after which the side effects neither recur nor are mentioned again.
Supernatural: Yellow Eyed Demon mentioned other generations of children he'd given his blood to; they're never brought up and none of them ever appear, which is very strange for a show with such a complex myth arc.
Especially that they made it quite clear that YED was still giving blood to kids. What will happen with these kids who weren't part of the Battle Royale we will never know.
Used occasionally on Mystery Science Theater 3000; not really a straight example, as it's generally only brought up to either highlight the crew's disinterest in the film-du-jour's "actual" plot (the nuns at the bus stop from Soultaker, the Mad Scientist's father from The Brain That Wouldn't Die), or gaping holes in same (John Saxon from Mitchell).
John Saxon did die when he flipped his dune buggy while trying to run down Mitchell. The scene was already cut from the TV print Best Brains used, but you can briefly hear it mentioned on Mitchell's radio in a later scene.
One moment took this trope to an extreme: in a 1950s short about marriage, a counselor uses a rubber band as a metaphor for relationship stress. After it snaps and the young couple expresses shock, Tom and Crow go into mock hysterics ("We're gonna die!" "But where's the rubber band?!")
Also, in the last episode, the SOL crashed to earth. Mike, Crow, and Servo rented a one room apartment, and Gypsy is mentioned as starting her own company, but neither Cambot nor Magic voice were shown.
They never stated what happened to the nanites, who had made occasional appearances during the Sci-Fi era, either.
Another resolution deleted example: In The Office (American) episode "Drug Test," you don't find out where the joint came from. The deleted scene shows that it was from two of Vance's deliverymen who commonly deliver things to the office.
Another from The Office is the final fate of legendary Jerk Ass Todd Packer. Jim and Dwight set up Packer so that he would go to Tallahassee and get in a great deal of trouble with the Sabre CEO, and eventually Michael decides Packer is irredeemable and allows him to leave (and walk into the trap), though what became of him is never mentioned.
In season 8, his fate was revealed.
In an episode in a later season, Jim is being interrogated by Dwight via a drug store blood pressure machine, when it is revealed that Jim has high blood pressure. Pam escorts him away and it is set up as a major plot point. As of the last season so far, this has been completely forgotten about.
The monstrous praying mantis is shown to have laid eggs in a school storeroom.
Marcie Ross, the invisible girl, was taken by the government to be trained in assassination and espionage. Fan outcry about her was so great that the writers of the comics have said they might include her in a future plotline, since the Slayers are now enemies of the United States government.
Willow and Tara's cat Miss Kitty Fantastico appears a couple of times in season four, before disappearing for the rest of the series. A throwaway line in season seven suggests Dawn accidentally killed her with a crossbow.
After Oz leaves the show, the fate of his band, Dingoes Ate My Baby, and its members (including its lead singer, minor recurring character Devon) is never revealed.
Although Tara is healed prior to Glory's defeat, no mention is made of her other victims.
In season 3 of The Sopranos, Paulie and Christopher try to kill a Russian man in the snowy Pine Barrens, shooting him as he runs off. He escapes into the woods and isn't mentioned ever again.
A commercial for the final few episodes of the series highlighting most of the notable character deaths up to that point makes light of Valery's (intentionally) ambiguous fate, by first including him in the montage of the dead but adding a question mark after his name before moving on to the next character.
Paulie and Christopher later tell that story as an anecdote, prompting them to fight it out. So while it remained a mystery, it's been mentioned.
In season 2 of Heroes, Peter accidentally leaves his girlfriend Caitlyn in a plague ridden future. This is a major motivation for him to try and stop the virus from getting released. Then in the season finale, he prevents the virus from getting released, and afterward doesn't seem the slightest bit interested in what happened to Caitlyn. The real reason for this is that before the writer's strike, the virus was going to be released, and when they reshot the ending, they didn't have enough time to shoot a resolution to Caitlyn's story. Word of God makes it clear she'll never be seen or heard of again either.
Also, the writers have apparently forgotten about Nathan's ex-wife and children.
But what happened to Audrey Hanson? She walked away from the scene of Ted Sprague's death and off the face of the earth without further mention.
Who healed Nathan in Season 3!? They revealed it to be Linderman, but then, a few episodes later, we learn that Linderman was just an illusion. BUT THEN WHO HEALED HIM!?
It's been offered that Nathan may have been right about it being some type of divine intervention or it may have been Future Peter drawing on a power that he did not know he had because of his empathy.
Forget Nathan being healed by Future Peter... The mystery is what happened to Angella Petrelli's sister after she left Coyote Sands?
Whatever became of the police officers who wanted to question Sandra Bennett after Claire's immune system suddenly fails in hospital when her healing powers disappear, prompting suspicion on the part of the doctors? Claire recovers and they leave the hospital, but the question of her powers being revealed to the police isn't addressed afterwards.
Hana Gitelman, Claude Rains, Monica Dawson, Maya Herrera, Molly Walker, Flint Gordon and many others simply disappeared from the series. If they were lucky they got a fate in the comics.
Leoben, the first humanoid cylon ever revealed to the colonial fleet. Leoben initially had a pivotal role in Starbuck's character developement. Its Leoben that first claims Kara is "special" and that she has great importance to the future of human and machine kind. Kara and Leoben's relationship goes through many phases as Leoben tries to convert her to his personal religeous theories. Starbuck even has a vision of Leoben which results in her finding the way to Earth. Then half way through season four, Leoben gets kicked to the curb. Starbuck remains important, but Leoben is not seen again except for a breif line in the final episode, and at that point there's no telling if that was even the same Leoben. Leoben and Starbuck are never seen together after the discovery of "Earth".
Although the scene was deleted, there was an occasion where the rebel Cylons are discussing things and Leoben says "The Sixes and I are in agreement," leaving the viewer with the implication that the other Leobens have all been killed by Cavil's forces and that Leoben is the last of the model, which is why there are no Leobens seen elsewhere in the fleet until the finale. Still doesn't explain why that one Leoban isn't seen until the finale, though, but that was probably more due to actor availability.
Bulldog, a recon pilot that was involved in an incident which may or may not have triggered the second Cylon war is released and allowed to escape to galactica. It is implied that the cylons let him escape so he'd find out the truth and kill Commander Adama. After his debute episode Bulldog is never seen again.
In the commentary, Ron Moore jokes that Bulldog was sent to the same ship Boxey was sent to (see beow).
And Boxey, a boy Boomer rescues from Caprica. He managed to last all the way into the first episode, with deleted scenes in the next. Plots for him attached to Sharon/Tyrol were derailed when Sharon's Cylon plot was jumped forward as soon as episode two, and the writers just seemed to not care about a possible artful dodger/black marketer idea. He very quickly disappeared never to be seen again.
The Young Ones, Bomb. This example goes hand-in-hand with Nobody Poops. The students have a TV detector man on the doorstep and no TV licence. Low on ideas they come up with a desperate idea... "Vyv -- eat the telly!" However, once the man from TV licencing barges into the living room and sees a electrical flex protruding from Vyvyan, he decides to go and wait for the unknown contents to work their way through. Once he heads upstairs, falls through the hole in the floor and walk back up, we never hear from him again.
In the second season of Life On Mars, we never find out what happened to DCI Frank Morgan and the rest of Hyde's C Division.
Father Ted: If Jack ended up wiping his nose with that puppy after his once in a leap-year good mood, we can only shudder to think what happened to that little girl.
Eh, maybe she ran off?
The West Wing is somewhat infamous for this, the most notable example being the character of Mandy, who disappeared with not a single reference afterwards after the first season. The website Television Without Pity uses "sent to Mandyville" as shorthand for indicating that a character disappears without comment or reference from the cast of a show.
She didn't just disappear, she was retroactively erased, failing to appear in flashbacks where the character should have been present. It was as if she had been swallowed by a crack in reality.
Done at least twice by Veronica Mars. In Season 1, Weevil breaks into the Kane house to steal a note he wrote to Lilly before she died. At the end of the episode, Veronica confronts him and asks what the note said. Weevil refuses to tell her, and the show never explains, or even mentions, the note again. And in Season 2, when Grace Manning's abuse is discovered by Sheriff Lamb, we never find out what happens to her or her parents.
In the Hogan's Heroes episode "Everyone Loves a Snowman", the latest scheme involves pretending to work on a snowman to hide the fact that the group is digging a tunnel underneath/inside the hollow snowman in order to smuggle a group of airmen back into the regular tunnels. When Schultz sees someone going inside the snowman and reports it, they quickly move the snowman so that Hochstetter will investigate it and find no tunnel. So...what did they do with the now-undisguised tunnel opening in the middle of the prison yard?
Will Scarlett and his brother Luke are saved by Robin from the gallows. Will escapes with Robin, but what happens to Luke? This is answered in a later episode in which Will tells a fellow outlaw that Luke went to Scarborough. But in the second series Luke and his father Dan Scarlett return to Sherwood to visit Will, and during the course of the episode Dan dies. The episode closes with Will and his brother standing beside one another...but in the next episode, Luke has completely disappeared again.
Like Luke Scarlett, Benedict and the man who claims to be a clergyman also disappear without a trace, as do all of Little John's gang except Roy
A similar fate happens with another guest-star: the Fool. He was one of the more interesting and useful allies to the outlaws, but after the episode is finished (during which the Fool has actually been brought into the outlaw camp), he's never seen or mentioned again.
In the third season a lion (don't ask) is set loose on Sherwood Forest in order to kill the outlaws. After the outlaws throw mustard powder at it (please don't ask) it disappears entirely. It's real shame that it never reappeared when least expected in order to eat someone.
In Wizards of Waverly Place, the Alpha Bitch, Gigi, supposedly set to be Alex's arch enemy appeared twice in the first Season, then once in the second and hasn't been heard from since.
Alex and Justin acquired an enchanted dragon disguised as a beagle named Dragon in "Curb Your Dragon". After that episode, Dragon never appeared again. It was lampshaded at least twice, once by Max and once by Zeke, who coincidentally debuted in the following episode.
A literal example takes place on House, with Dr. House's pet rat, "Steve McQueen". After an episode devoted to his capture and some time as a proverbial coal mine canary, he made background appearances until he and his cage suddenly disappeared from the apartment. Following fan speculation, Word of God finally admitted that he'd died.
Truth in Television, as even healthy rats don't live more than three or four years, and Steve McQueen wasn't a baby when House caught him.
Babylon 5: the character of Lennier abruptly departed from the show in the penultimate episode. He is the only major character whose fate at the end of the series remains unknown.
He is dead. In "Sleeping in Light" Sheridan offers a toast to absent friends, standard Unusual Euphemism for the dead. Mentioned are Londo, G'Kar, Marcus... and Lennier. JMS has made vague implications that it happened during the Telepath War. Another reason for us to hate Bester, maybe?
By "vague", the plot outline for the Telepath War bluntly states that Lyta and Lennier die during it.
This also includes a major character from the pilot episode, Lt. Commander Laurel Takashima, the station's first executive officer. She just vanished without a trace and was never referred to again (except once, not by name and obliquely).
People in the Real Life Military get moved around from post to post all the time. J. Michael Straczynski once said (in reference to Riker of Star Trek: The Next Generation) that anyone working as First Officer for seven years on the same ship meant that his career was over.
This was actually Lampshaded in ST:TNG itself where other characters would occasionally admonish Riker (Notably in 'Best of Both Worlds') for letting his career stagnate over not wanting any position other than Captain of the Enterprise.
Possibly the ultimate example is the apparently major and important character of 'Justin', an old human man who is Sheridan's opposite number amongst the Shadows and revealed to be the more familiar recurring villain Morden's boss, as well as the originator of the plan to use Sheridan's wife against him. Justin's character not only vanished without explanation (presumed killed in the nuclear blast, but since Morden survived Justin may have as well) but showed up with absolutely no explanation either except the foreshadowing dream that mentioned "the man inbetween". He is one of the show's biggest outstanding mysteries.
According to Word of God, it was planned that Justin would make more appearances, but Real Life intervened. Legendary character actor Jeff Corey, who played the character, was undergoing radiation and chemotherapy treatments for lung cancer, which rendered him too weak to work. By the time Corey was back in (relatively) good health, the show was off the air. Corey succumbed to the cancer five years later, in 2002.
Avoided by the character of Na'Toth, however. Originally a recurring character of some importance in the first two seasons, she vanished in late Season 2 with no explanation. Much later we got an explanation that she'd returned to Narn and been killed in the Centauri bombardment. Eventually and unexpectedly, she returned in a single episode of Season 5 that satisfactorily resolved her fate.
Sinclair is abruptly sent away from Babylon Five at the end of the first season, but the authors at least try to justify it in-universe. His fiancee (who he proposed to at the end of season 1!), on the other hand, is never seen or mentioned again.
She was lost in a mission, sent through a wormhole, lost in time and space, and it is IMPLIED that she ended up somehow meeting Sinclair in a spin-off book. All There Inthe Manual.
The League of Gentlemen: In the first episode of series 3, one of the plotlines featuring a woman who had to play party games with a Doctor in order to get her medication had the final scene of her plotline cut from the final draft, so we never saw if she actually won the game or not and got her medication. In the last episode of series 3, there's a variation on this where Dean Tavalouris has his camera stole of him which is then used to make a seedy striptease video. The next time we see him, Dean has his camera back and the whole camera being stolen story has been dropped and is never resolved.
Remember in Sliders when there was an evil slider who was going to follow Quinn Mallory to the ends of time and space to get her revenge? What happened to her? And don't say "the show got cancelled," because she already hadn't shown up for years.
Considering the show casually got rid of Colin Mallory who got "unstuck", it's not surprising that other characters would be forgotten too.
In one episode of Law & Order: Special Victims Unit, the initial plot is set up based on a serial rapist known for carjacking his victims - it's quickly discovered the case they're investigating isn't actually his work, though, and the subject turns entirely to that. The fact that they're focusing countless man hours on a solitary case while there's a serial rapist on the loose isn't brought up, and no closure is made on that plot.
On Desperate Housewives we learn that Susan's ex-husband Karl re-married. In a later episode the audience finds out Karl's second wife abandoned him and their young son Evan because she couldn't handle being a mother, thus making Karl a single parent. After a couple of episodes Evan vanishes and is never mentioned again. Not by Karl, not by Evan's half-sister Julie, not by Bree the woman Karl wants to marry and then Karl dies and Evan * still* isn't mentioned.
In a forgettable After School Special from The Eighties called Maricelanote about a young El Salvadorian refugee whose mother works for a middle class WASP family with a teenage girl of its own, it is established that the title character's mother is possibly an illegal immigrant who could face deportation, and that said title character's father is trapped in the middle of an El Salvadorian civil war. However, both of these potentially interesting plot points are forgotten at the climax of the special, when the WASP-y girl learns an important lesson about tolerance and rescues Maricela, yadda yadda.
LOST has Cindy, Zack and Emma. Some of the only tailies left, they joined the Others and appeared occasionally during seasons 2 and 3 before becoming regular recurring characters in the sixth season. They join MIB's group. Said group gets bombed, but some survivors scatter into the jungle. Problem is, it's never resolved what happened to these three-you never see clearly if they were in the group that got bombed, and the ones that scattered are never seen. Especially bad since Cindy was one of the few guest characters left over from the pilot.
On the night of the crash, the Others abducted 3 Tailies. A week later, they abducted 9 more, including Zack and Emma. Finally, on the 48th day, they abduct Cindy. So that's actually 13 characters unaccounted for. When Cindy, Zack, and Emma first appear with the Others, they're part of a small group that one might assume are the other abducted Tailies, but it's never spelled out. So that's 10 characters that get even less resolution than Cindy, Zack, and Emma.
Benjamin Linus has a childhood friend named Annie that appeared in one episode's flashback to his youth. The creators claimed she was going to be very important to his character, but she was never brought up again on the series, and it is never explained why she is no longer on the island when Ben grows up.
Sun conceives Jin's baby while they're on the island together. She's rescued, thinking Jin's dead, and gives birth to their child in their native Korea. Three years later she learns Jin's still alive and goes back to the island, leaving the baby in the care of Sun's mother. Jin and Sun are finally reunited and die together. Which led millions of viewers to ask (including people who'd accost Daniel Dae Kim on the street), "BUT WHAT ABOUT THE BABY???!!!??"
This happens frequently on CSI: Crime Scene Investigation. Two characters, Mia Dickerson and Riley Adams, get a passing mention in the season following their respected disappearances. However, two others simply vanish. Ronnie Lake was a CSI on swing shift who worked closely with Sara Sidle. When Sara quit, Ronnie was never seen again. The biggest one was Sofia Curtis. She had a substantial character arc from seasons 5 through 7. She appeared in the first episode of season 8, then poof. Gone. Not even mentioned by those who were apparently close friends and co-workers. That is, until season 11, where she did finally re-appear in an episode. She's Brass's boss now.
CSI NY is no stranger to it either. In the season 2 episode 'Run Silent, Run Deep', Danny Messer's brother, Louie, is badly beaten and left in a coma by Tanglewood boy Sonny Sassone, who discovered him wearing a wire to try and get information about an old murder case recently discovered by the CSIs. Louie was mentioned once in passing, in a season 3 episode, but nothing's been said on his condition, and he hasn't been brought up since then.
While not outright stated, it was heavily implied that he lost to the magician.
The second season finale of Californication saw Becca and her boyfriend Damien proclaim their love for each other; Damien is, in fact, the reason Hank and Becca stay behind while Karen goes off to New York. Damien is never seen in the third season, however, is never mentioned and has no bearing on the plot whatsoever anymore. This is never explained.
In As Time Goes By, Lionel's father, Rocky, is diagnosed as terminally ill during the third series; Lionel says "he'll be lucky to last a year". However, the writers apparently decided they'd rather not kill him off. The program lasted several more years, but the possibility of Rocky's death was quietly dropped.
Didn't this episode show a newspaper article about the mouse being the bane of cats all over the world?
At least a later episode showed the power transfer isn't permanent.
In Dead Like Me, at George's funeral she, as a ghost, watches her father hug one of his male students long and hard. She even asks Rube if its normal for two men to hug that long. There was an obvious implication of a homosexual affair, but it was never brought up again and towards the end of the series' run, its stated that he had an affair with a female student, not a male.
Lawrence Dominic. Partway through Season 1, he gets Put on a Bus (well, in the attic, actually). Echo meets him again a whole season later, when she gets sent to the attic—but when she escapes, he decides to stay in the attic and help the people there. He manages to escape in the next episode, but Adele sends him back, reasoning that it will be useful to have someone on the inside. And then the season ends without ever mentioning him again. He does show up once in "Epitaph One" to confront Adele, but he is never shown with the group again, and we neither find out what happened to him nor what happened to Clyde or any of the other human computers in the attic.
There's a brief mention that Whiskey/Dr. Saunders gets taken out of the building before it explodes, but we don't know in what state, how she gets to her totally different "Epitaph One" state (and how her scars get removed), or what happens to her afterward, since she isn't in "Epitaph Two."
Both of these can be attributed to the cancellation making it impossible to wrap up every plot point. If the cancellation confirmation hadn't come before the last few episodes were made, it could have been much worse.
Fans of Dexter have been asking this of the surviving Fuentes brother in Season Five. This might not have been such a big deal, except the first half of the season has several episodes dedicated to Miami Metro investigating the Santos Muertos murders, which, of course, leads to Deb shooting one of the Fuentes brothers dead, but there is still one running around somewhere.
And while we're at it, where did Officer Cira Manzon run off to?
Dexter also has a very strong tendency towards dumping its non-Dexter related subplots and any characters introduced that season between seasons, regardless of whether their plots have been resolved.
On Angel, a minor character working in Wolfram & Hart's mail room is revealed to be a follower of a powerful rival demon who, unlike the higher people in the corporate hierarchy, knew ahead of time that the baby Connor was the fulfillment of an apocalyptic prophecy. After the scene in which he makes this phone call, we never see him again, nor is the demon ("Master Tarfall, Underlord of Pain") ever mentioned outside this one incident.
Boston Legal is a terrible offender, the series constantly throw away characters without any resolution at all, one of the worst examples is when the new guy had been thrown away from his own office by an old lady, both of the characters were regulars and had been central to several plots, one episodes ends with the new guy making a resolution to get his office back from the woman, we never heard of them again (and this was midway through a season).
Even the otherwise awesome-beyond-all-belief Chuck Norris couldn't save Walker, Texas Ranger from falling into this trope: one episode where Cordell Walker was supposed to rescue a girl who was trapped in a Christian cult camp ended up having the last several minutes of it focused on Walker rescuing Alex Cahill from the cult camp, leaving the intended rescue target's status in question.
This as well as many of the team partners that joined Walker, seem to simply disappear with no explanation.
The Monkees: At the beginning of "Monkee See, Monkee Die", before the plot gets underway, the boys' landlord tells them that he is kicking them out. Once the actual story begins, this plot thread is completely forgotten about and never addressed further.
An episode of Victorious ends with the character Cat being locked up in a padded room in a strait jacket. No-one from the rest of the cast notices.
iCarly, in the episode iWon't Cancel The Show has Sam get arrested and put in juvie for assaulting an ambassador with a hot dog or shoving it down his pants or something. Whilst she'll get out of juvie in a day or two, the case wouldn't just be dropped. It's never mentioned again.
In M*A*S*H, both Spearchucker and Ugly John disappear without any notice or mention again. Likewise, Lt. Dish, who was in the theatrical feature and in the TV series' pilot. After that, she was gone. Justified, in that it's the Army where people come and go a lot as they get rotated between various posts. The unstated reason for the removal of the Spearchucker character was that the show's creators learned that there were no black surgeons (working as such in MASH units) in the Korean War.
The 4400. Throughout the first season, Diana drops hints about her bad relationship with her father. She even calls him in the pilot episode when she thinks the world is ending. You never find out what happened between them, and he is never mentioned again.
In the first series, Vorenus is made a senator by Julius Caesar. In the second series this is seemingly forgotten about, even though people were made senators for life. Caesar arranges a staged-election of Vorenus to the Senate. In fact, no one was elected to the Senate. Politicians elected to certain offices became members of the Senate for life, unless removed by the Censor. The point stands, of course.
Before his assassination Caesar is shown to have appointed several long-haired Gauls to the Senate, much to the consternation of his rivals. No mention is made of what happened to them after, so one is left to assume that they left Rome and returned to their homelands.
Many of her close friends like Crystal (a recurring character who appeared in the opening credits and later married Dan's father) and Anne-Marie stopped appearing with no further mention.
Crystal and Anne-Marie both appeared in the opening episode of Season 8 where Roseanne had a baby shower. It was explained when she was making a video for her unborn child that Dan and his father just can't get along, and that's why Crystal isn't seen around as much.
In Primeval there are a couple of these, especially after its return for series 4 and 5
What happened to the Diictodons, Sid and Nancy?
while it could be case of "No-body likes her anyway and she abused Connor, tried to get in the middle of some serious shiptease, and helped Leek and Helen with their evil plot before having a case of Heel-Face Turn" Caroline plays a major part in series 2's arc, appears at Stephen's funeral, and then disappears off the face of the Fandom.
What happened to Connor's front door key after it disappeared into the anomaly?
What ever happened to Abby's little brother Jack? How did the ARC explain his older sister getting trapped in the Cretaceous for a year? Pretty sure Abby would want to tell him she's still... you know, alive.
The Walking Dead manages to avert this humorously in the episode "Guts". Glenn and Morales are going down a storm drain, following after a rat while looking for a way out of the city only to discover large and impenetrable bars in their way. It's explicitly shown that a walker made a meal of the rat after it passed through the bars. Rather than what happened to the mouse we find our what happens to the rat.
Warehouse 13—MacPherson's wife plays a role in the last episode of season 1, and then . . . is never heard from or mentioned again. Including when Mac is, well, getting his mack on in the first episode of season 2.
An infuriating one in Miami Vice: At the end of the second season, Tubbs rekindles his relationship with Angelina Medera, daughter of murdered crime lord Esteban Calderone, and discovers that they have a son together. However, Calderone's son Orlando captures and kills Angelina and manages to convince Tubbs that his son was killed as well, when in fact he is secretly being kept alive. In season 3, Tubbs managed to get revenge on Orlando, but never discovered that his infant son's death was in fact faked, and this was never brought up for the remainder of the series.
A fairly minor one occurs in the Leverage episode The Carnival Job: the original mark is discussed as mourning for his dead wife by neglecting his daughter and spending lots of money on their house - he's $3 million in debt. By the end of the episode, he has come to his senses, reunited with his daughter, and returned the MacGuffin he stole, so everyone acts like this is a happy ending. There's no mention of the fact that he's still deeply in debt and probably won't be able to pay it off.
In the two-parter at the beach house, it's revealed that Phoebe's biological mother is very much alive. After this two-parter and one Season 4 guest appearance, she never appears again and arguably came down with Chuck Cunningham Syndrome.
Phoebe's father Frank (played by Bob Balaban) who made one appearance and was never seen or heard from again.
The fact that Rachel is a single mother is something that should, in theory, weigh in hugely on her decision to move to Paris or not. Emma should probably be on the plane with Rachel when she makes her dramatic exit to come back to Ross at the end. Yet, she's never even mentioned or seen in the finale.
The Mathnet episode "The Case Of The Calpurnian Kugel Caper": In the beginning, we find out the old king of Calpurnia was killed in an car crash that the CIA believes was sabotaged. Then nothing else. It's not even brought up in the mugshot sequence at the end. Our best guess is that Kaboom Pickens or his accomplice Carl Eiken-Meister (or possibly both) did it.
Whatever happened to Missy Cooper, Sheldon's sister from The Big Bang Theory? She only appeared in one episode, and got a passing mention in another, but she hasn't made a return appearance so far.
In the episode "The Cooper Extraction", Sheldon visits Texas when his sister is giving birth.
For that matter, whatever happened to Stephanie? Her relationship with Leonard got a whole arc of episodes devoted to it, and just when their relationship troubles had been resolved... she disappeared and was never seen again.
Kamen Rider Fourze plays a rather horrific version of this trope. In the show, the Horoscopes don't really die, and the people that turn into them can still fight. How do they get taken out for good? Well, one of them sends the failures to the Dark Nebula, a dark place that horrifies some of the most loyal of Horoscopes. There were only three people trapped in there in the series' run: Sonada, due to her failing Gamou for the last time, Kijima, due to Libra tricking Virgo into thinking he was a nosy police officer, and Sugiura, who went against the Horoscopes. It's eventually revealed that Virgo, the Horoscope that summons the Dark Nebula, was just sending them into suspended animation at his HQ. When he is killed by the Big Bad, the three that were sent to the 'Dark Nebula' were never seen again after that revelation, nor was there any telling that the heroes ever got around to saving them, even in the "Where Are They Now?" Epilogue movie which has a five-year timeskip. Rather unnerving for a fun show about friendship.
In "Starstruck," the episode begins with Jo putting on a fund-raiser for a scholarship student. By the end of the episode, the girl is never mentioned again.
In "The Four Muskateers," the girls have worked off their debt to the school and are told they have to move out of their room to make room for four more girls who are being punished. By the end of the episode, Blair, Jo, Natalie, and Tootie are in trouble again and must stay in the room as punishment again, but no mention is made of the other girls who were to be punished.
In American Horror Story: Asylum, Pepper vanishes partway through "Nor'easter". No one comments on this, or really seems to notice. She shows up again in "The Coat Hanger." It turns out she was abducted by the same aliens who abducted Kit, Alma, and Grace.
Ultra Seven Episode 17 features a literal example of this trope. In the beginning a miner named Jiro, whom is identical to lead character Dan Moroboshi due to Ultra Seven having saved Jiro's life and based his human appearance on him after the latter nearly died performing a similarly selfless act, runs back to save one of the miners' pet mouse from death during a mysterious cave-in. Both are trapped in the mine, and the plot of the episode centers on the Ultra Garrison attempting to free him. Throughout the episode Jiro talks to the mouse (named Lucky) for comfort and yet when Ultra Seven rescues Jiro towards the end of the episode...we never see what happens to the mouse. Ultra Seven didn't even know it existed, so in all likelihood it was just left to die in the mine or killed in the explosion that follows. Which means that not only does the audience (many of whom would have been children) never receive any resolution to that character arc, but for all we know the entire point of Jiro becoming trapped in the mine, and thus the entire point of the episode's plot, has been negated.
Community usually avoid these, but in the episode Custody Law and Eastern European Diplomacy, Troy and Abed have a new friend, Lukka, who they eventually find out did some pretty nasty things during the ethnic cleansing in the Balkans. By the end of the episode, they've stopped being friends with him and he's never mentioned again. It's of course good that they stopped hanging out with him, but there's still a Balkan war criminal on the loose in Greendale.
Degrassi's first half of season 12 had episodes revolving around Clare taking an internship at the Toronto Interpreter and trying to impress her boss Asher Shostak with her great journalism skills. She is then sexually assaulted by him in his car and keeps the incident a secret. Clare all of a sudden gets fired from her job because Asher accuses her of making false accusations about him and making her the bad guy in all of this. In the last episode of the first half, Asher's former intern tells Clare that she too was sexually assaulted by him and agreed that they both should go to the police together about the matter. We never find out if they did go to the police and Clare's former job at the Toronto Interpreter, even Asher's sexual assault on her, is never mentioned again throughout season twelve, nor in thirteen.
Technically, this isn't the show's first and only storyline to go unresolved. There are several that are still left hanging from previous seasons, but we're not going to name them all because there's one too many of them.
The first few episodes of MythQuest set up David as a romantic interest for main character Cleo. After the fifth episode, he's never seen again.
The cat in season 1. Caused a bunch of drama for the characters for about three episodes, then after one episode, was never seen or mentioned again.
The tear of gold from the season 2 finale. It was even the cliffhanger, and...it was never even mentioned in season 3.
A lot of characters have disappeared between seasons with no given explanation. The most notable will have to be Jason Winkler, who was a pretty major character and actually had a suggested romance with one of the main characters despite being a teacher. Then season 2 came, and...gone. Granted, with him he WAS dying of a degenerative illness but they never specified a death so much of the fandom refuses to believe he really died.
In Season 9 of Smallville, Mia Dearden (the second Speedy in the comics) appears in two episodes, over the course of which Green Arrow accepts her as his sidekick. She is then never mentioned ever again.
In Season 3 of Charmed, it was hinted that Cole was working for the Triad for something in return. At the end of the season, it was revealed to be his father's soul, which they had somehow acquired years earlier. However, after obtaining his father's soul, it is never stated what Cole did with it and is never mentioned again.
A rather literal meta-example from The Goldbergs: In the episode "Mini-Murray," after Adam's mother refuses to allow him to go see Poltergeist on his own, Adam tricks his grandfather into taking him, telling him that they are going to see The Great Mouse Detective. After a few short clips of the two watching the movie and being visibly terrified by it, finally they cut to the movie having just ended, and Adam's grandfather exclaims, "Where the f__k was the mouse?!?"
In one episode, Roz says she accidentally ran her car into a limousine that happened to have four high-powered lawyers in it, and that she'll be paying for the damages for a very long time. It never gets brought up again after that.
In another episode, it's revealed that Martin regularly goes to the parole hearings of the man who shot him. The episode ends on an unusually somber note, when Martin is asked if he wants to make a statement, he stands up and says, "I've got nothing to say," then leaves. He does this even though both the man and his mother had told Martin he's changed his ways and now repents his criminal actions. This subplot is never followed in later episodes, and Martin is never shown going to the parole hearing again. Then again, how often are parole hearings for someone convicted of shooting a police officer anyway?
Once Upon a Time, in the season three episode Going Home, the curse is destroyed, sending all the characters back to the Enchanted Forest, the only exceptions being Emma, because she's the savior, and Henry, because unlike everyone else, he was born in in the 'real' world. There is no mention of Ashley/Cinderella's baby, who was also born in the 'real' world back in season one.
The Thick of It: Malcolm's Ax-Crazysecond in command Jamie, after playing a fairly prominent role in the post season 2 specials, vanishes without explanation for seasons 3 and 4. Given the he was last seen siding against Malcolm in the leadership contest, though, it seems safe to assume that he probably doesn't have a job any more.