In Pokémon, Ash promised that he'd come back for Pidgeot when he returned. Twelve seasons later (including 4 returns to Kanto), he still hasn't come back for it.
Maybe an even more ridiculous one would be Ash's Primeape, which was Put on a Bus less than five episodes after Ash caught it, left in the care of somebody who promised to turn it into "a true champion." It was never seen again.
Almost everyone Ash meets, he promises to see again. Only a fraction of those end up being recurring characters.
The last above would also apply to the Kanto Gym Leaders, as Ash does travel to the cities where Sabrina and Koga hold court in the Battle Frontier season to fight in the facilities of the eponymous organization. This actually makes those two Gym Leaders, as well as Misty's sisters (as Misty left them at the Cerulean City limits,) an Inversion, as it would've been easy for Ash to visit them but he didn't.
At the end of the second arc (and second episode of the anime adaptation) of King of Bandit Jing, the nine-tailed fox Sherry swears to one day come back for vengeance against the titular thief. The series ended without her ever showing up again.
The Amazoness Quartet in the Sailor Moon anime. They mention that fate may bring them to meet the Sailor Senshi again, hinting they'd possibly return in the next arc. This wasn't unreasonable, considering that their manga counterparts did exactly that, as the Sailor Quartet, guardians to Sailor Chibimoon. However, they never appear in the anime again.
The character Gin from one of the earliest arcs in One Piece stated that he wanted to see the main characters again on the Grand Line. Over 500 chapters, half of the Grand Line, and a two year time skip later and he still has yet to be mentioned again. And this is in a series where the author seemingly never forgets about characters. Of course, the series is not over yet, but each passing arc (and dramatic increase in the power of the villains) makes it more and more difficult to imagine his return.
Kouga from the InuYasha manga appears less and less often until Naraku finally takes his Shikon shards, then he's gone. Even Houjou, Kagome's other Hopeless Suitor, gets an appearance in the final volume.
This is the case of Asuna in Mahou Sensei Negima!. As a result of the events of the Magic World, Asuna's fate was to sleep in the Gravekeeper's Palace for a century.
Subverted two issues later, after she wakes up and breaks down in one, and is brought back via Cassieopia at the start of the next.
Two sets of X-Men dove into what was essentially Hell to rescue their version of seven year old Illyana Rasputin from a demonic wizard named Belasco. One set got her out safely but were trapped themselves (the lucky ones died), the other set escaped but their Illyana spent seven years stuck there as Belasco's student while getting her soul sacrificed piece by piece before she overthrew him and returned to earth a heartbeat after Kitty lost her grip. A few years later, after Illyana's apparent Heroic Sacrifice, her eldritch armor was torn open to find a seven year old and wholly uncorrupted version of her. It was over twenty years before the X-writers were permitted to account for what was obvious to most readers at the time... it could easily have been the other Illyana.
Let's face it: for the X-Men, death is essentially being Put On The Bus.
The title character of the newspaper comic Robotman And Monty was written out of the strip, at the request of the comic's syndicate (tired of fielding complaints from readers confusing the absurdist humor strip with the much more kid-friendly "Robotman & Friends" toys and books). It would be easy to bring Robotman back (he's alive and well in space), but that's unlikely.
Elscol, a guerrilla leader who joined Rogue Squadron in the first arc of the comics, quit at the end of the arc following that. She knew that what she was doing was making a difference, but she knew any other good pilot could do as well, and she chafed under orders. She left without malice towards the squadron, hinting that she would try and talk Sixtus Quinn into joining the Rebel Alliance. But although plenty of other allies showed up again during the series, including a commando leader from that same issue, Elscol and Sixtus didn't. Not until the novels, anyway.
Dr. Andrew, a Delivery Guy OC who helps the Mistress deliver mixed-gender twins in Chapter 13 of the Superjail! fanfic Extended Stay, appears in this chapter only to never be seen again for the rest of the fanfic. The only thing that hints at his return is when he tells the new parents to call him back in when they're ready for him.
At the end of the third Artemis Fowl book, Holly Short tells Juliet Butler before she's mind-wiped that the bodyguarding business is too cold for her and she really should consider other employment. The epilogue says Juliet has quit and is trying out to become a professional wrestler, though Artemis hopes she'll fail and come back, and she's never seen again, though she's mentioned a few times in passing as her brother is still a fairly major character.
Averted by book seven, in which Juliet comes back in full force. As a professional wrestler.
In John Bunyan's Pilgrim's Progress, Christian (or Pilgrim, depending on which version you're reading) has to fight the beast Apollyon. When he's defeated, Apollyon swears vengeance on Christian, but aside from a couple of name-drops, Apollyon is never mentioned again.
Live Action TV
On the series Becker, the character "Reggie" (Terry Farrell) was put on a Long Bus Trip when Farrell was booted from the series under suspicious circumstances (she has told interviewers that it came as a complete shock to her and that she is unsure of exactly why it happened, although she reportedly led a strike among the show's cast). Farrell was even fired before the episode was filmed, and thus never even appeared on the episode where she was placed on the long bus trip, but was dismissed in a rather unflattering way where Farrell herself vanished but Reggie's continued presence was inferred (i.e. it was simply related that she slept with Becker, and it made her realize that she needed a change immediately). This was all done in a very implausibly slipshod way, obviously trumped up at the last minute: i.e. it was inferred that Reggie was popping in and out without Farrel ever being on-camera during the entire epsisode, and finally Reggie said good-bye to the cast in a phone-call that was dictated to them all by a complete stranger (don't ask— I said it was implausibly slipshod).
Power Rangers: What became of Trini and Zack after that trip to Switzerland is unknown, and is especially mysterious since Jason came back for Zeo and two guest appearances after.
A better example would be Rocky: he didn't even leave Angel Grove, he just stopped hanging with the Rangers.
Per Word of God, they tried to get Steve Cardenas to come back for "Forever Red", but he didn't respond in time — and even if he had, Rocky would have just been Tommy's butler, rather than being given the unused-in-America Gosei Sentai Dairanger costume as fans believed.
And though bringing them all back would naturally be problematic production-wise, there are many past Rangers whose powers still exist, so you'd think they'd drop by for certain global catastrophies. In particular, Countdown to Destruction could've benefitted from a few "stock footage shoots at other stock footage" type scenes from the Zeo Rangers and the (never stated to be in continuity, but why not?) otherSabanshows.
The departure of David Caruso from NYPD Blue was an example of this trope. Detective John Kelly doesn't even resign from the force. He simply moves to another precinct, leaving the option open for potential guest appearances in the future. There were even some hints later in the same season of his character coming back to be the best man at Detective Sipowicz's wedding, but a handwave was used to explain why it doesn't actually happen (in real life the actor simply would not have agreed to return, even as a one-shot guest appearance, nor would producer David Milch have wanted him back).
Implausibly; Kelly doesn't even visit his best friend Andy Sipowicz even when Andy gets married, no matter how badly Sipowicz obviously misses him. (Word of God says that the NYPD Blue producer kept a ''picture' of Caruso on his desk whenever any actor came to discuss a raise.)
Fer(r)etti in Stargate SG-1. Kawalsky's death in the first regular episode leaves him the only surviving member of the original Stargate team other than O'Neil(l) and Daniel, and he was shown to be leading an SG team of his own in his several-second cameo in the first season finale. He was mentioned once more in season three, then permanently and entirely forgotten, though as a team leader, you'd think he'd at least be seen or mentioned around the SGC every so often. (Meanwhile, via flashbacks, time travel, and parallel universes, Kawalsky has returned plenty of times.)
Of course, the really weird thing about this is that Ferretti was made leader of SG2 at the end of his last appearance, and SG2 is never shown or mentioned again. SG3 are recurring characters, and teams with numbers up to 25 are mentioned sometimes, but SG2 is never spoken of. It's almost as though the writers were planning to have him make an appearance at some point, but never got around to it.
Jonas Quinn, after being a regular cast member in Season 6, hasn't been even mentioned in the Stargate universe since Season 7, even when his planet was invaded by the Ori. Even an upcoming Stargate Universe episode involving said planet shows no sign of him.
Lt. Ford in Stargate Atlantis, in an example that crosses over with Never Found the Body, disappeared never to return following a hive ship explosion halfway through the second season. His conspicuous failure to return is more glaring in light of Sheppard's outright statement of relative certainty that he somehow survived and would turn up again at some point.
In Coupling (the original British version) Jeff disappears between series 3 and 4 as a result of his actor not wanting to return. His absence is explained by a hilarious one sided phone conversation with Steve "No, Jeff, the name of the island is pronounced Les-BOSS..... well that was fairly over-optimistic of you really wasn't it?". By the end of the show, Jeff is still "trapped" on Lesbos as a result of not being able get a passport... or something; Word of God has it that he ultimately decided to live there.
Shining Time Station wrote out Ringo Starr's Mr. Conductor by having him accompany Santa Claus to the North Pole. A relative, also called Mr. Conductor (and bearing a resemblance to George Carlin) took his place, and the original Mr. Conductor was never seen or heard from again afterward.
When Mama's Family transitioned from NBC to CBS, and the actors for the two characters were no longer available, Buzz and Sonja were said to have gone off to college...and were never heard from again. Not even at Christmas.
On The West Wing, Sam Seaborn left the White House in order to run for Congress, which would have made him an excellent candidate for future cameos in the series. Although it is finally revealed at the end of the series that he lost the race there seems to be little good in-universe reason for why he completely vanished.
Especially since they decided in the episode "Inauguration" to promote him to Senior Counselor to the President if he lost. This was just another subplot that was dropped entirely when Aaron Sorkin left.
On Dallas, Barbara Bel Geddes (who played the matriarch of the Ewing family, Mrs. Ellie) left the series in 1990 because she was quitting acting. The official reason in-show was that she was leaving to go on a tour of the Orient, and (off-screen) deeded the Southfork ranch to her son, Bobby. Despite her appearances in several of the Dallas reunion specials, she never appeared in either of the two television movies (J.R. Returns and War of the Ewings) made in the 90's.
Alex (who literally gets on a bus), Darcy, Mia, Ashley, and many other characters on Degrassi The Next Generation end their runs on the show this way.
It's actually pretty funny that Peter's last two girlfriends, Mia and Darcy, both took long bus trips straight to The CW to star in other teen dramas.
The Execs are not unaware of this bit of humor, and commented when 18ToLife got an airtime on The CW, it was the first time a Degrassi Alum got a CW series without dating Peter. Then talked about doing a Degrassi Mini where Peter and Ellie date.
Dr Goodman from Bones went on a "six month sabbatical" on the second season opener and that is literally the last time we've ever heard of him. He didn't appear, or even get mentioned, on the show's 100th episode which was specifically a "before the pilot" flashback.
Alice from Soap. She moves out of Jodie's apartment when Carol's mother said she isn't happy with her granddaughter (Jodie's child) growing up with a lesbian - the show was made in the 70s. She chose to move out so Jodie didn't have to make that choice but she never turns up even as a friend again.
Jenny, the Doctor's daughter, was shown to have revived and left the planet she was on to follow in her father's footsteps. Georgia Moffett has expressed interest in reprising the role, as well, but it seems doubtful now that anything will come of it.
As New Who progresses more characters come back from a long Bus Trip (not counting the Doctor Who Expanded Universe). Series 1 had the Autons after 34 years, Series 3 had the Macra after 40 years, and Series 7 had the Great Intelligence after 44 years. The Zygons will be returning in the 50th after 38 years.
The Pandorica Opens had a bus with a lot of passengers coming back for the Legion of Doom from the Classic Series, including the Zygons and Drahvins. However we only see the creatures that have been in New Who, meaning a lot of fans felt this was a missed opportunity.
The A-Team: Col. Lynch (Bill Lucking) is put on a long bus trip, after appearing in the first season. He's replaced by Col. Decker (Lance Le Gault), who in turn was Put on a Bus and replaced by General "Bull" Fullbright (Jack Ging). Decker returned for the A-Team's court martial in Season 5.
This was done to Brooke Smith's character Erica Hahn on Grey's Anatomy for no apparent reason whatsoever: they spent several episodes building up her relationship with Callie, including an excellent scene where she tearfully declares that Callie "is glasses," only to randomly fire her because the writers stopped liking their own story. Erica and Callie get in a fight about the Izzie LVAD-cutting debacle and about how Callie "can't bekind ofa lesbian," and she walked off into the parking lot...forever. She was then more or less replaced with perky Lipstick Lesbian Arizona Robbins. Further proof of Shonda Rhimes' ridiculous and fickle casting habits.
Neighbours' Marlene Kratz went on a three month cruise in 1997. She's due back any day now.
Spin City has a knack for dumping characters without acknowledging this. So when, towards the end of season three, Mike gets a temp secretary assisted to him because Stacy Paterno was sick, you'd expect the character to return. Que episode two of season four, in which James becomes Mike's new assistant because he still hadn't got a new one and James would be out of a job otherwise. In between, no statement whatsoever is made about Stacey.
Sergeant Scott of Midsomer Murders was mentioned as having called in sick in one episode. That was the last mention of him.
Will Scarlett and Djaq stayed in the Holy Land at the end of series 2 of Robin Hood. Fans hoped for a reappearance in series 3, but they never turned up again. No one even mentioned them ever again.
On Parks and Recreation, Mark Brendanawicz left his government job to return to the private sector at the end of season 2. This was originally intended to turn him into a Drop-In Character, but instead he simply dropped out of the show entirely (actor Paul Schneider wanted to return to movies.) Even though Mark should still live in Pawnee, a relatively small town, he is never seen, heard from or even mentioned again... to the point that in a season 5 episode when Ann, whom he dated for most of season 2, clears out boxes full of ex-boyfriends' stuff from her house, there's no box for Mark.
In 24, the fan favorite villain Mandy is last seen in the final of Season 4 making a deal in exchange for immunity from her past and present crimes, much like a certain other recurring female villain who did make a reappearance. While it could be argued that she left her life of crime after this, she was intended to return in Seasons 7 and 8, but the decision was dropped to avoid making "sensational moves", although there is a chance she might return in a spinoff.
Largo LaGrande is LeChuck's second in command and was last seen in Monkey Island 2: LeChuck's Revenge in a blown-up evil lair that... wasn't even really that blown up. He has not been seen at later points in the series despite LeChuck's constant appearances.
He did show up in an April Fools' Day "teaser" of Monkey Island 5, perhaps in recognition of this trope.
Half-Life: Adrian Shepard hasn't made an appearance in quite some time.
Likewise, Valve is directly invoking this trope for Chell, so as to stick to their desired plan of using a different protagonist in future games. Chell was only brought back for Portal 2 by playtester demand.
However, Gabe Newell has said that Chell will have some importance in the Half-Life universe, so who knows?
Nack/Fang is especially bizarre, as he didn't even get a card in the Twitter-based game Sonic Tweet, even though all of the games he made a major appearance in (other than Sonic the Fighters/Sonic Championship) had cards.
EggmanNega and Tikal as well. They didn't even show up in Generations.
Also Shade, who hasn't been referenced at all since her debut. This one is more justifiable in that Chronicles is both non-canon and supposedly the furthest event in the series chronologically speaking.
Emerl is another offender. Sure, he gets offed at the end of Battle, but given his interactions with the rest of the cast and the fact that Sonic Advance 3 all but outright states that he's been revived in some fashion as G-mel/Gemerl (who is repaired, reprogrammed, and shown to be playing with Cream in the epilogue), you'd think he'd show up again sometime.
The elemental masks, Zem, Zam, Rusty Walrus, Pasadena Possum and Von Clutch from the Crash Bandicoot series.
The Nerevarine from The Elder Scrolls III: Morrowind apparently decided to take a long trip to the distant continent of Akavir by the time of Oblivion. Note that while the latest game takes place over two hundred years later (beyond the lifespan of any non-Mer player race), the perks of being the Nerevarine include being The Ageless.
Before Pokémon Bank, Bridgette's last appearance was in Pokemon Box Ruby and Sapphire three generations ago.
Tariq Bhartti of KateModern left abruptly for India and never returned.
Amazo in Justice League Unlimited teleports himself away from Earth in the middle of a battle, after seeing the monster of the week absorb his attack and become more powerful for it. Though the foe is defeated mere minutes later, Amazo never returns.
Eddie from Rover Dangerfield. He didn't move out to the farm with Rover in the end so his fate is unknown.
An episode of Thomas the Tank Engine introduced a character named "Derek The Diesel" who suffered from "teething troubles" . At the end of the episode he was sent off to have the "troubles" repaired, but aside from a cameo (from the back) during a music video in the special "Calling All Engines" he has not been seen since.
Warren T. Rat and his gang in An American Tail are defeated by putting them on a ship headed for Hong Kong. Instead of having them come back for the sequels they instead brought in new villains for each movie.
One episode involves a rescue mission for Sokka's father Hakoda and girlfriend Suki. They also pick up a new character, Chit Sang. When Azula attacks the group in the next episode, Chit Sang is put on an airship with Hakoda, Haru, Teo and The Duke and is the only one not to return at the end of the series finale.
See also The Earth King, who takes off on his own at the beginning of season 3 and is never seen again. Presumably he comes back after Ba Sing Se is liberated, but he's never shown in the series, appearing only much later in the follow-up comics.
Also Hawkie, the messenger hawk Sokka buys while in the Fire Nation. Toph uses it to send a message to her parents and it's never seen again. Dear Toph and Katara, how were you expecting the hawk to find you when you are always travelling?
Happens to both Blackarachnia and Waspinator at the end of Transformers Animated based on the events of the episode "Predacons Rising." Adding to this was the fact that Waspinator's final lines in the show were "Waspinator has plans..."
Though this was mainly because the fourth season for the show was cancelled.
Brian's cousin, Jasper, and Jasper's boyfriend, Ricardo, has been subjected to this trope in Family Guy. Jasper appeared twice in a Cutaway Gag and then twice more in two episodes; the first one made Jasper and Ricardo a One-Scene Wonder and their next appearance several seasons later sets up the plot over gay marriage in Quahog. The duo has not been seen or talked about since then.
Jasper is seen talking to Brian on Skype in "Brian's Play. Jasper and Ricardo are both seen at Brian's funeral in "Life of Brian as well."