2 Broke Girls had Deke, a prominent character and love interest for Max in Season 3, and an investor in their cupcake business. A few episodes later, he disappears without explanation.
24 generally has no qualms about having characters cease to exist once their purpose has been served, even though its high body count means you'd think someone could spare a bullet for any of them. Worse, many of them were last seen in situations where death is likely but not a given. In some circles, this is called "Behroozing", after Behrooz Araz, a character who vanished in such a manner.* Two completely different scenes were filmed to wrap up his storyline, one right after his last episode and one in the season finale, but both were cut for time. Most notably:
The unnamed Eastern European assassin from the first half of season 1, who pretends to be an American photographer named Martin Belkin. The assassin plays a major role in the first half of the season, as he gets a face transplant and attempts to kill Palmer at a rally in downtown Los Angeles. After he is foiled by Jack Bauer, he flees and is never seen again (even though the other assassin hired to off Palmer, Mandy, is seen in several more episodes in different seasons afterwards).
Lynne Kresge (an assistant to President Palmer) is pushed down a flight of stairs late in the second season. Even though she's badly injured, she doesn't seem to be in danger of dying (and she's loaded into an ambulance, knowing damaging information about Mike Novick). Strangely, she's never referenced again, even when Palmer returns to the White House and talks at length with Novick.
John Keeler and Wayne Palmer both exited the show this way. While a reference to Wayne dying is made in a prop newspaper from Redemption (albeit, never shown on-screen), Keeler is never mentioned again after being listed in critical condition after Air Force One crashes. This actually has a justification—the writers were explicitly told that they weren't allowed to kill off a sitting president on-screen. Presumably since Daniels had already taken over and served out the rest of Wayne's term, it was okay for them to let it be known that Wayne was dead, and David Palmer's death happened long after he left office. This may not have been restricted to United States presidents, either. In Season 8, Omar Hassan, the president of a fictional Middle Eastern nation, is killed, but his death is not actually shown on screen; while CTU is trying to rescue him, the internet video feed that the terrorists have set up is shown, and then when Jack gets there and finds Hassan's dead body and realizes that the video was pre-taped, we aren't shown the conclusion.
Karen Hayes, the wife of long running character Bill Buchanan, who is sort of put on a bus with her husband at the end of Season 6 as they are both forced to resign. However, Bill is a main character for most of Season 7 and his wife is never mentioned not even after his death. Her not being mentioned is partly justified in that Bill spent most of his time in the field where there was less time for chit-chat than when he co-ordinated from CTU. This was due to actress Jayne Atkinson doing a play when the season was being filmed.
Daniel Dae Kim played a field agent in the first few seasons that was one of Jack's earliest field partners over the course of the show. In the second half of season 3 he worked with Jack and Chase in their attempt to capture that season's Big Bad Stephen Saunders, but after a botched attempt he completely disappeared and was never seen or heard from again, thanks in part to Kim moving over to Lost right around the time those episodes aired.
Dennis Ryland was a prominent character for the first three seasons of the show (though he did experience a brief absence), first as the head of NTAC, then as a notably more antagonistic character in a higher level of government. However, he disappeared entirely, and without explanation, for the show's fourth and final season, with only a single, fleeting reference being made to him as one of the people who exploited Isabelle Tyler. This despite the fact that the project he was working on (the development of promicin-enhanced soldiers) was at its height at the end of the third season.
Similarly, Nina Jarvis, the head of NTAC for the second and third seasons, disappeared with no further mention in the fourth season, her role being filled by new character Meghan Doyle. It can be presumed she quit, though no explanation is given.
Then there's Diana's boyfriend, Ben, introduced near the end of the third season and disappearing mid-way through the fourth with no further mention. The implication is that they broke up, though this is quite surprising considering how well they'd been doing as a couple... and the fact that this contradicts one of Maia's infallible prophecies.
One of the many abrupt changes in Airwolf 's 4th season was the disappearance of Caitlin O'Shannessy.
In All My Children, a teen named Bobby Martin went up to his family's attic to wax his skis in a 1970's episode. The actor was then abruptly fired and so Bobby was never seen again. Decades later, the show lampshaded this by having a character go into the same attic and find a skeleton with a pair of skis, wearing a ski hat with "Bobby" on it - a comedic example of a Bus Crash.
The Andy Griffith Show had Jack Burns' character of Warren Ferguson, who was brought in to replace Barney Fife as Mayberry's overzealous deputy. He lasted one season before being quietly dropped from the show and never mentioned again. (Even the '80s Reunion Movie forgot about him.) Another example would be Ellie Walker, the town pharmacist and Andy's first-season girlfriend.
Detective Kate Lockley, Angel's LAPD contact. She made her last appearance fairly early in the series, when she hit a Despair Event Horizon and was barely saved from a suicide attempt by Angel, and completely vanished from the story after that (despite it being implied there was some larger importance to Angel saving her, since he was miraculously able to enter her home uninvited). She was supposed to star in season 3, but the actress had already moved on to Law & Order and so a new character, Justine, was created as a replacement. Kate did return, though, in the comic-book series that continued the storyline after the show ended.
Nathan Reed takes over from Holland Manners in season two, but doesn't appear in season three where Linwood Murrow picks up where he left off. However, the fact that Lindsay revealed his dirty dealings that Lilah dug up could mean he was possibly terminated.
AP Bio: Two students vanish from Jack's class between seasons one and two without explanation. At their desks, one is replaced by a new speaking cast member, while the other is more of a Living Prop. The second season does introduce more speaking opportunities for the Living Prop background extras.
Are You Being Served? had a few regular characters vanish without mention, with the most memorable being Mr. Grainger and Mr. Lucas (actor left to pursue other interests).
G'Kar's assistant Na'Toth only appears twice in the second season (after an unsuccessful recasting and doesn't appear at all in that season's second half. A third season episode mentions that she'd been on Narn when the planet was bombed into submission by the Centauri and was presumed dead. Finally in an season 5 she reappeared for one episode as a P.O.W. on Centauri Prime and was sneaked onto a transport home. Interestingly, G'Kar's first assistant Ko'Dath also disappeared off-screen (though this was explicitly mentioned on screen as death due to an "unfortunate airlock accident"). Is it any wonder why G'Kar becomes something of a loner for most of the series?
The alien mantis-like mob boss N'Grath essentially disappears after the first season and is all but forgotten (although his demise is hinted at once in season five dialog).
Banshee: Despite being prominent in Season 1, the Moody family disappears entirely from Season 2 without a single mention.
Barney Miller dropped detectives with little or no explanation. One notable example was Eric Dorsey, the abrasive newcomer introduced in the next-to-last season (he lasted three episodes, same as season 4's Licori and one more than season 3's Batista).
In Battlestar Galactica (1978), Commander Adama's daughter/Apollo's sister Athena vanishes without explanation after the episode "Greetings from Earth: Part I."
Boxey completely vanished without explanation after a relatively prominent role in the miniseries and a smaller scene in one regular episode. He was meant to be a kid that the pilots had adopted, but the writers couldn't come up with uses for his character. It could be justified as Commander Adama may have decided that a military ship in a time of war was no place for a child and sent him to live with a foster family in The Fleet. It should be noted that he was featured in several episodes in Season One, but all of his scenes were cut out due to timing constraints or pacing.
His photo does appear on the Wall of Remembrance in later seasons, implying his death sometime between Ragnar Anchorage and Earth. It gets expanded upon in one of the tie-in novels, which has him living in foster care in the fleet, and getting fatally shot helping Helo and Starbuck stop an attack by an apocalyptic religious cult in the fleet (the novel also pins down his death as occurring during Commander Fisk's tenure in command of the Pegasus).
Prominent Quorum members such as Marshall Bagot and Sarah Porter simply disappear after the Second Season. It is possible that they died during the explosion of Cloud Nine or the Cylon Occupation or were simply not voted back into office in the new administration and thus lost their relevance, but it is never addressed.
Another character, Bulldog, was given an entire episode's focus when he was introduced, then was never heard from or mentioned again. The story is that he was intended to be a recurring character afterwards, but the actor playing him couldn't work out his schedule to fit the show. Word of God says in the podcast for Bulldog's lone episode, "Hero", that they decided to wrap up the character's arc within a single episode because Carl Lumbly was an "expensive actor".
Baywatch normally gives at least a line or two explaining the departure of major cast members if they weren't given a proper exit in the series, but some of the main characters and supporting characters still go through this anyway.
Trevor from Season 1 is last seen getting refused a sandwich. He walks away mid-episode and is never seen or spoken of again.
Supporting radio operator Sid was a "comic relief" character that disappeared in the shuffle between Seasons 1 and 2.
Harvey was added to the cast at the start of Season 2 as a "comic relief" lifeguard. He is missing in action by the start of Season 3 and the show never references him again.
Guido was an Italian stereotyped "comic relief" character that eventually even learned to be a lifeguard to impress his mother... and that episode turned out to be his last before he disappeared without a trace along with the other failed comic relief characters that polluted the show.
Megan was a lifeguard in Season 2 with three episodes to her name, including one with a central focus on her, before she vanished without a trace.
Becker: Bob was said to be "on vacation" in the first episode of Season 6, and never returned.
This happened by necessity on Best Friends Whenever: Marci, a supporting character from Season 1, disappeared without an In-Universe explanation between seasons, despite being part of an Official Couple with a member of the main cast. This is due to the fact that Marci's actress, Madison Hu, took a lead role on Bizaardvark and was thus unavailable.
The Big Bang Theory: Similarly, Leonard gets a love interest, Dr. Stephanie Barnett (Sara Rue) in Season 2. Fearing they're moving too fast, he tries to break up, but she keeps luring him back with sex. At the end of their third episode, he heads off for another booty call. Presumably he grew a spine between that and the next episode, when she's just gone.
The Big Valley, in addition to the four siblings who were series regulars (Audra, Heath, Jarrod, and Nick), had a fifth Barkley sibling in the first season, youngest brother Eugene (played by Charles Briles). Eugene only appeared in a few episodes and then went off to medical school. He returned home one time and then disappeared from the show.
Percy, of Blackadder fame, was completely excised from canon. Tim McInnerny, who played Percy in the first two series, didn't want to be typecast playing a buffoon, so he played different characters (Topper in Series 3 and Darling for all other appearances) afterward, and Percy was subsequently replaced in the show's main Comic Trio by George. His lack of appearances after series two can be justified and seen as a variation of this, at least due to the fact that each series takes place in a different time period with technically different people, and one can assume that the Percy of Series 2 never had any descendants. Not as much for specials that harken back to the timeline pre-Series 3 however which toy with this as they toy with time itself, which don't have him get a single mention or appearance even though other returning characters have, though it can still be handwaved as these were very brief and he could conceivably not be around during these moments.
The Bob Newhart Show had Margaret Hoover, a neighbor in the Hartleys' apartment building and friend of Emily's. She appeared in a handful of first-season episodes before being quietly dropped.
During the first season, Jonathan Adams plays Dr. Goodman, the head of the Jeffersonian Institute, who is Bones's direct superior and takes an active role in many of the cases. In the first episode of season two, Cam is head of forensics, and Goodman is said to be on a "two month sabbatical"; four years later, he's yet to be seen or mentioned again. (Word of God is that he may eventually pop up in guest appearances, however.)
This trope was even applied retroactively, as Dr. Goodman is never even mentioned during a flashback episode set before the pilot.
This also happens with a restaurant owner, including the restaurant. In the first season, the main characters frequent a restaurant where the quirky owner knows better than you want you want/need, so it's better to not order and just let him bring you whatever he feels is best, but in season 2 they suddenly start going to a diner instead and the restaurant is never mentioned again. Of course, they could have just decided they liked the diner better, but the switch is never explained in any way, even though Booth seemed to be on quite friendly terms with the owner of the restaurant.
Early episodes offer a possible explanation: the owner complained about the Squints bringing photos of crime scenes and bodies to the restaurant with the threat of not allowing them back if they kept it up. However, there was nothing confirming this as the reason they stopped going.
In one early episode, we see Topanga's older sister, Nebula. She is never referred to again and indeed Topanga is later stated to be an only child. However, this is back when Topanga was going through her Cloud Cuckoolander phase, and this makes her word a bit unreliable. Nebula could be anything from a "soul sister" in the hippie sense or just a friend she calls a sister.
In earlier seasons, Shawn had at least two siblings before Jack came into the picture. There was an older brother who Cory caught stealing or something on video, and an older sister he called after Cory had him help straighten his hair. However, because these people are rather scummy, it makes sense that they would not appear again. Then comes the matter of Shawn's mother revealing she did not give birth to him and one wonders if they were his brothers in the first place.
Minkus, a recurring character from the first season, also disappears without a trace. Later, when the show became more meta, they had him return for their high school graduation, pointing off-stage and saying that all his classes had been down the other hallway. The kids wave down the hall, shouting hello to their old teacher Mr. Turner, who had also been Chucked. (Ironically, the actor playing Minkus was one of the three Torkelsons to survive the Retool into Almost Home). None of this stopped Minkus's son from getting into Girl Meets World, though.
Cory's sister, Morgan, almost got the Chuck treatment. She disappears entirely at the end of season 2. Her character is brought back in the middle of season 3 (as a different actress), and remarks, on her first appearance, "That was the longest time out I've ever had!"
When Breaking In was (briefly) Un-Cancelled, team member Josh vanished without explanation. There was a minor handwave for another major character who was not technically a team member, Dutch, disappearing at the same time.
The Brittas Empire: The first series features pretty secretary Angie as a main character in all episodes. She vanishes without a trace from series 2 onwards, replaced with a brand new secretary, Julie, who is merrily treated as if she's been there since the beginning by all the other staff. Of course, the whole thing did turn out to be a dream, so...
The First Evil wasn't destroyed or trapped or anything like that, and is presumably still around, doing...whatever it does when it's not tormenting the good guys. Justified as it only had a very small window of becoming corporeal due to Buffy's resurrection and other events, and its army of "uber-vamps" were all killed. It's alive but not as much of a (direct) threat, since the most it can do is act as the evil version of a Spirit Advisor.
General Voll was dropped and never mentioned or seen. Word of God states that the writers forgot about him when replacing him with the second General.
Oz's band "Dingoes Ate My Baby" never performed again after his departure.
Eddie, owner of the local Bad-Guy Bar, vanished after season 5. Even the new owner doesn't say where he went.
British ensemble dramas are rather notorious for this, with regular characters vanishing in between seasons. Casualty and Holby City were noted for it in the days before they were on all year round. (Between the eighth and ninth seasons, a massive cull saw Casualty lose seven main characters, one of whom had been on the show four seasons.) But perhaps one of the most prominent cases was in ITV's military drama Soldier Soldier: Dave Tucker, played by Robson Green, was the only remaining original cast member and practically synonymous with the series. The last scene of the fifth season has him and his wife breaking up. He is never seen or mentioned again. (Neither is his wife.)
Central Park West had a number of characters randomly disappear, due to the freefalling ratings and Troubled Production causing all manner of continuity errors and bizarre departures.
Most of the employees seen at both stock broker Gil Chase's firm (including his boss and a woman he flirted with before being called in for a meeting) and the staff members at Communique magazine (most of whom are seen attending an editorial meeting) disappear after the pilot episode. Moreover, the sets changed too - Gil's firm goes from an expansive office with rows and rows of desks with employees to a much smaller, more dimly-lit office with a small conference room.
Nikki Sheridan, a major character in the first season, disappears during the jump from the first season to the second, though it wasn't entirely intentional. Due to the series going on hiatus after the ninth episode, Nikki's storyline randomly ends with her fleeing her apartment and going to Peter Fairchild for help, who tells his stepfather to stay away from her. However, the missing four episodes of the season and several deleted sequences from the second (which only aired internationally) would have continued her plotline, as Allan attempted to assassinate her by hiring a hitman. As a result, she randomly disappears until several episodes into season two, where she shows up at the Zinc Bar, summarizes her storyline for Carrie Fairchild's benefit and disappears for good.
This even occurred to the main character, Stephanie Wells. Due to the aforementioned hiatus, Stephanie's storyline randomly ends in the middle of her trying to investigate the leak at Communique. When the second season begins, it isn't even clear what happened to her until a later episode reveals that she moved back to Seattle with her husband (who subsequently divorced her and moved back to New York).
Played with on an episode of Charmed when a witch who seems to know the sisters ask for their help. She reveals she was their cat familiar Kit, who is prominently shown in the opening credits but vanished after a few episodes.
Dex is a prominent love interest for Phoebe in the first six episodes of season eight. Even though he and Phoebe decide to take things slow in the end, he isn't seen or mentioned again save for a clip show which doesn't provide any explanation for his departure.
Holly Ellenbogen was a main character who is last seen in the twelfth episode receiving a threat from Richie's wife Fern, who is also never seen again. There is no explanation given for her disappearance, leading to the unfortunate possibility that she may have been hurt in some way by Fern.
A later episode, made to calm fan worries that Holly was killed offscreen, features her husband Perry working as an interior decorator for some of the other characters, and referring to his wife as very much alive, and even having a phone conversation with her.
Professor Slater. A recurring love interest for Jeff throughout the first season, she was involved in a competition with Britta for Jeff's affections in the season finale and hasn't been sighted since. Lampshaded in the second season episode "Intro to Political Science": one of the news ticker headlines on Troy and Abed's election coverage reads "Professor Slater still missing".
Not to mention Ian Duncan of the same show. You'd think a character taking off to "get something in his car" would have him back by the end of the episode or the beginning of the next, but nope. This was due to John Oliver's commitment to The Daily Show.
This is actually Lampshaded in the fourth season. During the balloon episode, Troy asks if anyone else has noticed that Professor Duncan hasn't been around in a long time.
Duncan returned in Season 5.
Buzz Hickey (Jonathan Banks) essentially replaced Pierce as the "old man" in the group in Season 5. He does not appear at all in Season 6 (due to Banks' commitment to Better Call Saul), and this is not mentioned. This despite the fact that Shirley Bennett, who had also disappeared from the group in Season 6 (due to Yvette Nicole Brown's needing to take time off to look after her ill father), receiving her own explanation and "spinoff" at the end of the first episode of the season.
Hickey's disappearance is finally very briefly lampshaded by Ben Chang in the season (and series) finale: "...Shirley, Elroy, Troy, Hickey, living in the past."
The Cosby Show: Theo's best friend Walter "Cockroach" Bradley appeared frequently during the first half of the series, was very close to the family, and even started to get some individual development ("An Early Spring"), then abruptly stopped appearing without explanation. The episode after his final appearance is even focused on Theo and Cockroach's gang of friends' locker room antics, which Cockroach would normally have been present for. Word still has it that his actor, Carl Anthony Payne II, refused to cut his hair as per Bill Cosby's wishes (the kids all changed hairstyles regularly throughout the series) and was eventually fired or left the show as a result. And the haircut in question, that was apparently worth leaving the cast of what was currently the most beloved and popular show on television? Snip.
Crossroads gives us a weird and infamous example in the handyman Benny. He climbed up a ladder to fix the lights on a Christmas tree, and was never seen again.
From the first season up to the eighth season, there was a ballistics expert named Bobby Dawson. However, he disappeared about halfway through the eighth season with no explanation.
Catherine's sister appears at the very beginning of the pilot... only to never be mentioned or appear again. Later episodes that concern her family history imply that Catherine is an only child.
Season 8 brought us CSI Veronica "Ronnie" Lake for a few episodes leading up to Sara's departure. After "Goodbye and Good Luck", Ronnie is never seen again, without any explanation or mention of her since.
Former CSI and detective Sofia Curtis stuck with the crew for nearly 3 seasons, even making it into the opening credits briefly. However, after the season 8 premiere, she completely vanishes without a word. She does return briefly late in season 11, however, having been promoted to Deputy Chief of the LVPD.
CSI: NY had this happen with two characters: Det. Kaile Maka (who appeared in season 1 and 2) and coroner Evan Zao (who appeared in season 2).
Curb Your Enthusiasm had the main character as a father in the pilot episode. The kids were never mentioned again.
Miss King, a sexy female clerk at Mainwaring's bank whom the writers admit was introduced solely for the purpose of being Ms. Fanservice. She disappears after series two and is never heard from again, although she did play a very small role in The Movie.
A couple of platoon members also disappeared with no explanation: Private Bracewell, who appears in the first episode only (Word of God states he was cut because the writers felt his character was too much like Godfrey's) and Private Desmond, an Ascended Extra who goes on a mission with the main cast in the episode "Sons of the Sea", then is never heard from again.
Private Cheeseman appeared for one series and then disappeared entirely, with no explanation. Word of God states this was a case of Shoo Out the New Guy since neither the audience nor the rest of the cast liked him. Cheeseman's storyline was that he was a reporter who temporarily joined the platoon in order to write news stories on them, thus, it can be assumed he left at the end of his assignment; but there is no mention of this in the show, he doesn't get a goodbye and is never referenced again.
Bizarrely, recurring character Dusty Farlow suffered this fate by accident: He appeared in a few episodes at the end of the 7th Season, then left town a few episodes into the 8th. Unfortunately the 8th Season was also the infamous Dream Season and the producers apparently forgot about Dusty (despite his father being a main character), so that per canon he simply vanishes without explanation.
If a house counts, Dallas also did this with Southfork itself. In the pilot miniseries, Southfork is a huge mansion with two smaller houses attached by a breezeway. It is discussed in great detail in the first episode that J.R. and Sue Ellen live in their own little house, and the other house was built for Gary and Valene, and that that is where Bobby and Pam will live. After the first season, they switched to a different real life ranch for the exterior sets, and suddenly the Ewings were all living under one roof in a much smaller house.
This was actually an example of Real Life Writes the Plot (or setting, in this case): When the second season began filming, the owner of the ranch used in the first season had been indicted for insider trading, was preparing for his trial, and did not want film crews around. So the producers switched to a different ranch that didn't have separate houses. Also, having everyone under one roof (as unrealistic as it may be) made for a lot more dramatic tension.
Daniel Boone: Daniel's daughter Jemima, played by Veronica Cartwright, disappeared after the second season without explanation. Cartwright said in an interview that Patricia Blair, playing Boone's wife/Jemima's mother Rebecca, threatened to pull out of the show if they didn't drop Cartwright. (Jemima, as Cartwright reached her late teens, was being given more adult/romantic stories, and apparently Blair thought this made her, Blair, seem old.)
Days of Our Lives: Don Craig went to the mail to post a letter and hasn't been seen since.
Kendra was a recurring character in many second and third season episodes but vanished without a trace in the fourth season. This was particularly strange because her brother and (ex-)boyfriend were still on the show. One wonders why she wasn't there to react when her brother got expelled and re-admitted, found religion and abandoned it, got married, etc.
Some sources have it that Kendra was planned to have sex with Toby; the actress' parents disapproved and yanked her off the show.
To a lesser extent, Chester, who was introduced as a new main character and vanished after about three episodes.
Chris Sharpe, Emma's love interest in season 3; Derek Haig, a notable character until season 9 when he mysteriously disappears; Terri Mcgregor, whose sendoff is only explained in a deleted scene; and Principal Shepherd, former Lakehurst principal becomes acting principal of Degrassi after the merger, is fired due to an outburst at Clare, returns briefly after attending anger management classes, but is suddenly written off and replaced without explanation by old Degrassi principal Ms. Hatzilakos.
Ms. H herself vanishes without a word the following season, with Mr. Simpson, the Media Immersion teacher, being bumped up to the top spot.
Mrs. Kwan, who became one of the most prominent recurring characters, played this trope very well. She was the English teacher and a Recurring Character who appeared in several episodes from Seasons 1 to 9, but hasn't been seen in any of the episodes in Season 10. During that exact same season, Mrs. Dawes, the former art teacher, is now taking her place as the new English teacher with no word of Mrs. Kwan's disappearance. It's unknown whether she's no longer teaching at Degrassi or if she's simply teaching English class periods that the main characters aren't in.
This happens in Season 12 to Wesley. His actor revealed the writers ran out of ideas for the character.
This was also common in the original series too. For instance, a character named Susie Rivera was sexually assaulted before she disappeared. Another character named Scooter Williams did the same thing. Another example is Melanie Brodie. Stephanie Kaye was a major character in the first two seasons; at the beginning of the third, her brother mentions that she is now in private school.
In the original series, and the early seasons of the current series, could be considered Truth in Television. Children and teenagers do move away without notice, and they seldom have any say in the decision.
When Degrassi returned on Netflix and F 2 N, now titled Degrassi: Next Class, three characters were noticeably absent: Jack, Arlene and Principal Pill. Jack's absence could at least be explained away as her no longer being apart of Power Cheer and Imogen (her former girlfriend) having graduated the previous season. But Arlene was both Hunter's only friend and potential love interest while Principal Pill took the place of Principal Simpson. The end of season 14 hints that Principal Simpson's firing is permanent and he's stated to be headed for Africa to build houses. By the time Next Class begins, Hunter now has THREE friends, one of which is a potential love interest and Arlene is never mentioned again. Meanwhile, Principal Simpson has quietly returned and Ms. Pill isn't mentioned.
Dave Turner was one of the show's most prominent characters in Seasons 10-12. After helping organize Adam's memorial he's never seen again.
A Different World: When Lisa Bonet left, several characters disappeared with her, most notably Marisa Tomei's Maggie and Whitley's Girl Friday Millie.
In "The Time Monster", the Doctor recounts a story about his childhood on Gallifrey, about how he lived halfway up a mountain beside a hermit who helped give him his first understanding of the beauty of life. In "Planet of the Spiders", this character is revealed to be K'anpo, who helps the Doctor regenerate after his radiation poisoning. Despite K'anpo presumably being a very important figure in his life with a fairly profound and familial relationship with him, he is only mentioned once again after "Planet of the Spiders", in an off-hand reference by the Fourth Doctor seven years later. This may be because the revelations about what life on Gallifrey is actually like in "The Deadly Assassin" do not jar particularly well with this origin story.
Brendan Richards, from early Eighties spin-off K-9 and Company, is the teenaged ward of Sarah Jane Smith's Aunt Lavinia and it seems likely that he was intended to be a recurring character; indeed, he features prominently in the pilot. However, not only was a full series of K-9 and Company never commissioned, there is no further mention of Brendan in either Doctor Who or The Sarah Jane Adventures.
In the first story of Sarah Jane's Big Finish audio spin-off Sarah Jane Smith, it is revealed by 2002 he's gotten a job in Silicon Valley, and unable to attend Lavinia's funeral at the start of the story for that reason. The novelisation for 'The Lost Boy' (the finale for the first series of Sarah Jane Adventures) states he's still working there as of 2008, and working a project there with Mike Yates is why he didn't attend her wedding (this one from the novelisation for "The Wedding of Sarah Jane Smith").
In the classic series, Kamelion boards the TARDIS and promptly vanishes for a long time, because the one crewmember actually capable of operating the robot died, leaving no one any clues how to program it. Kameleon eventually returned (by way of an actor covered in silver paint) and was destroyed, rather than try to mess around with his character anymore.
Kamelion appears in a deleted scene for "The Awakening" that can be seen on the DVD, as well as several novels. One of those novels, The Crystal Bucephalus, gives an explanation for why he made so few appearances (he was influenced by strong personalities, such as the main antagonist of the story).
A instance so subtle that it is often mistaken for Killed Off for Real occurs with Madame Kovarian, the Big Bad of Series 6. In "The Wedding of River Song" she is left to die at the hands of her own minions by a vengeful Amy Pond... except the death occurs within a Time Crash which is undone by the episode's end, meaning Kovarian didn't die as shown yet the show never mentions her again.
In Drake & Josh, Drake's original band members, Scottie, Rina and Paul, are never seen after the 1st season.
The Drew Carey Show had Drew's Wacky Neighbors in the first season, who vanished when it became more of a workplace sitcom and focused on Drew's Wacky Friends. Occasionally, Drew got new neighbors—sometimes being written out, sometimes getting Chucked.
After leaving his job in disgrace, Doug (one-half of the show's Official Couple) simply vanishes until his unexpected return in the sixth-season finale, "Such Sweet Sorrow". After that, both Doug and Carol vanish for several seasons, missing key events including Mark and Elizabeth's wedding and, later, Mark's funeral before finally showing up in a final cameo in the fifteenth and final season. Part of this was caused by George Clooney's burgeoning film career, and others (like a proposed cameo during the aforementioned funeral) were nixed by Clooney on the basis that his appearance would take away from the nature of the situation. The writers sometimes tried to write explanations for Doug doing things off-screen (such as mentioning via dialogue that he was visiting Carol's twins), but most of the time, it was anyone's guess where he was.
Speaking of Doug, on two separate occasions in Season 1, he mentions having a son. Aside from the fact that the boy is 8 and that he's never met him, the audience is told nothing else. This is never mentioned again, not even during storylines where it would make sense—his abusive Disappeared Dad resurfacing, his and Carol's efforts to have a baby, etc. At one point in a later season, when asked if he has any children, he says "no".
Bob the receptionist, to the extent that some people now call this trope "being Bobbed". The thing about "being Bobbed" is the character has to have just become interesting when they vanished. In Bob's case the County staff had recently discovered the "simple" foreign janitor rather patronizingly nicknamed "Bob" because Doug couldn't be bothered to learn to pronounce her real name had actually been a skilled cardiovascular surgeon in her native Poland, and was only working as a receptionist until she could learn English well enough to gain Illinois state medical board certification. Also Maggie Doyle, who would disappear for long periods of time before reappearing, to the extent that this trend was called "being Doyled". Ironically, Doyle herself was eventually "Bobbed" when she left in between seasons after a sexual harassment lawsuit against Romano fell apart.
A huge number of characters were "Bobbed" over the years, with a scant few of them reappearing, in some cases with a decade having passed, just to let us know they still worked at the hospital. One of those who never reappeared, and whose disappearance wasn't even briefly mentioned, was Dr. Hicks, played by CCH Pounder. She was major enough that despite only being a recurring character, managed to be nominated for an Emmy as Best Supporting Actress in a Drama Series, rather than Guest Actress.
Ellen had a friend, Anita, who was abruptly dropped after the first seven episodes were produced. Holly was never seen or mentioned again beginning with season 2. Paige Clark was introduced at the start of season 2 as if she'd always been there, and an establishing shot of her apartment was the same building used for Anita's apartment in season 1!
Whatever happened to Spenser, Henry's assistant? (Possibly he was fired for hijacking an experimental satellite to watch pirated movies.)
Greg Germann's character from the pilot, who suddenly vanishes in the second episode even though his obnoxious assistant, Fargo, remains. Turns out, he was Reassigned to Antarctica - and returns for a single episode - for nearly ending the world, though this is hardly exceptional behavior for characters on this show.
Callie Curie, an apparent love interest for Carter toward the end of Season 2, doesn't come back, is never mentioned and never addressed as to what may have happened to her.
Let's face it, half the scientists of the week. Which is probably for the better, surely at some point someone would have just realized ten minutes into an episode that some guy from two seasons back could probably just save the day.
In the first episode of Everybody Loves Raymond a character called Leo is presented as one of Raymond's close friends. Leo was instantly dropped from the show, likely because he wasn't considered funny enough.
Judy Winslow, the youngest daughter, simply vanished after the fourth season due to a "budget consideration" for the series (they mean "the actress and her parents wanted a salary raise"). Word of God says she would have returned for the originally planned Grand Finale of the series, if it had lasted to show the marriage between Steve Urkel and Laura Winslow; however, there is an episode in which father Carl remarks that he has two children, which would have made her return a retcon of sorts.
Several minor characters also disappeared with no on-screen explanation: Laura's best friend Penny, Eddie's sleazy best friend Rodney, and Carl's boss Murtaugh.
Fire by Nite had a serial sitcom embedded, entitled Family First. The family originally had two boys and a girl. When the actor who played the younger son moved, and the parents were replaced by a different couple, the younger son, Robert, disappears. They refer to him as if he's off-camera for a couple of episodes, but eventually, through the 3-year run of the show after that, he is never mentioned again.
In Frasier, many characters who were prominent in Seasons 8-10 vanished after the writing staff for those seasons was fired en masse at the end of Season 10. Probably the highest-profile example was Julia Wilcox, who appeared prominently throughout Season 10 and rather improbably got into a relationship with Frasier at the end of the season. The new Season 11 writing staff had Frasier quickly remember just how much of an insufferable bitch she was and break up with her two episodes later, after which she was never seen or mentioned again.
Jackie appears at one point as a friend of Will's from back in Philadelphia. Throughout the season, she is built up as a potential love interest for Will, until an episode where Will gets into a drinking game with her date, during which Jackie disgustedly asks Carlton to drive her home. The rest of the episode is about Will's dream, delivering An Aesop about drunk driving and Jackie is never mentioned again in the show's run.
Lisa Wilkes. Will dated her throughout season five and she was the most important relationship he ever had. They became engaged but on the day of their wedding they decided they weren't ready to get married and called off the wedding. Lisa was never seen or even mentioned in season six.
Will had a close friend named Tyriq who appeared in the first couple of seasons, but completely vanished once Will and Carlton began attending college after season 3. Will's other friend, Jazz, stuck around at least.
Friday Night Lights: Santiago just seems to have disappeared from existence between season 2 and 3. Ditto Waverly from season 1.
Phoebe's family. After discovering her biological mother, played by Teri Garr, and their reconciliation being a major point of development in Phoebe's life, after a few appearances she never came up again. The exact same thing happened with her biological father, Frank Buffay, after his one-shot appearance. Phoebe literally spent years trying to track him down over the course of multiple seasons - entire subplots of a few episodes revolved around it. Then they finally meet, and after an emotional (if awkward) reunion, he never appeared again. When Phoebe gets married in the final season, no one from her family is in attendance, even her half-brother Frank who had appeared earlier in the season; granted, there was a huge blizzard going on, but no one came to the rehearsal dinner either.
Ross's son Ben is last seen in season eight, and even then his scenes are with Phoebe rather than Ross. His disappearance can be explained due to his mother Carol and her wife Susan—who in turn are last seen in seasons seven and six respectively—being his primary caregivers. Ben's disappearance was lampshaded in "The One Where No One Proposes": Ross's father, Jack, looks at Emma and says "Look at her, my first grandchild", when Ross reminds him of Ben he says, "Well, of course, Ben, I meant my first granddaughter!" and then turns to Monica making a "I totally forgot about him!" face. The fact that they never show or address Ben meeting his new sister is pretty egregious.
Terry, the original manager of Central Perk. He appears in one episode each of the first and second seasons, and after that Gunther (who had appeared earlier as a barista) was the manager and Terry was never mentioned again. Of course, as coffee shops go through staff quite regularly, even management, it would make sense that Terry would eventually move on and Gunther, the longest-serving and most qualified employee, would be promoted to the office.
Fringe: Agent Amy Jessup appeared in the first two episodes of season two, and hasnt been seen or mentioned since. This could be attributed to fan anxiety that Jessup would replace Olivia Dunham, who started the season comatose.
Despite being one of Robb Stark's key supporters (and the first to declare him King in the North in the season one finale), Greatjon Umber is conspicuously absent from seasons two and three because the actor portraying him was unable to appear due to scheduling conflicts. Season six finally reveals that he was Killed Offscreen.
Ser Ilyn Payne was removed from Arya's list of people to kill and never mentioned again when his actor Wilko Johnson was diagnosed with terminal cancer (though his cancer was later revealed to be not terminal and has since undergone a successful chemotherapy) No explanation is given for why Arya forgave his character even though he was the one who beheaded her father.
The actress who played Old Nan in the pilot died, so she disappears for good. It is only in the Blu-Ray season 4 lore that Bran explains that Nan has indeed died.
The mountain clansmen simply vanish and are never mentioned again after season 2. The most prominent member, Shagga, doesn't even make it into season 2. In the books, it's explained that they eventually left the capital, with some returning to the Mountains of the Moon with their booty and others raiding around the Kingswood.
Daenerys's remaining bloodriders, Kovarro and Aggo, have not been seen since Season 2, with Aggo disappearing after his first appearance, and despite Kovarro being one of Dany's closest protectors. In addition, Malakho has not been seen since Season 2. Her khalasar has essentially disappeared into the background starting with Season 3.
Daenerys's handmaiden Jhiqui is a combination of this and Adapted Out. She appears in only one scene, echoing Irri's proclamations of "It is known," and is never seen again. Only a reader of the book, who would expect her to become a recurring character, would even realize that she's gone.
Many, many characters, including the dog Mr. Needles, Accident Amy, Randy, and George's long lost sister Linda.
Linda most likely stayed away from George due to the revelations of how she was put up for adoption and how her birth family is, mixed with George's father-in-law's failed attempt at romancing her.
Randy, Benny's love interest played by Nick Offerman, first appeared in the first episode of season 3, and continued to appear regularly until he broke off his marriage with Benny. Randy only appears once more after this, in season 4, and is never seen again.
Possibly a Justified Trope with Amy, since she was played by Sandra Bullock, who possibly wouldn't have had time to appear consistently. Amy only appears periodically anyway.
The Golden Girls gave the roommates a live-in cook, a flamboyantly gay man named Coco, in the pilot episode. The character of Sophia, who was only supposed to have appeared periodically throughout the series, turned out to be so popular with test audiences that she was moved into the house to be a permanent part of the cast, and Coco got puffed. Arguably justified, since he was merely a live-in cook as opposed to a relative, it's likely he was simply fired or let go. In fact, the pilot episode had Dorothy complaining that Coco's cooking was too spicy.
Good Times: Esther Rolle left (temporarily) at the end of the 1976-1977 season, with her final storyline being her character Florida's wedding to Carl Dixon, an avowed atheist, and the new couple moving to Arizona (to allow Carl to tend to his failing health). Rolle — already upset about the perceived over-emphasis on Jimmie Walker's J.J. character — strongly objected to the storyline, contending that Florida was a devout Christian who was now being expected to live with someone with whom her religious beliefs would conflict. When Rolle agreed to return to the show in 1978, one of her demands was that there would be no mention of Carl or her ever marrying or even meeting him, period. The writers agreed.
Aaron, Serena's boyfriend and Cyrus's son. Aaron and Serena were heading off on holiday to Argentina, but you find out the next episode that they broke up on the flight so that Serena and Dan could get back together. Pretty glaring, considering that he's Blair's stepbrother and appears to be close with his father, yet he never shows up for family events.
Gotham: Nathaniel Barnes took over as Captain of the GCPD and was well on his way to making it respectable when he got infected with the Tetch virus, which made him so violent he had to be sent to Arkham Asylum. He's never been seen or mentioned by anyone since, not even after Gordon and several others were able to cure the virus.
The Greatest American Hero: Ralph's son Kevin disappears after the first season. He is mentioned in the the second season episode "Operation: Spoilsport", but not seen.
Grounded for Life: The dog was tied to a fence in the first episode and then seemed to vanish.
H2O: Just Add Water: A few characters from season 1 disappeared with no explanation in season 2, such as Miriam and Emma's love interest Byron.
Mikayla's (Selena Gomez) last full appearance has her becoming friends with Miley despite her hatred of Miley's alter ego Hannah Montana (of which she was unaware). This could have easily been played with after Miley outs herself as Hannah Montana. Mikayla is last mentioned on a TV show using (likely) archive footage from her earlier 2 appearances and is never mentioned again. Behind the scenes, Selena Gomez had been cast as Alex in Wizards of Waverly Place.
No one's seen, heard of, or even mentioned Monica since the season 2 finale. Doesn't look like anyone's missing her either. However, what she did during season 3 is being revealed in some graphic novels following Micah.
Hana Gitelman - though the graphic novels are doing their best to explain what happened to her.
Zach, Claire's friend in Season 1, also caught this syndrome. Admittedly, Claire permanently left his town 4 episodes after his last appearance, and his actor had other commitments, but it's still a little jarring how he's never even mentioned again.
There's also Caitlin, who is never mentioned again past season 2.
Anyone remember Lyle, Claire's brother? Don't worry, neither do her parents. When Claire's in college, her mother, mother's boyfriend, and HRG, and HRG's sort of mistress, all have a Thanksgiving with Claire, but there's no Lyle in sight. Lampshaded later on in this exchange where Sylar remembers the name of Claire's dog over Lyle:
Sylar: "Everybody dies. Well, almost everybody. Papa Petrelli, Mama Bennet, Mr. Muggles. What's your brother's name, Larry?"
Sylar: "Lyle, right. He's gonna die too."
Hey Dad..!: Nudge's disappearance from the series was never explained. So much for being Simon's best friend.
Hogan's Heroes: In the last season, Sgt. Kinchloe abruptly disappears. His role as radio man is taken over by Sgt. Baker (who previously had been one of many prisoners who basically loitered around in the background during scenes to show that there were more than five people in the whole camp) and no one even mentions him again. (Given the context of the series, however, it is very possible that Kinchloe could have escaped, been killed, transferred to another camp, or possibly even released by Klink.)
Home Improvement: During the first two seasons Jill had a friend named Karen, an outspoken feminist who enjoyed antagonizing Tim. She vanished without a trace after her actress, Betsy Randle, landed the role of Cory's mom on Boy Meets World. The role of Jill's best friend was taken over by a new neighbour character named Marie... who herself disappeared a few seasons later.
Jason Winkler is the most notorious example, as he was given good development, had some romantic subtext with one of the main characters, and was pretty handsome; but after season 1 he was never mentioned again. The fans haven't forgotten him, however, and constantly wait for his return.
Mick Campbell, too. He was a main character in season 1 then got demoted in season 2, but was still pretty important. Come season 3 he was never mentioned, though his actor did get a cameo in the finale, delighting fans enormously.
Other characters who have gotten this treatment, but aren't as missed by the fans, are Vera Devinish, Amelia Pinches, The character's family members, among others. It is in fact very rare for a character who isn't a secondary character to make a return or get a mention in this show.
Ted's sister Heather visits because she is interested in moving to NYC. The ending scene of the episode is Ted presenting her with a briefcase and a lease for her new apartment that he co-signed. And yet, she is never seen again, though her presence at her mother's wedding is acknowledged by Barney. We also see Barney's half-sister Carly once and never hear about her again.
Lily's mother, Janice, appeared in her daughter's bridal shower and was angry at Marshall for getting her cat killed. But in the later seasons, she's never seen again and is only mentioned when Lily dissuaded her Disappeared Dad, Mickey, that her mom raised her full-time. Unlike Mickey, who had some Character Development and even got to be with Lily when she gave birth to his grandson, Janice wasn't even there to see her grandson.
In the first season of I'm in the Band Tripp's best friend was Jared, who was never seen or mentioned in the second season while his role as Tripp's best friend was given to Ash.
iCarly had suffered from this badly in terms of minor characters:
The mean popular girl from Nevel's intro episode, "Germy" Jeremy and the Dumb Jock Duke had both only appeared in one episode and is never seen since.
Not just first season characters either. Wendy, a popular minor Ensemble Dark Horse character simply vanished after her last appearance in the final episode of Season 2 (somewhat downplayed as its mentioned time to time that she still hangs around the main cast).
Tasha, Gibby's recurring girlfriend from a handful of episodes in Season 3, appears to have suffered this fate along with a breakup, only in the 8th episode of the 4th season did a reference to her come, and it was that Gibby and her were no longer 'exclusive' in his words.
Brad, who was implied to have transferred to Ridgeway school with Carly, Sam and Freddie wasn't referenced in the first episode of the second half of season 4, despite it taking place in-universe only three days after iOMG which was the last episode of the 1st half. He doesn't show up in the second episode either, despite them doing a webshow in their usual time and place, which he was explicitly recruited to help with. In the third episode, Carly has to cancel a webshow broadcast specifically because Freddie and Sam aren't there. This is the exact thing Brad would be useful for, and specifically what he was hired for. So he's gone.
Basically, if your name is not Carly, Sam, Freddie, Spencer, Mrs. Benson, Gibby, or T-Bo you will not be coming back, especially if you aren't a villain. Only a handful of characters have even made second appearances, and three of them are Nevel, Nora and Chuck, who are all villains. The only other prominent one to come back was Griffin, who showed up in a second episode, Justified because he lives in the same building, and vanished again.
In I Love Lucy, Fred has a dog in an early episode, seemingly only existing for a single joke. The dog disappeared afterwards and is never mentioned again.
Imagination Movers: Nina's very boring (but oh so funny and entertaining) uncle Knit Knots: a beloved character who owned a business next door to the Imagination Movers' "Idea Warehouse" that created boring items and services for boring people. He appeared in every episode in the first season of the show, but completely disappeared, with no explanation for seasons 2 and 3. He did make one other appearance in the Imagination Movers' concert special, which aired on Disney during season 3, however, this was a live event, which was not intended to be canon with the series.
The Invisible Man was forced by the network in Season Two to add a new, attractive-to-males character played by Brandy Ledford. The fans didn't like her very much, and there were numerous complaints about how she ruined the Fawkes and Hobbes dynamic. So when the show's cancellation was announced, the writers took advantage of the fact that they had nothing to lose anyway, and just left the character out of the last few episodes with no explanation.
Jessica Jones (2015): Jessica's neighbor Robyn disapears in season 2. However given how unpopular she was, hardly anyone complain.
On The Jeffersons, George Jefferson's brother Henry Jefferson, who appeared on All in the Family, is mentioned in early episodes. Henry Jefferson never appears on the series, and in some episodes, George Jefferson states he is an only child. Henry's son Raymond Jefferson appears in season 5.
Jesse shows the title character and her son living with her two brothers in the first season, while she works at her father's bar. However, for the second season, the network retooled the series. While Jesse is shown getting a new job, her father and brothers are treated as if they had never been there.
Kickin' It: Eddie, one of the main cast, disappeared without a trace in season 3 and hasn't been mentioned since then.
Kids Incorporated references missing characters from previous seasons through season 5 — Mickey moves away after season 1, Gloria goes to music school after season 3, Renee and The Kid become exchange students after season 4. Even the characters dropped after the original pilot episode are said to have moved away in a scene added at the end of the VHS release. On top of that, each new character is introduced and has to audition for the band. However, when season 6 begins, Ryan and Connie have been replaced by Robin and no one mentions their absence or where Robin came from, then Stacy, Richie, and Devyn are replaced by Eric, Ana, and Haylie for season 7. At this point, more than half the cast is new this season, and only one of them has been on the show for more than a season. And yet we have not a word about this (they do mention, several episodes later, that Ana had only recently moved in with Robin's family after her parents' divorce).
Sara Spooner, the younger sister of Carrie, only appeared in about five episodes of season one and disappeared from the show without an explanation. It was later revealed in an interview with show star Kevin James that the writers had no idea how to develop her character so they just decided to write her out.
Doug's sister Stephanie and his friend Richie, both of them vanished without a trace. Doug and Carrie also had two dogs in the first few episodes which disappeared without an explanation.
Doug and Carrie also adopt Stanley, a dog that belonged to one of their neighbors towards the end of season 1. He was shown in the background for the rest of the season, but suddenly disappears for several seasons without any mention, only to suddenly reappear in an episode which featured him prominently in the fourth season, where he was again a background character....only to vanish once again without mention.
Kyle XY: More often than not, this became an Averted Trope in favour of putting people on a bus or killing them off-screen. In the second season, however, there is a glaring example of this trope. After the Madacorp plot is defeated, Julian Ballantine is demoted and replaced by The Dragon, Emily Hollander. The scene in which this happens has suitably ominous overtones, suggesting that the viewer hasn't seen the last of Madacorp. And then Hollander appears in one further episode, attending her company's stand at Kyle's school, and is never seen again.
Eli Duckett, a popular recurring character for 15 years, was never mentioned again after the actor playing him, Danny O'Dea, died.
Ros was never mentioned again after her actress left following the 26th series.
When Tom first arrives in town, he is accompanied by Mrs. Avery, a potential love interest and foil for Nora Batty, and her niece, Babs. Both characters were unpopular, and Babs disappeared after only three episodes, while Mrs. Avery was around for a series. Neither character has been mentioned since their departure despite their former relationship with Tom.
Earlier, the librarians, Mr. Wainwright and Mrs. Partridge, were regulars during the first series but disappeared completely during the second series without mention or explanation. Mr. Wainwright would return for a few episodes during the third season but disappeared completely following the third series and was never mentioned again.
Las Vegas: Sarasvati had been built up in Season 3 as a potential love interest for Mike. The last we see of her is the final episode of the season at Delinda's bachelorette party, where she asks Mike to come to her room. Mike never makes it, and the next we hear Sarasvati had gone home with all of the male strippers. She appears for about five seconds in one episode of Season 4, but other than that is never heard or seen from again.
Laverne & Shirley: Edna De Fazio, the girls' landlady and later Laverne's stepmom vanished sometime after the characters all moved to California.
Paul Robinette. A deleted scene has Stone telling Van Buren that Robinette quit and joined a private firm, but this scene wasn't aired. Robinette resurfaces for a few guest appearances, though.
Nina Cassidy. Considering her performance/Van Buren's reaction to her through the entire season, including her final episode, it's heavily implied she was transferred if not fired. Unlike the others, though, she never shows up again after her disappearance.
Alfred Wentworth, the DA in the pilot, "Everybody's Favorite Bagman," which was the sixth episode aired.
Two of Elliot's daughters haven't been seen since season eight, with a picture reference in season ten. All the more jarring because one of them has a twin brother who had a Day in the Limelight episode.
In the episode 'Totems', Elliot says he has five kids, so they still exist. The other two apparently have normal, uneventful lives.
Lie to Me: Torres' boyfriend served his part in her character development, then faded from the cast's collective memory.
Life With Bonnie: Samantha Molloy flat-out vanished between Season 1 and Season 2. This example is especially disconcerting since she was the main character's 12-year-old daughter in a show that had many, many "family at home" scenes.
Life with Derek: Where Noel only had three appearances that were rather influential (i.e. he was partially the reason why Casey broke up with Max in the episode "Allergy Season"). It was even set up in the episode "Just Friends" where Casey and Noel would become a couple... except not, apparently.
Cat Grant disappears without a trace after the first season, ostensibly because the network thought she was too risque. Increasingly important character Jack, who'd been the focus of some serious character development over the course of the season, showing Clark's positive influence on people, disappeared with Cat. Disappointing to say the least.
Lucy Lane felt like this - a regular in the first three episodes, then vanished without a trace. She would make a few more appearances, though, by a different actress and with a very different relationship with Lois, as if they'd forgotten about Lucy's earlier days.
The Loop is a particularly bad example of this. Between the first and second season, both female leads simply disappeared without a trace. The reason this is so unnerving is one of the female leads was the main character's love interest, and their relationship was left completely unresolved.
The character Isabel is introduced as "the sheriff" of The Others in a season 3 episode. She investigates Juliet after Danny's death, and seems to be a high-ranking member of The Others' hierarchy. She is never seen or mentioned again, and producer Damon Lindelof said that she was killed offscreen when the Others attacked the beach in the Season 3 finale.
In a bizarrely large-scale example of the trope, the Others themselves disappear completely after the season 6 episode "The Last Recruit" and are never seen or heard from again. They are last seen getting pulverized by mortars, making it unclear if any of them survived. The arguably canon Lost Encyclopedia claims at least three Others—kidnapped flight attendant Cindy and children Zach and Emma—survived the bombing, but the fates of the rest of the Others are never known.
Mad About You had Paul's friend Selby vanish, last seen at the end of season 1 (Lampshade hung in one episode when Paul, complaining about their lack of friends, yelled, "Like Selby, what the hell happened to him?").
Cynthia, a recurring character, originally has a one-sided crush towards Malcolm. She goes to Europe and when she comes back, is all grown up. Malcolm eventually re-considers her as a potential love interest. In her final episode, she loudly proclaims in front of the whole school that they had previously been intimate. And then, without explanation, she never appears again.
Some of the Krelboynes that appear early in series such as Lloyd and Dabney aren't seen again after season four, with only Stevie remaining by the end.
Married... with Children: Seven, a Cousin Oliver introduced in Season 7, was removed without explanation when he proved unpopular with the fans. Lampshaded in one episode when his face was seen on a milk carton and no one in the Bundy family noticed or cared. Another episode, "Kelly Knows Something", showed that Kelly could learn things, but for every new fact she learned, another fact would be forgotten. While cramming for a quiz show, a visual gag shows new facts going into her head as old ones exit... including the existence of Seven, apparently.
Rhoda has a sister Debbie who is not seen at all when Rhoda got her own series. Debbie appears on an episode where Rhoda and Mary go to New York for Debbie's wedding. Furthermore, Rhoda's sister Brenda is nowhere to be seen.
Rhoda also mentions once, on The MTM Show, that she has a brother, and therefore does not need to have the purpose of a bar mitzvah explained to her. It's one line, but her brother is never mentioned again. This could be explained as Rhoda lying, just so a person who tends to ramble on, doesn't go on and on explaining bar mitzvahs, but it doesn't sound that way.
Likewise, The MTM Show has possibly TV's first blatant, non-judgmental, declaration of a character as "gay," using that exact word; it's a punch-line that is built through an entire episode, in that Phyllis is horrified that her wonderful brother is spending time with Rhoda, whom she can't stand, and not with Mary, in spite of Phyllis's efforts to set up Mary with her brother. At the the end Rhoda tells Phyllis she isn't interested in her brother because "He's gay," and Phyllis says "Thank God!" When Cloris Leachman gets her spin-off, Phyllis, the brother is never mentioned.
What ever happened to Spearchucker Jones or Ugly John? The Real Life explanation is that Spearchucker was dropped when the writers were informed that there was no record of any African-American doctors serving in the Korean War. (There is now a Web memoir that mentions an African-American surgeon at a MASH unit.) Ugly John has no explanation, real life or in-universe; curiously, the actor who played him later showed up playing a different character (Muldoon) in the season 8 episode "Captains Outrageous."[[/note]]
Ho-Jon, presumably, went to college off-screen.
Matlock: Ben has a daughter, Charlene, played by Linda Purl, during the first season, who leaves to marry some prosecutor. He occasionally refers to "my daughter." Later in the series, Brynn Thayer comes on as his daughter, LeAnn, who just divorced her prosecutor husband of whom Ben didn't approve.
Merlin had Geoffrey of Monmouth, the court genealogist and librarian, who was often used for exposition purposes. A Recurring Character throughout the first four seasons, he was completely absent from season five without explanation. There was a time-skip between seasons 4 and 5, though, so it could be concluded that he died of an age-related illness off camera.
The Middle: Bob (played by Chris Kattan), one of Frankies coworkers at Elhert Motors in the first three seasons, was a very close friend of the Hecks. After Frankie was fired at the start of Season 4, it looked as if we would merely be seeing him less often. Instead, he hasnt been seen or even mentioned since. More egregious is that Frankies former boss shows up a few times since, and Middle has a richer sense of continuity than the average sitcom. Still, in a later episode where Frankie has to work at Elhert again to cash a paycheck, Mr. Elbert shows up for a scene, but Bob seems to have been... Bobbed.
Midsomer Murders: DS Scott, who is mentioned as calling in sick in one episode (Barnaby ropes in a random police sergeant as a temporary replacement). That was the character's last mention, with the 'temporary' replacement suddenly being permanent with no explanation. Especially jarring since both his predecessor and his successor got the same simple explanation for being gone — by being promoted and hence moving to another position within the police.
Mission: Impossible: All cast changes on Mission Impossible occurred without explanation, except for the switch from Terry Markwell's Casey Randall to Jane Badler's Shannon Reed in the revival, when poor Casey is caught and killed. (Needless to say, the Secretary disavows all knowledge of her actions.) But only Dan Briggs, the original IMF leader, is a genuine Chuck Cunningham (replaced by Jim Phelps because Steven Hill refused to work on the Jewish Sabbath). Other changes are explained by the way the leader chooses the rest of the team after he gets the assignment. The fact that he chooses the same people over and over is actually a kind of reverse Chuck Cunningham.
The first half of Murder, She Wrote season one had the recurring character of Ethan, a Cabot Cove handyman who served as The Watson to Jessica. He disappears from the second half of the season, and season two introduces Doc Hazlitt in much the same role.
My Family: Alfie disappears after series nine and is never mentioned again.
One of the early television masters of this art. It happened more than once, and in a deliberate fashion. First, William Frawley, veteran comedic actor (I Love Lucy's Fred Mertz) played Fred MacMurray's father-in-law, Bub O'Casey, the boys' grandfather and housekeeper. When Frawley (very begrudgingly) left the show when his poor health meant he could no longer be insured, William Demarest's Uncle Charlie was brought on, with Bub sent home to Ireland. Eventually, any and all references to Bub simply vanished. When the show moved from ABC to CBS and started color episodes, eldest son Mike married his sweetheart and moved away. Orphaned neighbor Ernie was adopted after some wackiness - and again, eventually, both Mike and the fact of the adoption vanished from mention. Steve's new wife and her daughter joined late in the show's run - but the signs were already there and references to a pre-Douglas life dried up for the two. So: Uncle Charlie was always their housemaid/gruff mentor, Ernie was always the third of three sons and no more, and the new Mrs. Douglas and child had always been there as well. This is the word of Fred MacMurray.
It was established long before that episode that Chip didn't remember his mother. One episode (from the black and white era when Chip was still quite young) centers on him admiring a photograph of her and asking lots of questions about her.
There was, in fact, a single line, when Steve was adopting his new wife's daughter. Ernie says at the breakfast table, "You know, I was adopted, too," and it drops right there.
Mike's vanishing was slower than all that - in one ep, when it seemed Steve might end up in a circumstance where he couldn't have them around, the younger boys discussed possibly living with Mike. As to Ernie in that later sequence, it could be a nod to his adoption, or it could be a way of saying, 'If the older Chip doesn't recall their mother, then Ernie has no chance at all.' No one ever states that Bub/Mike/the adoption never happened. They just avoid all instances in which it might be brought up, to the point where certain Bub flashbacks now have Uncle Charlie edited in.
Naturally, Sadie: Tad, a friend of Ron Yuma and Rain is a recurring character during season 1; he's never seen or heard from after the Retool.
Ned's Declassified School Survival Guide let a good few characters fell victim. You can possibly view it as Truth in Television, as in grade school classmates and teachers you see one year you may see less frequently the following year, due to schedule changes and making new friends to replace the old.
Bitsy, Suzy Crabgrass's best friend disappears after season 1, with the introduction of Missy Meany.
All members of the Huge Crew are never mentioned after season 3.
The Scoop after season 1.
Shandra Taylor, Moze's friend after season 1.
This nearly happens to Mr. Monroe. He was in the opening credits in season 1, replaced with Gordy in season 2, and starts appearing less and less until he disappears. However, he finally returns in a late season 3 episode. and was promptly re-added to the opening credits.
NewsRadio: "Efficiency expert" Andrea (a.k.a. Planbee, played by Lauren Graham pre-Gilmore Girls) disappears after appearing in four early episodes of season 4 without an explanation, as does intern Walt after four episodes at the end of the same season.
During the first two seasons, this happened several times - starting with the second episode. The original public defender, Sheila, vanished from the show and even failed to make the listing for the show on IMDB!
Also happened to a popular pair of recurring characters, Bob and June Wheeler, a married couple of hard luck hicks. What made their disappearance feel especially abrupt is that in their last episode, it was implied they were about to have a semi-regular presence on the show, since they just bought and began running the Court cafeteria. And to make their departure all the more jarring, their last episode was a Season Finale that ended on a Cliffhanger. In the next season premiere, the Cliffhanger is resolved, but with no sign or mention of the Wheelers. The Real Life reason for their disappearance is that the actor who played Bob, Brent Spiner, was cast as Data on Star Trek: The Next Generation in between seasons.
Also happens to their little girl Caroline. She's with them in their first appearance, but not present or mentioned in their second. Given the Wheelers' role on the show as The Chew Toy, it's probably best if we don't inquire too closely as to what happened to their daughter.
NUMB3RS, as often happens with pilots which are picked up. Originally, the Rob Morrow character wasn't Charlie's brother, nor was he Rob Morrow.
NYPD Blue: Det. Lesniak just stopped appearing after season 3. No explanation was ever given. The series lost quite a few leading characters, but usually they were described as moving on to other jobs, or were killed of outright.
A few extras from the pilot of disappeared after the cast began to fill out the workplace.
Erin's foster brother is introduced in one episode, and promptly drops from the face of the Earth afterwards.
Jevon from Only Fools and Horses, who was Mickey Pearce's trading partner in the sixth season, vanished without a trace after "The Jolly Boys' Outing". Unlike most of the other semi-regular characters that left the series, he was never mentioned again after his disappearance.
Once Upon a Time: Due to the "closed campus" nature of the Storybrooke setting, the series is prone to this trope whenever a recurring character suddenly disappears without having been killed off or sent to another world (due usually to an actor being cast in another series or only contracted for guest appearances), and situations later emerge where the absence of that character is very noticeable.
Sidney Glass (aka the Magic Mirror) is established as a major supporting character throughout Season 1, established as being a confidante of Regina in both worlds (and also in love with her). However, in real life actor Giancarlo Esposito became committed to a series on another network, Revolution, for the 2012-13 season. As a result, Sidney is last seen incarcerated in Regina's private asylum, prior to the curse being broken, and is conspicuous by his absence in post-curse Storybrooke and flashbacks rarely feature the Mirror (though he does make a voice-only cameo in a season 3 flashback). After The Bus Came Back for two episodes in season 4 (after Revolution bit it), Sidney is freed from his cell and promptly trapped back in the mirror; once the Snow Queen frees him, he suffers this again, as he has not been seen since.
Kathryn Nolan is absent in Season 2 as her actress (Anastasia Griffith) joined the series Copper. After disappearing for about the length of two seasons, she reappears briefly in the season 3 finale, still in Storybrooke but off-camera. Like Sidney, she vanishes again afterward.
The character of Ruby/Red disappears from the series suddenly, several episodes prior to the Season 2 finale; the creators had decided to drop the character, and the actress (whose role had been reduced substantially since completing a major character backstory arc midway through the season) had been cast in another series. Ruby's absence is particularly noticeable in the closing episodes of Season 2 when Emma and the others are frantically searching for the kidnapped Regina, yet for some reason do not employ Ruby's previously and well-established ability to track people. As with Kathryn Nolan, she cameos in the second half of season 3, then is absent again; she finally returns in season 5, which wraps up her storyline by explaining that after the party at the end of season 3, she'd used a magic bean to go back to the Enchanted Forest in search of other werewolves, and was taken captive by a witch; once freed by Mulan, the two traveled together for a time. In the second half of the season, they turn up in Oz, where Ruby finally finds her true love Dorothy Gale; she has not been seen since the two became a couple.
Mulan joins the Merry Men early in season 3. When the company returns in the second half of that season, she is nowhere to be seen or heard. As with Meghan Ory's case, Jamie Chung was also cast in another series at the time. (Incidentally, both series lasted only one season.) As with Ruby, she returns in season 5, having left the Merry Men under unrevealed circumstances and later saving and traveling with Ruby, last being seen in Oz after Ruby became a couple with Dorothy Gale.
The ex-giant Anton disappears after Season 2, despite joining the dwarves. His actor Jorge Garcia was cast as a recurring character for the 2013-14 season of Hawaii Five-0, and became a regular the following season. While Anton has not been physically seen since, season 5 eventually explains that he, like the others in Storybrooke, was returned to the Enchanted Forest for a year and then came back to the recreated town with the rest of the cast, where he and Ruby tended the bean fields together, producing the single bean that Ruby later uses to return to the Enchanted Forest.
Jimmy Edwards is one of the guys Lucas hangs out with at the River Court, and creates RavenHoops.com with Mouth, but vanishes with no explanation, even being missing Season 2's "Lifetime Piling Up" which retold the events of the pilot with Lucas & Nathan's roles flipped. Deconstructed in Season 3's "With Tired Eyes, Tired Minds, Tired Souls, We Slept" when Jimmy takes a gun to school, utterly devastated by all of his friends abandoning him over the past year.
Tim Smith is Nathan's best friend at the start of the series, and the only named Raven who isn't a major character, growing in prominence after Jake was Put on a Bus & becoming the show's resident Butt-Monkey, before vanishing halfway through Season 3. It's later revealed during Season 4 that Tim transferred to a school for slow students. He returned in Season 5, commenting that he misses high school since no-one's kept in touch with him.
Pair of Kings: Season 2 ends with the Evil King resuming his life as Lanny's pet fish Yamakoshi but Season 3 has no appearance of Yamakoshi or an explanation for his sudden disappearance.
The Paper Chase: In the pilot, the study group included a woman (Linda O'Connor, played by Katharine Dunfee Clarke) who never appeared again. In the second episode, she was replaced without explanation by a different woman, Logan, a major character throughout Season One. When, after cancellation on its original network, the series returned for Season Two on a cable network, Logan had inexplicably vanished, never to be mentioned again.
Desiree (Mari Marrow), Nikki's best friend, simply vanished about halfway through the first season and is never mentioned again. A few episodes later, she is replaced by Andell, a character from parent series Moesha. This is especially jarring considering that Desiree lives next door to the Parkers. Her disappearance from the show can later be justified as Nikki eventually moved into Kim's apartment...but that still didn't happen until a season after she disappeared.
Symone, the fourth member of Freestyle Unity. Curiously, she disappears once the group achieves a comfortable level of success.
Susan Campbell, friend of Larry Appleton and Balki Bartokomous, appeared in several of the early episodes, but vanished midway through the second season.
In the third season, Harry Burns was introduced, whom was Larry's boss for his new job at the Chicago Chronicle. He was never seen or heard from again after the third season, and Mr. Wainwright would become Larry's boss instead.
Poirot: Chief Inspector Japp, Captain Hastings and Miss Lemon, who had previously been the show regulars, disappeared at the time of Poirot's 10-Minute Retirement at the start of Season Nine. Since these characters were often used for comic relief, that was a sign that the show started to take itself more seriously. The characters also appeared far more frequently in the series than they did in the books, Miss Lemon and Japp in particular.
Scorpina double subverts this trope, disappearing the moment Lord Zedd shows up, only to appear in one episode fighting the Rangers, surviving and never being seen or heard from again. Behind the scenes, she had been scripted to return, but Saban could not get the American actress back and the plot line was dropped instead. The graphic novel Soul of the Dragon eventually explained that she'd betrayed Rita and was banished to the Talos Dimension (home of Rita's ally Lokar) as punishment, eventually returning to Earth in order to capture Tommy's son JJ as a sacrifice for Lokar so he would give her more power. He kills her when she fails.
Angela, Zack's love interest, disappeared after the first season. Richie, Trini's love interest, and Curtis, Zack's cousin, both disappeared after Trini and Zack were written off the show.
Richie and Curtis were very weak attempts at creating red herrings to the identity of the White Ranger. It was so clumsily done that Richie being a martial artist wasn't even mentioned until his last scene, after the White Ranger had been revealed.
During the transition between Power Rangers Turbo and Power Rangers in Space, Lt. Jerome Stone, as well as the Angel Grove Youth Center and Juice Bar, disappeared and was replaced with Adelle and the Surf Spot.
It was previously averted with the previous Juice Bar owner Ernie whose absence was explained as a trip to the Amazon.
Mr. Caplan and Ms. Appleby make similar departures in the episodes "Alarmed and Dangerous" and "Carlos and the Count", respectively.
Sometimes, Bulk and Skull would occasionally be seen with various nameless thugs, as part of what we are led to presume is their 'gang'. They only appeared for certain early season 1 episodes, and then seemed to disappear forever.
When Jason returned in Zeo, he met Emily who became his love interest. He departed at the end of Zeo, going off with Emily, and appearing again in the Turbo movie but without her. He then disappears until the tenth anniversary special.
That's not even the true mystery. It's Emily that's the mystery. She has one more on-screen and named appearance in Turbo, which lasts for a few seconds and without Jason, then she's never seen again at all.
Squatt and Baboo, where are they in Zeo? They used to cling onto Rita, and yet when she, Zedd, Finster, Rito and Goldar are forced out of their home, and made to travel around in their caravan, it seems Squatt and Baboo just kinda...vanished.
They went to stay with Master Vile to regroup; maybe they stayed with him when the others went back to the moon.
Squatt actually does make an appearance in the Grand FinaleSpace, as one can see here. However, it's alongside a whole bunch of other previously seen monsters, so he's easy to miss. No reported sign of Baboo, though.
Rito was never seen again after Zeo. This was reportedly due to his suit constantly falling apart and needing to be patched up.
Eddie is nowhere to be seen in Raven's Home and isn't even given a passing mention. During That's So Raven, Eddie was the third major character, was best friends to Chelsea and Raven, and had a fair amount of Ship Tease with Raven. His actor, Orlando Brown, was having troubles during the show's production so Eddie is just never mentioned.
Reaper: Sam's brother appears in the first two episodes and then is never seen or mentioned again for the rest of the show's two-year run.
Red Dwarf: Threatened, then averted with Kryten. He appeared in the first episode of the second series, and by the end of the episode he looked like he was going to stick around. But he didn't appear again and was completely forgotten about for the rest of that series! But then in Series 3, he came back as a main character with a quickexplanation about where he'd been and why he now looked different.
The end of Series 7 implied that Lister's best friend Olaf Petersen was on the ship and would return in Series 8 but he never did. In reality this was cause his actor was unavailable, though it is worth noting that he never made a return in any other series, unlike several characters that did.
The second season introduced a host of new characters, none of whom were ever seen again afterwards, save for the odd reference here and there.
There were also Glen Brachston, the lazy marina owner; Garth Harble, the first animal control officer before Ed Frid; Earl Battersby, the bait shop owner; Dwight Cardiff, the other lazy marina owner; Dougie Franklin's brother, Benjamin; Bob Stuyvesant, golfer/ministry of natural resources worker; Arnie Dogan, accident-prone roofer/aspiring country singer; Young Walter, who substituted for Bill in the later Adventures With Bill segments; Dale, a teenaged boy who worked at a local gas station; Kevin Black, a Yuppie cottager; Brian Jacobs, funeral parlor owner; etc. At least one was justified, as Red mentions in one segment that Garth got bit by a toad and "lost his nerve," and thus was replaced by Ed.
This happened repeatedly over the course of the series with family friends and neighbours. The most egregious example was easily Roseanne and Jackie's best friend Crystal Anderson, whom they'd known since childhood and was an official main character for the first few seasons, appearing in the opening credits and everything (a rank never even granted to the Healy brothers, despite them living with the Conners and appearing prominently in almost every episode in the second half of the series). After she marries Dan's father Ed (a recurring character played by Ned Beatty) and bears two children with him, they all vanish for a season or two before prominently appearing again in a two-episode arc about Dan reconciling with his father. After that, Dan's father did not appear again and Crystal returned for one last appearance at Roseanne's baby shower at the beginning of the 8th season. Neither of them appeared after that, even at extremely notable events such as Darlene's wedding, or in the final season when the Conners won a hundred and eight million dollars in the lottery. Crystal finally re-appeared in the 6th episode of the revived 10th season.
One neighbouring family introduced a few seasons into the series got tons of episodes and development, including one daughter pursuing main character David Healy and her overweight wallflower sister catching Roseanne's attention as somebody who needed support and guidance. The Conners even all traveled to California with them in an RV at one point. Unlike their previous set of neighbours, who'd had a proper send-off, they eventually just stopped appearing.
Jackie's husband/ex-husband Fred stopped appearing altogether a couple episodes after they divorced, although was occasionally referenced as taking care of their infant son in various episodes. Like other characters, his absence in the face of the Conners winning the lottery (including his best friend and boss Dan, his ex-wife Jackie, and his infant son Andy) is nigh-inexplicable.
There's also the absence of Bonnie, the waitress from Rodbell's with whom Roseanne worked. Somewhast justified in that everyone seemed to have gone their separate ways after the Rodbell's diner closed down - Leon, her boss, vanished for a while before resurfacing after Roseanne and Jackie opened their own diner, and stuck with the show afterward. Bonnie, meanwhile? Just gone.
Anne-Marie and Chuck also vanished. Anne-Marie was a friend of Roseanne's from high school, while Chuck became friends with Dan (or friends again, since it was suggested he and Dan played football together) through their wives and regularly participated in the men's poker games. Chuck did appear again in the 8th and 9th seasons, as well as the revival, as a fellow contractor and business contact for Dan.
Damn near everyone from the plastics factory disappeared after the mass walkout. Vonda stuck around for a little bit into the second season, even having a singing part in the musical episode and setting Roseanne up for a job interview. Then she was replaced by Anne-Marie as the Token Black Friend. Possibly justified with Sylvia, who was much older than the others.
The salon crew got plenty of screentime and development during Roseanne's tenure, and the whole setting felt like a possible test for a spinoff launch. But, after Roseanne getting into an accident with one of the regular customers, she's never shown working at the salon ever again, and no mention is made of why. (Presumably, her job was filled while she recovered.)
Apparently she did keep working that salon job offscreen, because in early Season 3 when she got the Rodbell's job, Dan mentions something to the effect of she won't have to sweep hair anymore.
Two characters at the end of season 1 disappear without explanation: Sabrina's supposed "best friend" Jenny (who might have been referred to in passing as "Jennifer" in a later episode) and Mr. Poole, the science teacher.
Mr. Poole may have a bit of an excuse, as he wouldn't be Sabrina's teacher due to her moving up in the grades.
It's also never mentioned what happened to Dreama, the girl Sabrina was supposed to be coaching for her Witches' License.
The character of Miles also vanishes quite suddenly.
Saved by the Bell: The New Class had a good deal of Revolving Door Casting during its entire run. Scott, Weasel, and Vicki disappeared after Season 1 and were forgotten about by the remaining characters; same for Brian, Bobby, and Megan after Season 2. Starting in Season 4, the writers tried to address what became of departing characters, though that didn't stop them from forgetting about R.J.
It happened in the original Saved by the Bell too, but to a lesser extent. The most egregious example was the replacement of Jessie and Kelly with Tori for the last season. That is, until the graduation finale, where the process was reversed. Neither was given any explanation. What really happened was after the series finished production, the network ordered more episodes. The actresses declined to return for them, thus necessitating Tori. The finale was filmed before this happened.
Saved by the Bell is actually one of very few shows where the main character got Chucked. In its first season, the show focused on the kids' teacher, Miss Bliss, and the school faculty in general; the kids were meant to be supporting characters. This setup was quickly abandoned once it became obvious that the students had a lot more potential for comedy and stories than the teachers. Miss Bliss vanished from the show between seasons one and two. Even the school itself fell victim to this trope — in season one, it's a junior high school in Indiana. From season two on, the same cast is attending a high school in California. No explanation is ever given.
Saved by the Bell is sort of an re-imagining of a different show, Good Morning, Miss Bliss, which was then shown in later syndication as though it were the first season of Saved, despite the resulting oddities in continuity.
Scrubs: Dr. Grace Miller was introduced with much fanfare in season 3, then promptly vanished off the face of the earth. Series creator Bill Lawrence later explained that this was because Miller had been a failed attempt to create a female Dr. Cox character. This didn't work because A) it was redundant, as Jordan more than adequately fulfills that role, and B) Dr. Miller was an unfunny, unlikable shrew.
Dr. Jeffrey Steadman appeared regularly during the first season as J.D. And Elliot's original resident, as well as Dr. Kelso's lackey. After the fifth episode, he dropped off the face of the earth until making one isolated appearance in season 3 and another in J.D.'s last fantasy in season 8.
Secret Diary of a Call Girl: In one of the earlier episodes Hannah goes to the hospital to visit her sister Jackie who just had a baby boy. Later she goes to her new nephew's christening. But when Jackie separates from her husband and stays for awhile with Hannah, the baby is not only nowhere to be seen - he isn't even mentioned once.
Kramer's pet dog is never seen or spoken of after the original pilot.
Also, their friend the saucy waitress at Monk's, intended to be a main character.
Elaine's father Alton Benes appeared for only one episode and was quickly shuffled off the show and never mentioned again. This was due to him being played by infamously volatile actor Lawrence Tierney, who during filming of the episode scared the shit out of both the cast and crew, to the point that he tried to steal a knife prop from the set and, when called out on it by Jerry, responded by pretending to stab him with it.
Sesame Street: With the exceptions of David and Mr. Hooper, the departures or absences of most human characters have not been explained.
Additionally, numerous Muppet characters have come and gone for various reasons and are now no longer on the show. One was Don Music, a piano player who bangs his head against the piano in frustration, who was dropped from the show when kids at home started doing the same thing. Another was Harvey Kneeslapper, who was let go because his signature laugh was too much of a strain on Frank Oz's vocal cords. Then there was Roosevelt Franklin, who was arguably one of the first breakthrough Sesame Street Muppets, but who was dropped since he was considered to be a negative cultural stereotype (he was the only African-American Muppet at the time and was seen mostly in detention after school). Lefty the Salesman may have vanished due to his criminal nature, something that was probably later deemed inappropriate for a children's show. Finally, Professor Hastings, a teacher whose lectures were so dull that he would put himself to sleep while giving them, was discontinued because he was too dull.
Many of Richard Hunt's characters (such as Forgetful Jones and Placido Flamingo) disappeared upon his death in 1992. Similarly, a good number of Jerry Nelson's characters were phased out as his health problems started to worsen and forced him to limit himself to mostly just performing The Count.
Sleepy Hollow has Luke Morales, a semi-regular character in the first season and ex-boyfriend of Abbie Mills, who is knocked unconscious by a demon in the second to last episode. His fate has yet to be revealed.
Space: 1999 had quite a few characters disappear between its first and second season. From the regular cast, Paul Morrow, David Kano and Victor Bergman were suddenly gone without explanation. The Moonbase Alpha Technical Notebook explains that all three died... and apparently lines of dialogue were written to that effect but never used, making their absence all the more glaring as there was nowhere they could have gone (Not quite. Morrow and Kano's absences are never explained, but the Season 2 opener "The Metamorph" confirms in dialogue between Verdeschi and Sandra Benes that Victor Bergman died due to a malfunctioning spacesuit). A recurring character, Tanya Alexander, also went missing. Dr. Robert Mathias, Helena's assistant in the medical center, was briefly used in a much smaller role but then disappeared (again, the tech notebook "explains" that he changed sections). This was compounded when Tony Verdeschi started in like he'd always been there in their place at the beginning of the season.
Spin City was well known for this — of all the characters who left, only Mike (Michael J Fox) actually had an exit storyline. This meant that, over the course of the series, Stacy, James, Nikki, Janelle and Angie all disappeared without trace, often with only the barest of mentions (Catlin: I fired James.).
Stacey, Mike's secretary, is a notable example in just how she vanished. Season 3 ends on a cliffhanger and season 4 picks up exactly where the last episode left off. Jennifer Esposito left the show after season 3, so when season 4 picks up, all traces of Stacey have disappeared completely from city hall in a matter of hours and no one notices or mentions it. When Caitlin fires James, Mike takes him on as his new secretary, which is the closest the show came to acknowledging that Stacey ever existed.
Stargate Atlantis has Hermiod, a recurring Asgard on the Daedalus introduced in Series 2. He later vanishes without explanation, though Word of God claims that he died in the Asgard mass suicide in the SG-1 finale.
Re'tu Charlie is never seen after going to live with the Tok'ra in "Show and Tell", despite his closeness to Jack and despite many further Tok'ra episodes. Sadly, it was stated onscreen that he was so close to death that even a symbiote may not be able to save him.
Nyan is never seen after being becoming Daniel's research assistant in "New Ground".
Teal'c's love interest, Ishta vanishes after season 8 and is never mentioned again.
And of course, Jonas Quinn, a member of SG-1 for a whole season, completely vanishes after season seven's "Fallout". He isn't mentioned when when his homeworld Langara is said to have been conquered by the Ori, nor does he reappear when they actually show Langara in Stargate Universe.
The Tok'ra Anise was introduced to 'sexy up' SG-1 for ratings, but was removed without any fanfare after a few episodes when the show runners decided the ratings were fine as they were.
Star Trek: The Next Generation: Guinan appeared numerous times per season, starting with season two, but was completely absent from season seven despite numerous scenes set in Ten-Forward where she was the bartender/proprietor, and many scenes where the characters might have sought her age-old wisdom had them instead going to some other character. Guinan was, of course, played by Whoopi Goldberg, who was a very busy film actress during TNG's run. Her absence in the final season could possibly be explained by her filming schedules for Sister Act 2: Back in the Habit, Made in America, and numerous voice roles. She did come back for Star Trek: Generations and Star Trek: Nemesis.
Star Trek: Deep Space Nine: Remember T'Rul, the Romulan who was part of the command staff of the Defiant as a stipulation of the Romulan Empire's agreement to let Starfleet use one of their cloaking devices? No? No surprise. This was in part due to the actress who played her, Martha Hackett, getting cast in a recurring role on Voyager shortly afterwards, and the Deep Space Nine staff simply not feeling like replacing her. While it would've been technically possible to have her play both roles simultaneously (each character had a different rubber forehead which would've kept it from being too glaringly obvious), for whatever reason they didn't want to do that. (The rules T'Rul was there to enforce also conveniently disappeared without a comment, other than one episode where they simply remembered one of the rules, then broke it. Why the notoriously secretive Romulans no longer felt a need to keep watch over their cloaking device was never explained.) It became a moot point after a single season; the crew tried to use it in combat conditions and immediately broke it, never to be replaced or mentioned again.
Shakaar Edon, Kira's love interest and the Prime Minister of Bajor, also disappears; at various points afterward, Bajor is represented by Kai Winn at various conferences and such (e.g. the abortive signing of Bajor's admission to the Federation in "Rapture") where it would make more sense for the civil government to be involved. The real life reason was scheduling problems for the actor, Duncan Regehr, who lived in Canada.
Star Trek: The Animated Series featured M'Ress and Arex, who never appeared or were even mentioned in any following shows or movies. Arex's species never made any further appearances, and M'Ress's species only had a few cameos in the movies. At least in the case of Arex, this is probably because he wasn't a Rubber-Forehead Alien; since it was animated the show wasn't limited by what could be done with prostheses, and Arex was too inhumanly thin and gangly to be portrayed by a person in a costume (and that's leaving out his three digitigrade legs). For most of, if not the entirety of, the run of TNG and DS 9, CGI wouldn't have been up to the task of inserting an Arex who didn't look jarringly fake, and even now it would probably still be too difficult and pricey for a weekly series.
Elizabeth Cutler on Star Trek: Enterprise was a prominent secondary character during season 1 to the point of her shaping up to become Phlox's love interest. She vanishes without trace between seasons one and two due to the actres's death.
Dr. Ben Samuels was a major character who simply stopped appearing after the first season. His plot arc was never resolved, and none of the other characters mention him again.
This was also the case for Dr. Hugh Beale, a southern-fried psychiatrist who had a very prominent arc in the first season. As the only one from his department in the main cast, however, a bulk of his action kept him away from the other regulars and out of the main arcs for the series, so he was gone as of Season Two, without explanation.
Carol's sister and mother likewise disappeared after the first season.
Cody also went missing in the sixth season due to real-life issues with the actor's wife, but managed to return for the final season. His absence was handwaved by saying that he was off wandering the world on his bike.
The Suite Life of Zack and Cody: The old maid Muriel appeared a lot in season one... and disappeared in the next two seasons. It was revealed in the series finale that she retired two years earlier... and Zack and Cody brought her back because Mr. Tipton was planning on firing somebody, and who better to fire than someone who's already retired?
Supergirl changed production location from Hollywood to Vancouver between seasons 1 and 2, resulting in the loss of several cast members who didn't want to relocate to Canada. One, Calista Flockhart's Cat Grant, had her absence explained, but Lucy Lane (played by Jenna Dewan Tatum), who up to the end of season 1 had been depicted as one of the people in charge of the DEO, vanished without trace or mention when the second season began.
Maxwell Lord also vanished without mention in season 2, though in this case it may have been because he'd seemingly accepted Supergirl as an ally and was implied to be working with the DEO; without him being an antagonist, there was little reason for him to stick around.
The writers managed to do this in the span of a single episode in season 5. It introduced Jesse Turner, a young boy explicitly identified as the Anti-Christ. This resulted from a union between a demon and a human, which somehow imbued him with high-level Reality Warper powers, an ability neither species displayed in any way. Possibly realizing how little sense it made that this would result in the most powerful character depicted in the show up to that point (with the possible exception of God) and the Story-Breaker Power it entailed, the writers immediately sent the character off to nowhere, and he's never mentioned afterwards. It's technically also Put on a Bus, but it goes straight past even Long Bus Trip because everyone immediately forgets he ever existed at all.
Never to be forgotten, of course, is the infamous Adam Milligan. Introduced in Season 4 as the Winchester's long-lost half-brother, Adam was only in a few episodes himself, but still managed to become a beloved character in that time. At the end of Season 5, he was dragged into The Pit by Sam, locking the two of them up for supposed eternity with the archangels possessing them. And then Sam got out. Sorta. Admittedly, Adam was mentioned a grad total of once following his dramatic and highly involuntary fall: when Death told Dean he could only rescue one soul from The Pit and made him choose between Adam's and Sam's. Adam Milligan has not been mentioned or heard from again, to the point where his being forgotten has actually become a fandom-wide meme. So, once again, never forget: Adam Milligan... Still In Hell.
This became an ascended meme in season 10, where Sam and Dean run into a school production of Supernatural, and Dean asks who the character dressed up as Adam is supposed to be... only for the director to tell him that he's Sam and Dean's brother, Adam, who's still trapped in hell. Sam and Dean can only share a "oh, right, forgot about that" look with each other.
As of Season 11 Adam has even gone missing from Lucifer's cage.
Kate Keys, Russell's (eventual ex-)wife and Jesse's mother, plays a major supporting role in "Beyond the Sky" and "Jacob and Jesse" and Jesse briefly speaks to her over the phone in "High Hopes". After that, however, she is never mentioned again. Given that Jesse went into hiding in 1962, it is not clear whether Kate even knew that he was still alive after that or whether she ever found out that she had a grandson, namely Charlie.
After the opening scene of "Charlie and Lisa", which is set in 1980, Becky is never seen again. The last reference to her is later in that episode in a scene set in 1992. Unlike her brother Tom, she does not return for the final episode "Taken" and there is no indication whether she is alive or dead in 2002.
Taxi, in which John Burns disappears after the first season without on-screen explanation (though he may have been fired for crashing the beloved Cab 804 beyond repair; Word of God is that he was just too boring a character.)
Teachers had Jenny, JP and Susan just evaporate without explanation in-between seasons 2 and 3, though Susan is mentioned at the beginning of season 3 so her existence wasn't totally erased. Their disappearance is made even more egregious as it occurs only a few episodes after the very dramatic departure of lead character Simon, downplaying the emotional goodbye between him and Susan.
Teen Wolf had Danny, who recurred for the first three seasons of the show and played an important role during several episodes. Despite being a fan favorite, he was cut from the show and never mentioned again right after revealing that he knew about werewolves.
In the fifth episode, Donna's sister Tina is introduced... only to never be seen again. Later in the series Donna is referred to as being an only child. Tina's disappearance is lampshaded at the end of a season two episode called "Vanstock." A narrator announces a bunch of character questions in a dramatic fashion, such as "Will Donna and Eric ever consummate their relationship?" The final question is "And whatever happened to Midge's other daughter, Tina? Find out next time on That 70s Show!" However, this is the last time Tina is ever mentioned.
Donna's older sister Valerie was mentioned as being at college, and then was never mentioned again. Considering That '70s Show gave many nods to Happy Days, Tina and Valerie may have been intentionally introduced just to have this happen.
The most prominent semi-example is Laurie, Eric's older sister. She was a recurring character in season one, and then a regular in season two and three. Her actress then left the show, and Laurie wasn't mentioned at all.(At least not by name, though Red mentioned having "kids"). Laurie came back (played by a different actress) for recurring appearances in season five and six before disappearing again, though she was mentioned in passing several times. When Kitty considers Donna part of the family at the end of the series, they lampshade on Laurie's disappearance, wondering where she is.
Randy Pearson is another example. He was a prominent character in the show's final season who served as a replacement for Eric. However due to the character's unpopularity, Randy's appearances became less frequent as the season progressed. He appears in only one scene in the series finale with just a single line before disappearing and never being seen or mentioned again.
Chrissy Snow. One of the main trio of Seasons 1-4 was dropped from the show during Season 5 after Suzanne Somers was unceremoniously fired following a nasty pay dispute. At the end of her contract, her character would appear in only one scene per episode talking on the phone. It was explained that she had moved back to Fresno to take care of her ailing mother. After her contract was up, Chrissy was never spoken of ever again in the remaining three seasons.
Stanley and Helen Roper had it a little bit better, but not much. After Norman Fell and Audra Lindley left the show in Season 3 to star in their spinoff The Ropers, they were "replaced" with Don Knotts as Mr. Furley; but after the spinoff was canceled after two seasons, they made one final guest appearance on Threes Company before being dropped for good, and never mentioned ever again.
Cindy Snow, Chrissy's cousin, played by Jenilee Harrison, also ran into this after she herself was dropped from the show after Season 6.
The neighbor Lana, played by Ann Wedgeworth, was a regular cast member... for all of four episodes, during Season 4. She was dropped from the show following poor reception of her character, and vanished without a trace.
Jeff and Steph Woodcock (Eddie Kaye Thomas and Kat Foster) were lead characters, on equal footing with Eddie and Joy Stark (Brad Garrett and Joely Fisher), and the whole basis of the show seemed to be about contrasting a newly-wed couple and a long-time married couple. After the first 2 seasons, however, they vanished without a trace, and Jeff's sidekick role was taken by Kenny, played by J.B. Smoove. This was further confused when unaired episodes from Season 2 aired in the middle of Season 3.
Kenny disappeared himself at the beginning of the fourth and final season. His place was taken by their daughter Ally and her fiancé/husband Doug. Like the season before the episodes were aired out of order and had some leftovers thrown in. Since Ally was recast three times during the show it was especially confusing.
This gets a lampshade when Doug remembers all of the above, plus a random guest arc by Gilbert Gottfried, late in the fourth season, due to him getting Medium Awareness as a form of mental illness. Don't worry, it Makes Just as Much Sense in Context. At any rate, none of the other characters have any idea who he's talking about when he brings them up.
Stephen, one of the original cast members, just disappeared from the series without explanation after the fourth season. According to some sources, Executive Meddling was the reason for his departure, and writer Roger Price didn't feel like writing the character out... so Stephen is gone from the series without any sort of explanation or acknowledgement that he ever existed. Very jarring, considering he was one of the first people we were introduced to, and was one of the two longest-serving cast members up to that point.
Tyso disappeared at the same time as Stephen (also with no explanation). However, during the fourth season Tyso had been Demoted to Extra due to confusion over whether he'd be returning to the series.
In The Tomorrow People (2013), Lisa disappeared after the first season and Kevin vanished after the second, with no mention made of either of them by anyone.
Torchwood had Detective Kathy Swanson, whom the team reach out to when locked in their Elaborate Underground Base. She disappears after the first series and is never mentioned again, even in episodes that involve the police or take place in the police station (although she does make it into the Tie In Novels.)
The Torkelsons was completely retooled as Almost Home for its second season, famously losing two of the family's five children in the process.
Trailer Park Boys: Treena Lahey (Ellen Page) appears in the second season. She and her mom, Barb were staying with Lahey over the summer. Barb became a Recurring Character, but Treena vanished after the season's end and was never referenced again.
The Tudors actually had a rampant problem with this, resulting in an extensive rewriting of history. Among the important characters who disappear without a trace (and often have their historical roles delegated to someone else) are the Duke of Norfolk, Sir Anthony Knivert, Archbishop Cranmer, Pope Paul III, and Sir Francis Bryan.
Ultraman: Science Patrol's Tagalong Kid Hoshino vanishes without further word after the series' 25th episode due to his actor having to leave the series after breaking his leg in a skiing accident. However, a few episodes where he was supposed to have a role were still remaining, so the writers had to create a few one-off child characters to fill in the position.
Ultraman Ace had this problem too. TAC's resident scientific adviser Kaji vanished without explanation after episode 31. Then Hokuto's friends Dan and Kayoko Umezu (both introduced after Minami was Put on a Bus) disappear after episode 44.
The Vampire Diaries has Jamie, who was taken in by Bonnie's mother and started a romantic relationship with her. Last time we saw him, he and Bonnie were asleep together when Esther compels her to complete Alaric's transition into a vampire hunter to destroy her children. He hasn't been seen since.
Before Jamie, there was Lucy, a distant cousin of Bonnie, who was last seen in season two, telling Bonnie she'd see her again, and implying it be soon.
Harrison's girlfriend appears in the pilot and expresses doubts over why he's bothering to investigate an urban legend (the radiated Martians). After he kisses her and heads out on assignment, she's never seen again, and Harrison never mentions her any time afterwards.
Mrs. Pennyworth, whose estate Harrison, Ironhorse and the others stay at through the first season. Although her partner, Tom Kensington, dies in the episode "Among the Philistines", she is still alive at the end of the episode. She disappears without explanation between the two seasons, and isn't present when the estate is breached and destroyed in the second-season premiere.
Katara, an android from the planet Synth, who helps the Blackwood team repel a large Mortaxian force, heals their critical injuries and tells them she is leaving the planet to get help from her own people. She never appears in the series again.
Recurring antagonist/Mortax alien Quinn (played by John Colicos) is last seen in an episode of the first season, "My Soul to Keep", attempting to gain more information on the Blackwood Project from Suzanne's ex-husband, a journalist. After this scheme fails, he simply disappears, and isn't present in any following episodes or the second season. Interestingly, Colicos was originally tapped to play the lead villain for the second season, but talks fell through and he was replaced by Denis Forest (playing a different character than the vengeful widower he portrayed in a first-season episode).
Early Waterloo Road character Lewis Seddon disappeared immediately following a major storyline halfway through series three. He is never mentioned again, despite his girlfriend remaining on the show and the school kitchens, where he worked, still regularly being shown on the show.
In Welcome Back, Kotter, Gabe's wife was pregnant, but then it was suddenly forgotten; apparently they were trying to write the actress's pregnancy into the script, and it became a running gag for some time — until the actress had a miscarriage. A year later, however, they repeated the same gag, and she had twins.
The West Wing had publicity relations manager Mandy, who disappeared after the first season. According to Rob Lowe, the writers referred to any character who had disappeared and not been used when they seemed they'd be more important as having 'gone to Mandyville.' Not only did she disappear between season one and two (despite the opener of season two following directly on from the end of the first) but she does not appear in any flashbacks to Bartlett's initial campaign, despite having been established as both working on it and being romantically involved with Josh at the time.
A similar fate was shared by Elsie Snuffin, stepsister and close associate of Will Bailey, who appeared with him in 8 episodes of Season 4 but then vanished thereafter. Will never even mentioned her again.
Add to the list Joe Quincy, played by Matthew Perry, a Republican lawyer who is a major character in seasons 4-5 before evaporating.
They did it again with Agent Garrett Fowler. He was a major villain in Season 1 and the first half of Season 2. He and Neal face off in the mid-season finale, he gives them all the information he has, Peter brings him back to the Bureau, and... nothing. Absolutely nothing. We never find out what happened to him. He gets a passing reference at the beginning of Season 3, but it's only a mention of his and Neal's confrontation. Word of God doesn't even seem to know. When asked, Jeff Eastin replied, "Peter killed him and buried him in the backyard." Needless to say, this inspired a lot of fan fiction...
Wings brought in Brian Haley to play Budd Bronski, the replacement character for Thomas Hayden Church's Lowell, after Church left. However, Budd's personality was neither as memorable nor as well-defined as Lowell's had been, so after a few appearances, he disappeared from the series without explanation, and the writers decided to build up the show's other supporting characters (chiefly Antonio and Casey) instead.
The Wire: The actor who played one-time character Commissioner Frazier died so his position is suddenly open and eventually taken by Burrell without explaining what happened to his predecessor.
Neither Alex's rival Gigi nor her Girl Posse are seen or mentioned again after season two.
Also, Dragon. Last time he was mentioned was in an episode where Max says he told his girlfriend everything except the dog-dragon, as he didn't know WHAT happened with that.
Also Brad, who said he'd find out why the Russos were so weird then was never mentioned again.
In the television adaptation of Wolf Hall, Mary Boleyn vanishes after episode three with no explanation despite having been an important source and confidant for Cromwell. In the books (and Real Life) she was banished from Henry's Deadly Decadent Court because she married a man she loved from a lower social station. Although scenes were filmed, they were ultimately cut.
Scully's invisible brother, Charlie. He is seen once in a flashback to when they were children, mentioned perhaps twice, and then never again. And though we see Scully's other siblings, older sister Melissa and older brother Bill who have a moderate impact on the plot, Charlie is never seen as an adult in the show's nine year run. (At least not definitively: He may be one of the silent mourners at their father's funeral.)
There's also Senator Matheson, who's set up in "Little Green Men" as a replacement for Deep Throat. He appears only twice, and is mentioned a few times, in Seasons 2-3 before disappearing. He reappeared for a single episode in Season Six then vanished completely.