What Happened to the Mouse?/Newspaper Comics

Comic Books

 * In the Tintin story "Red Rackham's Treasure", it is mentioned early on that the villain of the previous story, Max Bird, has escaped from prison and may be hiding aboard the ship. In the end, this plot point seemingly just serves as an excuse to get Thomson and Thompson into the story, since there is nothing to indicate that Max was aboard the ship, and this is Hand Waved near the end when the Thom(p)sons claim he was "discouraged by their presence". Max never appears in any later stories, either.
 * In The Blue Lotus, it is mentioned that the evil fakir from the previous story had escaped from prison. This was never mentioned again. Hergé wrote these comics as serials, which took months to finish, so it could well be that he set this up and forgot it.
 * The Fakir subplot IS resolved in the original version published in serial form. Tintin refuses to leave India until the matter is resolved, and they later receive a message that the fakir has been captured. Not much, but still... The fakir's escape is mentioned to explain who just fired the poisoned dart that made the emissary from Shanghai go mad in the installment before, i. e. in order to solve what otherwise would have been a mystery that required Tintin staying in India to solve.
 * In the 1970s, Doctor Strange was going through a series of writers all within less than two years. During one of the early writers' runs on the book, Doctor Strange had a house guest staying at his manor, and he had gone insane and fled the house. This was never followed up on, because the writers that followed were usually fired or left the book before they could pick up on the dropped plot line.
 * Things like this used to happen constantly in Marvel titles. In 1993 X-Men titles alone, there was Scott Summers' long-lost second brother (mentioned but never found, and soon forgotten by everybody) and Wolverine's recurring vision of a tree with radiators growing out of it (never explained).
 * The "3rd Summers brother" plot was finally resolved 13 years later in "X-Men: Deadly Genesis" which revealed a new character Vulcan to be born in Shi'ar-member of forgotten second X-Men team-Ax Crazy Gabriel Summers. Long before that there was a common belief that it was Adam X-Treme, son of Shi'ar emperor D'Ken. So it's more a Brick Joke.
 * Technically, it's still implied that Adam X-Treme is in some way a Summers half-brother, as he's half-human.
 * Apocalypse almost ended up being made into the third Summers sibling but thankfully editorial squashed the plotline. On the other side, as part of the "X-Men: The End" trilogy, Chris Claremont made Gambit the third sibling in said non-canon storyline, but with the twist being made that Gambit was a test tube baby and that Sinister mixed in his own DNA as part of his plot to use Gambit as a host body to kill Apocalypse. Claremont's storyline itself was largely seen as a "Take That" against Wizard, which heavily pressed in the pages of its magazine, for Gambit to be revealed as the third brother.
 * In a 1989 issue of Uncanny X-Men, Chris Claremont introduced a group of Badass Normal ex-SHIELD agents-turned-mercenaries called the "Harriers" led by a man named Hardcase, an old friend of Wolverine's. They were made to look like a potential group of recurring characters even going so far as having a "created by" credit caption in the splash page with Claremont and penciler Marc Silvestri. So far, though they have only appeared in that one issue.
 * Numerous X-Men titles introduce characters who are prominent for a while then generally disappear. For example, X-treme X-Men introduces Lifeguard and Slipstream, who are important members of the team until Slipstream goes nuts and runs away. Lifeguard leaves with Thunderbird to find him, putting them on the proverbial bus. Thunderbird and Lifeguard resurfaced in the early 00s "Excalibur" series; Slipstream didn't.
 * In Planetary, an early issue has the team invading a secret installation where scientists are attempting to create a fictional Earth and give it substance. They succeed, but someone escapes from the fictional Earth and goes on a killing spree. The issue ends with a caption telling us that he is still at large. He is never seen or mentioned again.
 * Except for a quick mention in the final issue, where Elijah basically says they've given up on looking for him.
 * In Mark Waid's mid-90s reboot of Legion of Super-Heroes, the first arc's Big Bads were a group of psychopathic, xenophobic Daxamites. (Daxamites are related to Kryptonians, each one roughly as powerful as Superman.) To underscore how powerful and awful these villains were, a dozen of them destroyed a planet in minutes by flying over it and blasting it with their heat vision. Those are the key facts: very evil, very powerful, and there are (at least) twelve of them. The next issue four of them attack Earth and it's an amazing fight, with Crowning Moments of Awesome and Heel Face Turns and heroic sacrifices and deaths, and at the very last moment the four Daxamites get defeated, the Legion proves its worth to the skeptical authorities and the universe rejoices. And the other eight Daxamites? Each with the power to destroy a continent? Responsible for the deaths of billions? They never, ever, ever get mentioned again.
 * Star Wars: Tales of the Jedi included a subplot in which the Dark Lord of the Sith Exar Kun poses as a Jedi Master and brings a group of Jedi over to his side, infects them with shards of a broken Sith holocron and sends them out to assassinate their own masters. After the montage of successful attacks on Jedi Masters, none of these dark Jedi reappear and their fate is left unmentioned at the end of the story.
 * Near the end of his surreal run on Animal Man Grant Morrison created "Comicbook Limbo" for this sort of character. Not dead. Not in prison. Not retconned out of existence. Just gone.
 * In the first issue of the Archie Comics: The Married Life, Cheryl Blossom  and has not been seen or mentioned since despite.
 * "The pious Helene" by Wilhelm Busch. First her husband and shortly afterwards her cousin/lover (who's the real father of her twins) die, she goes mad, becomes an alcoholic, and dies too. It's never mentioned what happens to the twins.
 * In Captain America (comics)'s "Midnight In Greymoor Castle!" story, the final World War II period story in Tales of Suspense in the 1960s, had Rogers leave his unit in the field to help Bucky who has been captured and held in a castle in Britain. Although Cap later saves his besieged unit by smashing their attackers with a V-2 missile, the fact remains that Rogers is suspected of desertion and thus in serious trouble. In the next story, the series returns to the present day with Cap telling that story and the Scarlet Witch asking if he got in trouble and Cap simply explains that US military intelligence covered for him.

Other Comics

 * A mid-story issue of the Super Mario Adventures comic strip, which ran in Nintendo Power during 1992, featured this. Toad uses a Cape Feather to fly up to a pipe sticking out of a cloud (allegedly the one Mario and Luigi entered at the beginning of the story to unknowingly wind up in Dinosaur Land), and gets "help" -- which is actually Bowser's Koopa Troop in disguise (the cloud was actually a disguised airship). After the Princess gets kidnapped, Toad is shown being held hostage by two Koopas, tells her that the Koopas have taken over the Mushroom Kingdom, and is then never seen or mentioned again for the remainder of the comic.