An odd one in Aliens. Ripley confronts Burke about the fact that he ordered the investigation of the alien derelict ship. This should be a major plot point, as it essentially says that Burke knew that the aliens were out there and deliberately set the colonists to investigate so they could be infected, which should make him the villain of the movie and responsible for all of the evil that's happened. However, Burke offers an excuse for it that makes him come off as careless rather than deliberately villainous, and it's never mentioned again, even though it should be a major bit of information to bring up to the Marines. This is compounded because the investigation scene in question was cut from the theatrical release, so it's never explained when the colonists first encountered the aliens, and it's assumed that the aliens just coincidentally decided to show up not long after Ripley was rescued. As a result, the conversation is a bit confusing in regards to what Ripley is talking about. The Special Edition restored the missing scenes, removing the source of confusion.
Pretty much everything left on a cliffhanger in Prometheus was completely abandoned in Alien: Covenant when David bombed the Engineers into extinction. Shaw's quest to find them and the answers as to why they created and forsake humanity is abruptly ended by her dying offscreen and becoming another test subject for David. The sole surviving alien born from the engineer on LV-223 is stranded there with no way to escape and will eventually die. In short, aside from David, almost everything that happened in Prometheus wounded up not mattering.
In three scenes, we see the Deltas stealing the exam answers (unbeknownst to them, the wrong ones, planted by the Omegas), the Deltas taking the exam as they wink to each other and Marmalard smirks to himself and then Boon saying that he heard from "the Jewish house" that the answers they had were the wrong ones. They seem to be building up to a payoff where the Deltas learn that they did in fact have the wrong answers and that's why their collective GPA has crashed, which would give Wormer what he needs to revoke their charter. While this plausibly happened it only seems to be implied by later scenes in the movie.
The bust of John F. Kennedy's head that would have been damaged in a way that mirrors the entry and exit wounds of the last bullet to hit him during his assassinationnote until Landis felt that gag was too far to go even for this movie is also visible briefly in one scene as the Omegas prepare for the homecoming parade.
Hannigan smugly informs Annie that she's arranged for her to be moved to another home very early in the film after she messes with her in front of the social worker. This is never mentioned again, because before it can happen, enter Stacks.
Annie catches Stacks without his hairpiece, but other than a one-off joke with Grace about she "shouldn't bank on the hair", it's completely dropped.
Despite being in the title, the killer barracudas in Barracuda are completely forgotten when the conspiracy behind their behavior is uncovered halfway through the film, and the rest of the film is spent unraveling it.
In the Syfy original movieCamel Spiders, one of the two major stories involves a group of four college kids, two boys and two girls, trying to survive the spiders, sharing screentime with another group of survivors. A little more than halfway through the movie, though, after the two boys die, the movie completely forgets about the two girls who were still alive.
Charlie and the Chocolate Factory: Early in the tour, Violet and Veruca agree to be best friends. This goes absolutely nowhere and their alleged friendship is never mentioned or shown on-screen again.
Child's Play 2: Chucky was originally going to reincarnate right away, as an epilogue present in both the TV cut and the film novelization shows pieces of Chucky (most notably one of his eyes) falling into the machines making a new Good Guy doll. This epilogue was scrapped, as the sequel reveals Chucky ultimately had to wait eight years before getting a new body.
Cheaper by the Dozen (2003) had a few scenes where Tom Welling's character has problems with a bully. This is never resolved.
In the 2004 Zack Snyder-directed remake of Dawn of the Dead, after establishing that the zombie infection is spread through contaminated blood, a "very big deal" is made of one character washing off infected blood in a water feature in the mall. Later, a similar "very big deal" is made of one character falling into this pool of supposedly infected water, cutting open his arm on the way in. Both shots are done with Snyder's trademark "this is important" slo-mo, but the incident is never discussed nor followed up, and the character in question does not suffer from zombie infection.
In Django Unchained, Zoë Bell plays a mysterious tracker who always wears a scarf over the lower half of her face. She gets two significant close-ups, including a lingering scene where she's shown looking at an old photo of two children. What, if anything, this is meant to indicate is never resolved, since she soon ends up being killed off without contributing much to the plot. Bell has confirmed that the film was originally going to go more into her backstory and explain exactly what the photo scene meant, but this subplot was cut for time. Some have theorized that she was going to be revealed to be Django's former childhood friend.
Eighth Grade appears to be taking a darker turn when the protagonist, Kayla, attracts the attention of her crush, Aiden, by telling him she has naked selfies on her phone that she plans to give to the next guy who becomes her boyfriend; but after he asks her whether she does blowjobs she finds she cannot even practice on a banana, and the plot arc disappears from the story.
It's explained that they made Sean Archer sound like Castor Troy (and vice-versa) by implanting a small microchip in his larynx that will cause his voice to sound different. The doctor who performed the surgery takes great pains to explain that the chip is very fragile and easily dislodged, mentioning that even a strong enough cough could cause it to fail. This little tidbit never factors into the plot, not at all.
When Archer and Troy are fighting in the Archer's front yard, both are yelling to Archer's daughter that he is her father. Due to a throat punch, Archer (in Troy's body), suddenly has John Travolta's voice, and tells Jamie to listen to it. This is, however, gone by the next scene, and Archer in Troy's body sounds like Nicolas Cage for the rest of the film.
Shocker classic Freaks had a pair of reoccurring acrobat characters who kept setting up their great act as something that was going to be amazing, but we never get to see it. In the available cut of the film, their bragging feels more like padding to show off the eponymous stars' unusual traits and features.
In The Final, when the outcasts are setting up their torture chamber, they mention how they are rigging the place with webcams in order to send a message to high school students all across the country. This is never brought up again.
At the end of The Ghost of Frankenstein the Monster is given Ygor's (late Dr. Frankenstein's assistant, played by Bela Lugosi) brain, enabling the Monster to speak once again. This portrayal was supposed to be continued in Frankenstein Meets the Wolf Man, but the negative reaction from the test audiences made the executives to cut out all the Monster's dialogue and returning him to the Man Childish brute for the rest of Universal Horror movies featuring the character.
Halloween 4: The Return of Michael Myers has Michael's young niece Jamie Lloyd seemingly inherit whatever evil drove him to kill, as at the end of the film she stabs her stepmother with a pair of scissors in a manner almost identical to how Michael killed his sister Judith in the original film. The prospect of Jamie replacing Michael as the main villain of the series was abandoned by producer Moustapha Akkad, much to the disappointment of Danielle Harris (Jamie) and Donald Pleasence (Doctor Loomis). In the fifth film Jamie is back to normal and is retconned into merely wounding her stepmother in the previous film, instead of killing her.
Hellboy (2004) builds up Sammael as an unbeatable enemy due to his ability to duplicate himself every time he is killed. If you don't kill him then he lays eggs all over the place, which hatch into even more clones. By the end of the movie there are at least dozens of Sammael-clones and more hatching — so, how do they deal with him? Well, a whole raft of other plot points had come up, including the Big Bad and his chum, so they just set all the ones they could find on fire and called it a day. We already know from earlier in the film that Kill It with Fire doesn't stop him duplicating, and they only bother looking for clones in one room of a very large underground complex halfway around the world from his last hangout. Once they leave the room, Sammael is never so much as mentioned for the remainder of the film. It does however appear in The Stinger, so it wasn't forgotten.
In I Am Legend, Robert Neville lays a trap that captures a female dark seeker. Shortly after, a male dark seeker goes to look, even briefly exposing himself to sunlight. Neville theorizes that the dark seekers have started to lose their remaining higher brain functions, and with them some of their basic survival instincts. However, the next day Neville is caught in a trap very similar to the one he set, hinting that the dark seekers may be more intelligent than he thinks. In the original ending, the dark seekers come to rescue the female dark seeker and spare Neville's life; due to bad test audience reactions and Executive Meddling looking for a Sequel Hook, the ending was changed and the implication ignored.
The opening scene of Johnny Mnemonic establishes that the protagonist needs to do One Last Job in order to have enough money to afford an expensive "procedure" that he can have done to restore lost childhood memories. This character motivation sets the rest of the plot in motion, but the importance of needing money for the procedure and needing the procedure itself to restore the protagonist's lost memories is abandoned as soon as the scene ends.
In Jurassic Park, the subplot of the sick Triceratops doesn't come to anything (beyond getting Dr. Sattler off the tour) or get mentioned again, while in the book it was revealed the dinosaur got sick because it was swallowing gizzard stones to help with digestion and accidentally ate some poisonous berries with them. The movie leaves out this explanation (in fact it's explicitly stated to be wrong), so it's never made clear why the Triceratops is sick.
In Parking (1985), subplots like Orpheus's love for Calais and Persephone's contract with Orpheus don't really go anywhere once Eurydice dies, as he expends every effort to save her and dies shortly after failing.
Dead Man's Chest has Weatherby being captured by Beckett and forced to work for Beckett and use his power for the East India Trading Company's own ends for the sake of keeping keeping Elizabeth safe. This subplot amounts in the following movie to having Weatherby as a background character for the first act before Beckett and Mercer have a 15-seconds conversation about Weatherby asking too much questions and deciding to kill him...off-screen. This subplot did have a more fitting conclusion in the deleted scenes where Weatherby discovers about the sinking of the Black Pearl and Elizabeth's possible demise and tries to kill Davy Jones which leads to Beckett ordering his death, but even then Beckett alliance with Weatherby doesn't amount to much in either version.
Like the second movie, On Stranger Tides begins with Jack losing his iconic hat once things start going south but unlike the second movie he doesn't recovered by the end of the movie.
On Stranger Tides ended with three major Cliff Hangers: The first is Angelica being trapped in an island with Jack Sparrow's voodoo doll. The second is Philip being severely wounded, and Syrena spiriting him away underwater. And the third one is Jack and Gibbs having Blackbeard's ships in a bottle (including the Black Pearl) and planning a way to release them. Only the latter one is resolved in the next movie (and even then only the Black Pearl is released and the other ships are neither shown or mentioned). Philip, Syrena and Angelica are neither mentioned nor appear in the next movie. A number of other new side characters are also abandoned after this film.
In Romance on the High Seas, Oscar Levant's character is continually seen working on a jazz composition called either "Cuban Rhapsody" or "Brazilian Rhapsody." After the characters finally disembark in Brazil, there is a short ballroom dance which looks like a teaser for an epic Busby Berkeley Number (choreographed by Berkeley himself). Instead, Doris Day's character reprises "It's Magic," thereby wrapping up the movie at a much lower energy level and undoubtedly much reduced budget.
The Room (2003) uses this trope at least three times: In one subplot, Denny has a brief run-in with a drug dealer (a Voodoo Shark to explain the presence of the gun at the end). In another, Michelle's boyfriend Mike is shamed by Lisa and Claudette walking in on him with Michelle in Johnny and Lisa's living room; and another one - and here's the kicker - has Claudette telling Lisa that she has breast cancer, something that nobody else mentions. Not even Claudette herself. All of these subplots are introduced and immediately forgotten. In an AMA on Reddit, Mark's actor stated that Claudette's actress asked Tommy Wiseau several times whether the breast cancer would come up again, only to be told, "It's a twist". Eventually she dropped the subject and moved on.
Star Trek (2009) and Star Trek Into Darkness had a plotline going on about the increasing militarization of Starfleet (thanks to Nero's incursion and Klingon border skirmishes) and a looming conflict with the Klingon Empire. Star Trek Into Darkness even had Starfleet wearing Nazi-ish uniforms and was full of The War on Terror parallels. In Star Trek Beyond, we get a bright, colourful standalone adventure where Scotty explicitly says "Starfleet is not a military organization," the opening scene is about diplomacy, and the main plot is a big-budget version of TOS's many "stranded on an unfamiliar planet" episodes. Furthermore, the technological advances from the last two movies (transwarp beaming, using augment blood to cure death) have been forgotten.
Rogue One introduces Jyn Erso and the Force-sensitive Kyber crystal amulet given to her by her mother, who tells her to "trust in the Force". Kyber crystals power lightsabers and Death Star weapons, and there usually is a link between a Force-sensitive character and their personal crystal. This one, however, never has any impact on the story whatsoever (other than Chirrut starting contact with Jyn due to his sensing of it); originally Jyn's mother was supposed to be a Jedi apprentice in hiding (hence why she wears Jedi-like robes in her scenes). It gets destroyed along with Jyn and the other characters. The end.
Some plot points seem to have been dropped between The Force Awakens and The Last Jedi. The Last Jedi goes into more depth exploring why Kylo Ren went rogue, but never once are the Knights of Ren mentioned. Finally Snoke alludes to finishing Kylo's training at the end of The Force Awakens, which would make sense as he has just been defeated by Rey and would need to grow stronger. However, in The Last Jedi this only amounts to finding Luke Skywalker rather then any actual training and in the end Snoke ends up dead.
A lot of run time in The Last Jedi is spent building up a relationship between Rose and Finn, and at the end of the film, Rose declares her love for Finn. In The Rise of Skywalker, Rose is Demoted to Extra. Other than Finn watching over Rose while she recuperates, the pair barely interact, and their relationship is never addressed.
This Day has a near-literal example. At the end of the previous film, Laura had discovered she was pregnant by Massimo and hadn't yet told him. This movie immediately drops that plotline, revealing that Laura had a miscarriage due to the attempted assassination (and it never really explains how that played out either). It causes a little bit of angst for Laura and there's her internal conflict over whether or not to tell Massimo, but ultimately the pregnancy plotline has almost no effect on the story.
Underworld: A big part of the fourth movie's story showed that humanity found out about the existence of vampires and lycans and started a extermination war against them. Come the sequel which is set a few years after the previous one and not even a mention of this war as if it never happened in the first place, with humanity going on their way unaware that the two races are back and resuming their old secret feud like they did in the previous movie.
The X-Files: I Want to Believe features a controversial paedophilic priest with "psychic" powers around which most of the publicity hinged. However, about halfway through the film goes off at a tangent about a different character, the only reference to Father Joe being his death announcement at the end.
In the beginning of Zombieland: Double Tap, some new zombies are introduced: The Homer which is so stupid it's zero threat whatsoever to the survivors, The Ninja which is very furtive, and The Hawking which is a zombie shown to be intelligent enough to comprehend what a retinal scanner is and use a corpse's eyeball to open it. Ultimately though, this is a concept that basically doesn't go anywhere: The Homer appears only once more as a Brick Joke in the very end, and the others are not brought back as eventually a fourth type of zombie, the T-800, that is veryhard to kill, is the one that takes the forefront.
Claudette's actress asked Tommy Wiseau several times whether the breast cancer would come up again, only to be told, "It's a twist". Eventually she dropped the subject and moved on.