Forgotten Phlebotinum

""The issue in an ongoing series is once you've done it [used Time Travel] and it wasn't a fluke, it's like you've shown that one of your characters got Superman powers. And then in the next episode when a building is about to fall over on someone, Superman's running around in circles saying "Oh no what do we do? Frig frig frig" and the audience is sitting there, furrowing their brows, one hand on their chin.""

- the Alt Text for Dinosaur Comics, 9/18/08

A situation, most common in Speculative Fiction, where an amazingly useful power or device is revealed in one episode, and would be amazingly useful in later episodes, if it weren't for the fact that nobody seems to remember it. Sometimes the power or device is remembered under circumstances where it proves mostly useless, but not remembered when it would do any good.

This is for powers or devices that are forgotten in general. Something which the character does use a lot and only is forgotten this one time is an example of Forgot About His Powers.

If the device is remembered, but there's some contrived excuse as to why it isn't available or won't work, that's Holding Back the Phlebotinum or It Only Works Once. If it only existed as a single instance of a device or program that was utterly destroyed with no hope of recovery or reconstruction, that's No Plans, No Prototype, No Backup.

Not to be confused with We Have Forgotten the Phlebotinum. If they (finally!) remember to use it in the end, it's a Forgotten Superweapon.

This trope does not necessarily denote bad writing. It can be (as noted in the Order of the Stick example) convenient writing instead. If the protagonists have some piece of phlebotinum that makes them invincible or at least very hard to so much as injure that is both reliable and accessible, vast numbers of plots have to be thrown out the window. Some would call this unwillingness to change the Status Quo and then adapt to the new order of things "lazy", but when one is working on a regular series, changing the status quo (interesting though it can be dramatically) is not something to be done lightly. When it's a Shared Universe this is even more pronounced. In such cases, "Forgotten" Phlebotinum is a subtrope of Real Life Writes the Plot.

Larry Niven is extremely critical of this trope, and coined Niven's Law, which states that once a technology or discovery has been introduced into a fictional setting, it must continue to exist in all chronologically later stories in that setting. The secret may be lost for a variety of reasons—society enters a dark age, the discoverer deliberately covers it up, or there really were No Plans, No Prototype, No Backup—but Niven would maintain that this smacks of lazy writing and is best avoided. At the very least, the precedent that such a machine is physically possible in the setting must be maintained—which makes it likely that older, Higher-Tech Species will possess it even if it never became prevalent in the protagonists' society.

When it's not forgotten and is used in a later episode because a writer wants to acknowledge continuity, it is Chekhov's Boomerang.

Anime and Manga

 * Mazinger Z: Often it was played straight. Many times Dr. Hell came up with a Mechanical Beast equiped with a weapon put Kouji or Mazinger-Z in a serious disadvantage: Gromazen R9 shot an acid could melt Aphrodite A's armor (that was made of Japanium, although it was less tough than Mazinger Z's), Kingdan X10 projected mirages, Holzon V3 set eathquakes off, Jinray S1 flew at Match 5, Aeros B2 could absorb Mazinger's attacks and hurling them back, Desma A1 caused hallucinations, Gumbina M5 was nearly invulnerable... and they were not used again. However, sometimes Dr. Hell reused and improved some strategies or weapons, or deceived the enemy in believing he was using the same trick.
 * In Cowboy Bebop, the crew seems to conveniently forget that they have a nigh-omnipotent hacker on board who could take over other ships at will in her initial appearance but never, ever does that again, even though that would not exactly be the least effective way to catch their bounties. It was particularly bad in the episode where the villains are immobilizing ships through a computer virus. Gee, if only they had someone on board who could counter that...
 * In the first episode of Sailor Moon, Sailor Moon's hairclip things (on her odango/buns) can magically allow her to hear people in distress. This comes in handy, as she hears her best friend Naru being attacked by the Monster of the Week and goes to save her. This power is never shown again in later episodes, even though it would probably have come in handy. Similarly, in the first chapter of the manga, the costume included a mask in which she could see the monster attacking Naru by looking in the goggles. The goggles were quickly phased out in the manga, last seen in one transformation sequence where Usagi discarded them as she transformed, though why this happened was never explained. Considering that the monsters of the week/chapter were usually close by anyway, this power wasn't really that necessary in retrospect...
 * Another forgotten ability was Sailor Moon's disguise pen. It was commonly used early in the series, but forgotten during R and appeared only once later off screen, to explain why Venus was disguised as Moon. This may have been due to the lack of need for Usagi to actually use her disguises in later storylines as she gathered a team of fellow heroes and many of these disguises often appeared superfluous to the plot anyway.
 * Sailor Moon used a special attack, "Moon Tiara Stardust", in episode 5, to heal a group of transformed humans. She never used this again, despite it possibly being useful in many storylines, though she eventually acquired the Moon Stick which had the same abilities. This is likely because the former attack never showed up in the manga, while the latter item did.
 * In early Yu Yu Hakusho episodes Hiei has telepathic abilities and has the ability to transform into a more powerful demon form. These are never shown in battle again.
 * The demon form was seen again in the second movie, but it seemed less powerful than Hiei with the Black Dragon Wave. The telepathic abilities such as hypnotism still appeared in the show and manga near the ending.
 * Kurama also has a number of really nifty tricks that show up only once, when they would have been incredibly useful at other times (such as during Round 3 of the Tournament), like the smokescreen and the Petals and Thorns attack.
 * In Kurama's case, though, many of the techniques he can use are dependent on which plant seeds he has on his person at the time. Since we, the viewers, have no way of knowing his current inventory, how would we be able to know what he could and couldn't do at any given time?
 * Kuwabara also showed in his first fight in a while that through training he's gained the ability to create a second Spirit Sword, manipulate his sword to extend and bend to hit opponents from long distance, and during his fight against Elder Toguro, create a colossal tennis racket-hammer thing out of the same energy, all of these abilities are never used outside the fight they were introduced in.
 * One of Doraemon's early gadgets of the week (chapter 54, "Lies Become Truths") was a beak-like toy which one could wear, and anything uttered while using it will be spontaneously proven as fact. Nobita lied that his father can shatter a huge rock with his bare hand, and then he can do it with ease. Quite frankly this should have make any other gadget Doraemon had introduced, or will ever introduce, completely and utterly obsolete. It was never mentioned again ever since. Particularly frustrating in Doraemon feature films and volume-length comics, which featured life-threatening situations.
 * This is not uncommon in Doraemon. There are several predicaments that Doraemon and co. face that one of his gadgets that have been mentioned in previous episodes could have easily get them out but for some reason Doraemon seems to have to use the gadget that was introduced in episode they were in.
 * Tekkaman Blade gives us the Hi-Coat Voltekka, an upgrade to Blade's Voltekka. It's used once in it's introduction episode to defeat Evil and then again three episodes to shoot down a nuke. Then the time skip happens and it's not seen again.
 * Tengen Toppa Gurren Lagann: The Gurren Lagann is equipped with a powerful Deflector Shield, which stops Combat Pragmatist enemies from attacking it in the middle of its formation, as well as stopping any powerful attacks thrown at it. It only appears in episode 3 and is promptly forgotten for the rest of the series, where it could have been very useful. (It reappears in the Compilation Movie, however.)
 * In Digimon Adventure, there were several instances in which Gomamon's Marching Fishes technique could have come in handy, especially since he was seen in the first episode carrying the entire group down a river on the fishes.

Comic Books

 * Silver Age Superman's lead-glass suit. It's flexible, bulletproof, doesn't cover up the "S", and is impervious to kryptonite radiation. It shows up in one issue.
 * Superman: The Animated Series uses it, but makes it even more useful by turning it into an effective (if short-term) spacesuit (Superman can survive in a vacuum, but can't breathe in one, in the animated series). It is, however, noticeably more fragile than Superman himself, so while he uses it often, he's not reliant on it.
 * The suit reappeared for a story arc of Batman/Superman somewhat recently.
 * The Silver Age had tons of Forgotten Phlebotinum. For example, there is Action Comics #252, an issue otherwise better known for being Supergirl's first appearance. In the lead-in story, though, Superman is being menaced by kryptonite, and he escapes by melting it with his heat vision, at which point he learns the liquid kryptonite is no longer harmful. (In a real head against wall moment, he even says that it's because when items change their state, they lose other properties, like how ice, when it melts into water, stops being cold. Of course, Superman forgets that liquid kryptonite is harmless to him thereafter, and in fact, liquid and gaseous kryptonite are shown being harmful to him in later stories. So maybe he found the only chunk of kryptonite in the universe that would be harmless to him if it were liquid.
 * In those days, Superman also collected all manner of exotic gadgets in his Fortress of Solitude, in addition to all the Kryptonian gizmos in the Bottle City of Kandor. Generally, DC Comics's Superman continuity cop (and world's biggest Superman fanboy) E. Nelson Bridwell was the only writer who consistently remembered what a fantastic array of machines Superman had access to.
 * Shows up in one of the many Avengers stories (the relaunch with Kurt Busiek). Justice, sidelined with a broken leg, goes on an Archive Binge and realizes that the best way to defeat an Adamantium robot on a homicidal rampage is with Antarctic (type B) Vibranium, a metal that somehow destroys any other metal within range when it exposed to the air. Fortunately, the Avengers destroyed an AIM base with stocks of type B vibranium four or five issues previously; but in all the long history of Ultron's rampages, some of the finest minds (Stark, Pym et al) in the world never linked the "really tough metal" and "destroys metal on contact" dots together.
 * One serious cause of this in comic books is the variable access to technology between different books even where there shouldn't be any. Batman operates in a much lower-tech universe than the rest of The DCU, despite hanging out with Superman in the Justice League half the time. This becomes wall-banger material when you consider Barbara "Batgirl" Gordon, shot through the spine and confined to a wheelchair for the rest of her life. If this happened to Lois Lane, you know Superman would scour the galaxy for a cure and have her on her feet by sunset, and that's before considering the number of people on the planet who have healing powers! Call in a favor, Bats, you've earned it! But, if the characters in the DCU, or any other standard comic book universe, actually treated the technology and superpowers they encounter daily in a realistic fashion, half their problems would be solved before they turned into interesting stories.
 * In the case of Barbara, she explicitly refused to seek a cure from her various superpowered contacts, preferring not to benefit from medical technology not available to the everyday populace. Of course, there's no good reason that Krytonian/Martian/Thanagarian/Amazonian technology couldn't be made accessible to the public, making this a case of Reed Richards Is Useless.
 * X-Men: Rogue can't have a relationship with anyone due to her powers. Gambit's mutant powers used to allow him to touch her without an issue, which has since been forgotten. In addition, there have been numerous items that temporarily disabled mutant powers, many of which have been captured by the X-Men, these are never mentioned in relation to this issue.
 * In Spider-Man, Peter makes a gas mask for himself that is completely concealed by his Spider-Man suit; he uses it once and never again, even though he is regularly hit with gas attacks several times during the early run of the comic.
 * The loathed One More Day storyline. Aunt May is dying (well, she's only been in her mid-80's for a few decades now, but she was actually injured). Subverting this trope, Peter scours half the mainstay Marvel cast looking for someone that can heal her. Playing this trope straight, nobody can. There's very, VERY thinly implied instances where it's the fact that she's already so old and frail that conventional medicine can't heal her, but considering the fact that those who he approaches include the X-Men (who had no fewer than 3 people at the time whose powers could explicitly heal any wound), Reed Richards, and Doctor Strange - who is both the Sorcerer Supreme and a former neurosurgeon, there's no reason that SOMEONE couldn't have helped him before he ended up letting Mephistopheles wipe out the entire history of his marriage in exchange for Aunt May's life.

Fan Works

 * As pointed out in Yu-Gi-Oh!: The Abridged Series, Kaiba never used the cards given to him by Pegasus ever again, even in episodes set later, and even though they are able to defeat the Egyptian God Cards. Nor does anyone in the series ever mention either the Pyramid of Light or the Blue-Eyes Shining Dragon. Ever. (Granted, the movie wasn't part of the original storyline, neither anime or manga.)

Film

 * The "throwing 'S' shield" in Superman II. During a fight which occurs just before the climax, Supes rips off a copy of the 'S' emblem on his chest and uses it to temporarily incapacitate Non (one of the escaped Kryptonians). It's never explained how Clark does this, and he never uses it again in the following films. It sure would have come in handy against the evil Clark or Nuclear Man, even if it was a cheap-looking effect.
 * In Star Trek: First Contact, the invading Borg are able to create a "temporal vortex" to travel back in time to the 21st century. At the end of the movie, the Enterprise is able to easily recreate this effect to travel back to their own time. This method of time travel seems easier and much safer than the other established method of sligshotting around a star at warp 10, but it's never mentioned again.
 * At the end of Star Trek: Nemesis, all the transporters on the Enterprise fail after Picard is beamed over. Their only recourse is to have Data jump over and use a never-before-seen one-person mini-transporter badge to get Picard back and them die with the enemy ship himself. Everyone seemed to forget the shuttles have their own independent transporters.
 * Insurrection and Nemesis also subverted this with the Captain's Yacht, a large auxillary starship (attached to the underside of capital ships) that was designed for both The Next Generation and Voyager, but was never used in either series. Despite many situations where a craft like this could be useful (as it could carry more crew members, have a larger cargo area and generate tachyon bursts), the craft wasn't utilized until Insurrection (where the main cast go down in the yacht to deliver weapons to the Ba'ku) and Nemesis (where Picard arbitrarily decides to take it down to the planet where B4-4's parts are located).

Literature

 * In The Inheritance Cycle Eragon learns that Brom's ring contains a massive store of magical energy, enough to rip castles apart. He keeps outright, explicitly, forgetting that he has it. He's not supposed to be an Idiot Hero, is he?
 * To be fair to Eragon, he had no idea that there was anything special about the ring until Arya told him, and explicitly warned him not to use it unless he was really in an extreme, life-or-death situation, which really has not presented itself anywhere in Brisingr when the ring's powers were introduced..
 * The Grey Griffins books forget their phlebotinum all the frigging time. All the time. Other times they hold it back. Max can sense portals and enter them... wait, now he can't, except when he suddenly needs to warp into one much later. Max has a pet "spriggan" that he cares deeply about. Where'd it go, and how come neither Max nor the book cares? You get the idea. Contributes to the randomness of the plot.
 * A very subtle version of this happens in L. E. Modesitt's Spellsong Sorceress cycle. In the first chapters of the first book, a spell is cast that teleports the main character in from Earth. It's implied that although the lady casting this spell isn't a very strong sorceress, she can still send people to locations halfway across the continent with a bit of help. This use of magic is never mentioned again, despite the fact that it would be tremendously useful in a variety of circumstances.
 * Harry Potter: Sirius' two-way mirror, which is basically a magical walkie-talkie. In Harry's defense, he was never told exactly what it was, and swore never to use it for fear it would cause Sirius to come to Hogwarts and get arrested and/or killed. It still qualifies as forgotten phlebotinum, however, because even after Harry has gone through great risk to speak to Sirius through Umbridge's fire, it didn't occur to Sirius to tell him "Next time, use the mirror I gave you." which would have.
 * In Harry Potter/Harry Potter and the Deathly Hallows, Harry and his friends escape from the  mansion. At one point, Harry ends up with three wands in his hand, which he holds bundled together. When he attempts to Stupefy someone, his target is "lifted off his feet by the triple spell." However, every character is usually content to wield a single wand. No one habitually Spellotapes a few wands together for extra blasting power, no wandmaker designs multi-core wands - nothing of the sort.
 * Science Fiction author Larry Niven coined "Niven's Law," which states that once a technology is introduced into a setting, it must continue to be present in all later stories in that same setting.
 * Civilization-wide Forgotten Phlebotinum can be somewhat justified if there are No Plans, No Prototype, No Backup, or if the civilization enters a Dark Age when lots of stuff is forgotten, but Niven's Law is still a good rule of thumb. At any event, once the technology has been proven to work once, then the natural laws which permitted it to happen must remain consistent from then on, regardless of whether the tech is ever rediscovered.
 * In the Star Trek Novel Verse, the phase-cloak seems to go through this a lot. After its introduction (and successful use) in an episode of Star Trek: The Next Generation, it's largely ignored by the books (as well as later TV series). A short story in a Star Trek: New Frontier anthology eventually suggested the prototype was destroyed soon after the episode. By the time of Star Trek: The Genesis Wave, the Romulans are making use of the technology again, or something very much like it, but then it drops off a second time, and when Star Trek: Titan comes round no-one's using it. Finally, in the Star Trek: Typhon Pact series, we're explicitly told the Romulans have finally perfected it.
 * At least in The Next Generation, it was mentioned that the Federation had negotiated away its right to use cloaking technology in a treaty with the Romulans, making the Federations research into the phase-cloak illegal (i.e., a treaty violation that could lead to war with the Romulans). The episode showing the Romulans were working on it themselves showed that it was giving them trouble, and the illegal Federation project Riker had been a part of had not ended well either.
 * The Grav Lance in the Honor Harrington series is a key plot point in the first book, and is then never mentioned again. Considering how much other technology advances over the course of the books (about 20 years in universe), you'd think they could have worked out the glitches of a weapon that can one hit the shields of any size of ship, up to and including a superdreadnought.
 * A combination of a very short range, massive size, which requires a use that doesn't fit with the prevailing tactical ideology—and a political climate which would prevent time and money being spent on fixing those problems? Not really surprising it never reappears.
 * However, recent technological advances have solved most of these potential problems: the state-of-the-art in space combat has expanded to include drone warfare, the power problem is potentially solvable with recent advances in small, high-power-limited-endurance wer plants, and using stealthed drones carrying one-shot grav lances would entirely obviate the need to risk a ship at suicidally short range (and space warfare missile doctrine has already been oriented for years about the concept of getting a devastating one-shot beam weapon into short range on a missile, then detonating it -- the bomb-pumped laser warhead), so fans are starting to seriously ask questions again on the Baen forums about 'So, when do the grav lance torpedoes come out?' So far, Word of God has been resoundingly silent on the issue.
 * Also, its original sponsor (Vice Admiral Hemphill) is not only well back into favor, but has just teamed up with the series' other mad scientist of space warfare, Shannon Foraker, who has already displayed a willingness to reach back into R&D history for 'obsolete' and 'unworkable' designs and find new ways to use them to devastating effect. (Moriarty, anyone?)

Live-Action TV

 * Star Trek: The Original Series was notorious for this. When the Enterprise crew discovered kironide, a drug that gives people psychic powers (in "Plato's Stepchildren"), why wasn't this made a standard part of the medical kit, even if it is too dangerous to use all the time?
 * Then there was the subcutaneous transponder, which gave the ship the ability to lock onto and beam up the landing party if they were out of contact. Its actual purpose in the plot was to give Kirk and Spock a Cool Escape, rather than pull the whole trick-the-one-inept-guard bit again. Despite the number of times they were separated from their communicators, the thing was never seen before or since. You'd think it would be standard issue.
 * The Forgotten Phlebotinum was once the shuttlecrafts. The B story in "The Enemy Within" was a landing party trapped on a planet whose nighttime temperatures are Antarctica-ain't-got-nothing-on-it cold. The idea of sending down a shuttle never comes up. (Although, to be fair, the shuttlecraft hadn't been introduced yet.) The crew don't even bother to use the transporter to send the away team any survival gear, either.
 * They did actually try to send down heaters; Spock mentions that they duplicated just like the Captain and were useless. No explanation for why a coat or a blanket sent the same way would be adversely affected by the process, though.
 * The original series did this many other times with Scalosian water ("Wink of an Eye"), spores that can regenerate lost body parts, restore the human body to perfect health and give immunity to radiation ("This Side of Paradise") and Warp 11+ speed without strain ("By Any Other Name").
 * The movies introduce the Genesis device (a form of instant terraforming that may bring people back to life as a side effect), which is so much further advanced than anything the Federation possesses before or since that it might as well be magic. The planet it creates disintegrates within a couple weeks, but surely there would be a way to work the kinks out within the next century, and the research that went into it could at least be applied to other projects. But in The Next Generation and Deep Space Nine terraforming is a long and arduous process that yields modest results. Not to mention it would have made a handy-dandy anti-Borg weapon.
 * A Justified example of No Plans, No Prototype, No Backup. The research facility developing the Genesis Device deliberately destroyed all copies of their notes in an attempt to avoid capture when their facility was raided by Khan. Khan then killed most of the research team and used up the only existing Genesis prototype model in his attempt to kill Kirk. In the immediately following movie, the one surviving scientist capable of recreating the Genesis device is killed before he has a chance to write down the formula. And so, lost technology.
 * A major problem with Star Trek: The Animated Series being considered canon is that the enormously useful life support belts never appear in any later Trek works. The belt surrounded the wearer with a glowing forcefield within which breathable air was provided. The real reason, of course, was that it was cheaper to animate a glowing outline than it was to draw spacesuits on everyone.
 * This was justified in the FASA RPG by having them very vulnerable to damage - one good hit could deactivate them leading to "messy" results.
 * Star Trek: The Next Generation also indulged in this far too often. One example is the "dimensional transporter", that could transport things straight through even a Galaxy-class starship's shields (or any other shields) without trouble, but had a cumulative and lethal side effect on people who used it repeatedly. And while that's obviously a sane reason not to use it in normal service, it does nothing to explain why they didn't use it for, oh, one-way trips by inanimate objects straight through enemy starship shields... objects like armed anti-matter warheads, for example. (Or as a Plan B for when crew members are in danger on the planet but can't be beamed up due to an attack on the ship that requires them to keep their shields up or a Negative Space Wedgie that blocks the beam.)
 * Or the episode "Lonely Among Us" where the transporter brought the dead back to life! Although it's possible that this was only feasible in that one case, since the person's consciousness had been converted into energy by the being that had possessed him. Still, the episode seems to imply that they can always rematerialize a previously saved version of a crewmember.
 * The Galaxy class has Saucer Separation capability because the Saucer section contains the civilians, laboratories, families, etc., while the lower section contains the warp drive. It allows the civilians to be moved out of harm's way if the ship has to go into a firefight. Saucer separation was used often in the first season of the show, but after that it was forgotten and only sometimes referred to, just to drop the idea afterwards.
 * Star Trek: Deep Space Nine came up with a holographic communications array, installed it on the Defiant's bridge and Sisko's office, used it all of three times and then forgot all about it. Other than looking cool and saving some money on blue-screen usage, it really served no purpose at all.
 * They also had an easily replicable gun capable of shooting through walls (a combination of x-ray goggles and micro-transporter). Like SF Debris mentions, that weapon could have been useful on many occasions.
 * In Star Trek: Voyager, the crew conveniently forgot several gadgets that could have gotten them home, or at least closer to it:
 * Q Jr, depowered, retains enough Q knowledge to use the Delta Flyer's [insert Techno Babble here] to create portals, without any unpleasant Star Trek Shake-inducing side-effects that we saw. The crew could have done whatever it was that they did and gotten home via a series of portals, or at least - as was often the case with Voyager's non-deadly shortcuts - shaved a decade or two off their trip before the Applied Phlebotinum gave out.
 * Borg warp coils, in a season-five episode, allowed the crew to jump hundreds of thousands of light years, cutting several years off the journey. For the rest of the series, however, the crew never bother to get any of these warp coils, despite coming across several opportunities to obtain some (most notably in an episode where the crew come across an entire graveyard of half-destroyed Borg ships).
 * In "Threshold", where the otherwise successful test of an experimental transwarp engine turns Janeway and Paris into newts. An imposing side-effect, to be sure, but one which they have cured by episode's end, leaving them in possession of a magic new transportation technology which could get them back to Earth almost immediately, and a cure for its inevitable side-effect. So rather than using it to return to Earth, or even send a message back to the Federation (this was before the Federation discovered that Voyager and her crew had survived), they roll end credits and never mention it again. Even the producers try to forget that episode, so it's no surprise the characters forget it too.
 * Even ignoring the lizard-fication, "Threshold" mentions that the experimental shuttle's computers were jam-packed with helpful navigation aids and maps—which are never mentioned up again.
 * Seven of Nine once brought Neelix back from the freakin' dead after several hours via (what else?) nanoprobes. Apparently, the technology must only work on main cast members. And even then, only that one time...
 * Throughout the series, Voyager uses several technologies (the aforementioned Borg warp coils, the quantum slipstream drive from "Timeless", Kes' gift to Voyager in "The Gift", wormholes and other assorted devices) to cut a collective thirty to fifty years off their journey. As the (non-altered) future of the series finale "Endgame" shows, after the crew ignored the Borg temporal node, they supposedly spent the next twenty-six years merrily skipping along on their way to Earth without the aid of any of the aforementioned technologies. It's like the crew just gave up and decided to go the traditional way, even though Janeway wouldn't have hesitated to use an advantage if one presented itself.
 * The complaint about the Nemesis movie applies to several Voyager episodes as well—the show gives a reason ship-board transporters won't work, but they neglect to explain why they can't use the shuttle's independently-powered transporters.
 * Buffy the Vampire Slayer: One episode had the minion of the First Evil falsely claim he had kidnapped a proto-Slayer. Nobody thought of using the "detect proto-Slayer" spell discovered a few episodes before.
 * The issue of the Adjoining Spell from the end of Season Four. Arguably the most powerful spell seen in the entire series, although with the drawback of . It is never mentioned again.
 * At the end of Season 5, Buffy uses a hammer that (somehow) allows her to pulverise Glorificus, a literal Physical God that had shrugged off absolutely everything that was thrown at her before. Said hammer is never seen or mentioned again, despite how useful it would have been against, say, the Turok-Han or Caleb.
 * Stargate SG-1 usually avoids this, with plenty of Chekhov's Boomerangs shown years apart, but it still has its examples.
 * Kull Warrior Armor. That stuff shrugs off claymore explosions, and is light enough to wear, yet while Vala is able to get her hands on a suit and capture a starship—a United States starship, no less, so you'd think they learn—with it, the US military doesn't even seem interested in it. Plus it looks really Badass.
 * Season 3, "Past and Present": They discover a drug that reverses the effects of aging. Next episode, it's forgotten.
 * Atlantis is on Earth. Atlantis. The Ancient city-ship with sensors that are capable of picking up even cloaked ships in practically one third of the galaxy away from wherever it happens to be. The database of which contains truly obscene amounts of information on Ancient technology. And yet not only do they barely touch on the database in Stargate Universe (no sending a scientist back to, say, research useful ways to get help), but the . Word of God is that Atlantis had a fail-safe that required it be returned to the Pegasus Galaxy a few weeks later (this was to have been the plot of the Stargate Atlantis movie had it been greenlit), which addresses only some of these omissions.
 * In Farscape, Zhaan is capable of camouflaging herself like a chameleon, but only uses this ability in one episode ("Bone to be Wild", Season 1). She is a plant and this is one of the few times she is in a forest, but one would think it would be harder for her to camouflage herself against something as complex as foliage, compared to the relatively uniform interior of Moya.
 * D'Argo's super-long tongue and anesthetic saliva gets forgotten every fifth episode or so. His arms and legs are bound, while a sole villain gloats nearby without a helmet, whatever shall he do? The funniest is when John asks him to knock him out in "A Prefect Murder", and D'Argo pistol whips him. And it doesn't work. John asks him to hit him again harder.
 * Doctor Who, both Classic and New Series, did this a lot.
 * At the end of the first Christmas special of the new series, the vast alien starship that had been menacing the whole planet is utterly destroyed by a colossal laser fired from beneath London. This fantastic weapon devised by Torchwood London from captured alien technology for the defense of the kingdom from extraterrestrial perils is never again mentioned, despite London being menaced by aliens so frequently that its citizens get into the habit of evacuating over Christmas so as not to be there when the monsters turn up. However, the same technology (or, at least, similar special effects) seems to now be incorporated into so even if the main weapon was destroyed, it's not all gone to waste. At least, until "The Stolen Earth", when.
 * The TARDIS has had many features used over the decades that were completely forgotten soon afterwards; drifting back to its owner if separated from them in time ("Revenge of the Cybermen"), The Space-Time Visualiser ("The Space Museum" and "The Chase"), the Hostile Action Displacement System ("The Krotons"), the macro-kinetic extrapolator ("Boom Town" and "The Parting of the Ways"), among others. Considering that the TARDIS was a museum piece even before the Doctor stole it almost a millennium ago and is highly temperamental even at the best of times, it's entirely probable that these things literally don't work anymore.
 * Companions sometimes call the Doctor out on this in relation to the TARDIS. But it's conveniently stolen, missing, or can't be used due to the danger of crossing their own timestreams, which is apparently very bad.
 * In "Partners in Crime" the Doctor obtains a sonic pen which can open deadlock seals one of only two types of locks his sonic screwdriver can't deal with. At the end of the episode he simply throws it into a bin on 21st century Earth.
 * A machine that creates candy-bar-shaped Food Pills appears once in the 1963 season and is never seen again.
 * The Chula, first mentioned in "The Empty Child", produced nanobots capable of healing any injury and even reviving the dead, and in enough numbers to work over whole planets worth of people. Why hasn't the Doctor simply gone to Chula and got some for himself?
 * Ever since Disney took over the franchise, teams of Power Rangers have been getting single special abilities while untransformed. Except in Ninja Storm and Jungle Fury, where these powers were highly plot important, the Rangers would generally completely forget they had these powers for a dozen episodes at a time.
 * In the original series, this happened far more frequently, with MacGuffins being introduced regularly and never being mentioned again. The worst was the Sword of Power, summoned by a Brainwashed Tommy as part of a ploy by Lord Zedd to steal it. After regaining his mind, Tommy goes to great lengths to get it back, taking on the Monster of the Week single-handedly. He retrieves it... and it's never seen again. For that matter, it wasn't clear why it was so desirable in the first place.
 * Claire's blood in Heroes. It can heal people. It works on anything, and nobody even mentions it in situations where it might be useful (for instance, on Nathan at the end of season 3).
 * Claire's blood had previously restored her adoptive father to life. A shame he didn't mention this when her biological father needed it, and her grandmother was frantic to preserve him.
 * Even stupider, her grandmother should have already known about it because of Adam.
 * Happened so often in Knight Rider that it became one of the jokes of the series. Aside from the common stunts, Bonnie/April would mention off-the-cuff that they'd added some cool new feature to KITT... which just happened to be exceedingly useful for that episode's problem. Then, it would never be heard about again despite the gadget being a solution to a later problem. Only a very few added features went on to be regularly featured without being implied to have always been there (which invoked the reverse of this trope, why hadn't they been using it?)
 * A few gadgets were explicitly mentioned as being failures and being removed in order to avoid this trope when they were a little too powerful, such as the laser and device that let KITT drive on water.
 * In an episode of the 1950s The Adventures of Superman, Superman learns from a swami (or somesuch) how to divide himself into two by using the power of his Super-will. It was only used once. Each is only half as powerful as the full Superman so it makes sense for him not to use it all the time; but it would have come in very handy during all of those "you never see Clark and Superman at the same time" bits.
 * Another episode has him develop the power to walk through walls without smashing through them by brute force and doing major property damage. Like the splitting power, gets totally forgotten from then on.
 * In the movie of Wizards of Waverly Place, a bad wish of Alex's ruins her parents marriage. Too bad they didn't learn a spell that allows them to reverse time to correct such mistakes, like they were taught during an episode of the series.
 * This spell would have been very useful in the fight between Juliet and Mason to keep both characters from getting permanently transformed once Mason scratched Juliet.
 * The improv spell, which does basically anything as long as you can make up a rythme for it.
 * From the Television Without Pity forums: "If a non-expert actress can figure out in a few days that she can make Angel vamp out into Angelus/pseudo-Angelus for a night by spiking his drink with an easily available drug, why didn't Wolfram & Hart ever consider the same method when they wanted to screw with Angel? Why didn't Wesley consider it (even simply to reject it) when they needed Angelus to fight The Beast?"
 * In one episode, Wesley uses a flamethrower against a bunch of mooks. It is awesome AND effective, yet despite fire being deadly to most things, especially vampires, this is never seen again. It would have been particularly useful when Los Angeles was being swarmed with vampires and there were too many for them to attack one at a time.
 * In Eureka, the cryo sleep chamber that was used to put Fargo's grandfather in suspended animation could have been used many times to buy time during emergencies where people are mutating or dying of some horrible disease.
 * The Colt in Supernatural seems to have become this. Sam and Dean spend the first part of season 5 trying to recover it, only to discover that Some fans believe this is because Dean dropped it when Lucifer threw him, but since he wasn't shown to have dropped it and neither brother mentions losing it afterward, it's a bit of a stretch.
 * They do use it when they travel back in time to kill a phoenix in season 6, though, so it's not completely forgotten.
 * From the reimagined Battlestar Galactica: In the middle of season 2, Arguably, this is also an example of They Wasted a Perfectly Good Plot, since they could have done an entire episode about the ethics of
 * Supposedly they tried it again offscreen. It just didn't work.
 * The Blackbird would be another example. Admittedly, they have to scrounge up a lot of supplies and spares to assemble it together but considering that Pegasus had Viper production facilities, it would not be impossible to construct additional stealth ships once the original was destroyed. It would have come in handy during New Caprica or the battle of The Hub.
 * The Big Bang Theory has a rare non-F&SF example: Sheldon can be persuaded to do some things he finds ridiculous or inexplicable by telling him that the thing is a "non-negotiable social convention". This has been used a grand total of once in the show's history.

Toys

 * The ability of various Bionicle characters to form a Kaita or a Nui has been all but forgotten, and had only ever been used a handful of times early on in the series. This can be attributed to the set designers not coming up with combinations for the later sets, though a couple of already existing combinations still didn't get to be used, even when they would have come in really handy. There is no in-story explanation for this: the writer simply doesn't want to use them.
 * Another seemingly forgotten "power" is the ability for a character to rebuild itself (since they're Built With Lego). Granted, this ability apparently requires the character to have an amount of secret knowledge, have pieces lying around and having strong enough muscles to support a new body, but still... the ability exists and has been used to make the characters stronger, but only on one occasion (because the toys said so).

Video Games

 * In Phoenix Wright: Ace Attorney, when Phoenix first meets Maya and finds out she's a spirit medium, he comes up with the obvious idea - why not just summon the victim and ask them who did it? Maya replies that she's just in training and can't do that. In the next two games, not only is Maya more experienced, but Phoenix also meets an even more talented medium, Pearl, yet he never thinks of that idea again.
 * This might be because.
 * In addition, the time this * was* tried (albeit indirectly),
 * Freelancer, period. "Cloaking ships? What do you mean they were mounted on fighters during the Alliance/Coalition war centuries ago? They take more power than a battleship can provide! Besides, what war are you talking about? I've never heard of it before."
 * They also forgot the fighter-sized warp drives. They used 'em about the same time as the fighter-sized cloaking devices. Those Libertonians really ought to pack some Phlebotinum next time they go somewhere.
 * In The Elder Scrolls V: Skyrim many guards tell you "I used to be an adventurer like you, then I took an arrow in the Knee." but in a world of Magic healing spells and instant health potions you would not think such a minor injury would be so debilitating.
 * The Phase Transit Cannon from Wing Commander II is never mentioned again outside of a brief note in the manual for the Kilrathi Saga compilation mentioning that it was discontinued due to technical problems, and the flash-packs from Wing Commander IV isn't mentioned anywhere at all in later Wing Commander games, as if the tech has vanished.
 * World of Warcraft could be said to have this. When questing it is not uncommon to be given an incredibly powerful item to help with the quest, for example a crystal that can fire a beam to shrink down giants, making them much easier to fight, to never be used again.

Web Original

 * In the first episode of Chad Vader, he is shown to have the ability to force choke people, and he uses it on a guy who annoys him. In later episodes, his nemesis repeatedly humiliates him, and he just fumes impotently.

Web Comics

 * A particular device in Special School is designed to make people (except certain psychics) to forget all about it.
 * Early in The Order of the Stick, when going to face Xykon for the first time, Durkon enchants Roy's sword with a disruptor spell, which would have destroyed Xykon completely with one shot if Roy managed to land a hit. That spell would probably have come in handy the next time Xykon showed up, but nobody even thought to mention it.
 * Not to mention that the Giant, the author of said webcomic, specifically said that he prefers to do things that way. Taken directly from his FAQ, "Q: In Strip #X, why didn't character Y take action Z? If they had done so, they could have avoided a whole lot of trouble. A: You just answered your own question. The strip is about the trouble these characters get in; if a tactic would result in an effortless solution to their latest problem, there would be little point in showing it, see?"
 * In Sluggy Freelance, an early Story Arc had Riff and Dr. Schlock work together to build a time machine. After the machine is destroyed by a potato chip (it was balloon based), neither of them ever tries building one again, despite Time Travel having more Deus Ex Machina potential than just about anything else.
 * Though considering how much trouble they get into dealing with Alternate Dimensions and how big a mess they caused with the last time-travel jaunt, even Riff would hesitate to use it.

Western Animation

 * Events surrounding the second season finale of Avatar: The Last Airbender left many wondering why Katara, who had theorized that her vial of water from the sacred oasis retained healing properties, didn't make so much as an attempt to whip it out and use on the mortally wounded Tragic Hero Jet a few episodes beforehand. The DVD commentary has them admit that they forgot it, but said it wouldn't have worked anyway.
 * This is particularly jarring given how good this show normally is at avoiding this trope. Every time Aang can't use the Avatar State or bending won't work, there's a perfectly logical explanation. It even gets Lampshaded a few times.
 * This isn't always something big... It creeps in on a smaller scale, too, like Waspinator's Eye Beams that he shoots at Cheetor with in the Transformers: Beast Wars pilot. He never uses them again, even in the Season 2 opener, when Cheetor knocks his gun out of his hand and those Eye Beams would have been a nice alternative to running away. Of course, Waspinator's never really been the sharpest saw in the toolshed.
 * He did actually use them at one other time, when he and Terrorsaur (who was also using Eye Beams) were trying to cut into Tigatron's stasis pod. Basically, if a character on the show had that ability and wasn't named Dinobot, they only got used on very rare occasions, and got overlooked numerous times that they could have been helpful.
 * Transformers Generation 1 had both sides constantly creating weapons that would be a Game Breaker in the hands of non-idiots. Instead of being used for what they could be, they'd be used to create/stop the problem of the day, and then never be seen or heard from again. Also, the many, many, many one-shot powers displayed by individual Autobots that would never be used again. (Most iconically, the Pure Energy flail and axe used by Megs and Prime, respectively, in the series premiere only and never again.) They also suffered from No Plans, No Prototype, No Backup: If the ultimate weapon whipped up in the days since the previous episode gets smashed at the end, just making another is apparently never an option.
 * It wasn't only Transformers that did this - in Super Dimension Fortress Macross, we see that the Humongous Mecha piloted by humans have head-mounted cannons - that got used exactly four times during the entire show. Only twice as a weapon - both of the other times they were used as cutting tools.
 * The Shard of Lightning in Xiaolin Showdown is an example of this. It can freeze time, and Jack used it to steal most of the monks' Shen Gong Wu, and cause various havoc. The monks won it by the end of the episode. They could have used it to freeze time and just kill Jack, Chase, Wuya and Hannibal all at once.
 * Similarly, the Golden Tiger Claws, wich allow a person to create a portal to any location. It's introduced and done away with in Season 1, but returned to the heroes in Season 2. Despite the fact that Dojo can sense the location of any newly active Wu, the heroes never decide to have Dojo use the Claws to warp there, instead of flying there slowly enough to let the villains reach the Wu.
 * Added to that, the Reversing Mirror, which is restored at the end of the episode "Citadel of Doom," could easily be used to restore Wuya to her full powers and body throughout all of season 2, yet she is content to simply look for all of the other Shen Gong Wu.
 * The supreme example of this in Xiaolin Showdown is the Emperor Scorpion, a Shen Gong Wu that can control any other Shen Gong Wu. After being used to, it is sealed away in the vault. Neither the monks, nor the villains (who raid the vault every six episodes or so) ever take or use this supreme Shen Gong Wu ever again, presumably because it would make the show very boring.
 * In one episode of the Dungeons and Dragons cartoon, "Beauty and the Bogbeast", a magical river was introduced. At a particular time every year, it could take the heroes anywhere they wanted to go - Earth included. Naturally, they are forced to turn around, at literally the last minute, due to extenuating circumstances. They never seem to consider that there's nothing stopping them from coming back (it's not clear how long the series takes place over, but it can't be that much less) next year and making sure no one gets turned into a bogbeast this time. Of course, even considering the source, if there was ever an episode to throw on the Fanon Discontinuity pile anyway...
 * In Ben 10, Ben's Evil Counterpart Kevin 11's original power was the ability to absorb energy, useful for shorting out/controlling machinery and creating instant lightning blasts. As a side effect, he was also able to use it to absorb alien superpowers via physical contact. After using his ability to steal Ben's 10 superpowers, however, Kevin seems to completely forget about his original ability, even though he names himself "Kevin 11" specifically because he has 1 more power than Ben. He fails to use energy control in situations it would have been extremely helpful (i.e. when being held captive by robots), and also fails to absorb any more alien superpowers despite apparently spending a few months roaming the galaxy doing nothing except beating random aliens up. The alternate future episode "Ken 10" shows how useful this would have been, as Future Kevin finally uses his power-stealing ability to become a formidable combination of Sylar and Naraku.
 * In the sequel, Ben 10 Alien Force, Kevin's powers are even more limited, as he can only absorb the properties of solid matter. This pretty much limits his options in combat into turning into something tough like metal or stone, then running around punching stuff. However, absorbing the properties of energy makes him go insane, which usually makes it a bad idea.
 * Also in Alien Force, the DNAliens are all infected humans, but Ben finds out the Omnitrix is capable of curing the people and is able to help Gwen's brother this way. He then proceeds to forget that he can do this until the Season Finale.
 * Incredibly, Ultimate Alien appears to have forgotten about the Ultimate forms, which give the new show its title in one of its first episodes.
 * The Fairly OddParents. Sometimes Timmy Turner wishes for superpowers. Sometimes he doesn't unwish them. These actually show up later and affect the plot. Same with magical items, handwaved by saying that they were neglected or that Cosmo was screwing with them.
 * Space Ghost Coast to Coast. The episode "Snatch" has a segment that is pseudo-satirical of this trope. Although the device in that case is a mind erasing device, so at least it is feasible that the device is never remembered.
 * An episode of Winx Club has a double dip of this: A segment of a season 3 episode opens with Icy boasting about a new fire power Valtor gave her. In the Trix's battles against the Winx, including the one just a few minutes later, she doesn't use it (and in fact, it's Darcy who sets a library on fire in a later episode, not Icy). And during the battle in the same episode, Layla sneaks up behind Darcy and Stormy to tie them up, even though way back in the second ever episode, she sensed Bloom's presence behind a garbage can, even though Bloom was well out of her view. (It should be noted, however, that the non-4K version does mitigate the stupidity in both cases somewhat.)
 * A season 1 episode saw Icy destroy Red Fountain by freezing it with a nifty ice dragon. Season finale, Icy doesn't even make any effort to use it to freeze Alfea, or to take on Bloom, who has an fire-energy dragon of her own.
 * Bloom was shown to be able to use her powers to revive the dead. It's not explained why is still dead. This was changed to breaking a sleeping spell in the 4kids version, which removes the contention entirely.
 * In early season 3 there's nothing stopping a blind Layla from waiting to use Queen Ligea's healing staff the next sunset after she uses it on its owner.
 * The Charmix from season 2. Sure, it was literally So Last Season, but there's nothing else preventing the fairies who hadn't earned their Enchantix from using it in season 3, especially like when they're battling the Trix. Yet the only time it's even mentioned in season 3 is when the school headmistress talks about Enchantix.
 * In the 21st episode of the third season, Nabu claimed that the reason he had stowed away on the Specialists ship was so he could practice his invisibility spells against monsters living in the area the Winx were travelling to. Nabu's ability to turn invisible hasn't been seen again since this episode. (Although it's possible that he used this ability off-screen during season 4's episode 20 when he went to the nature fairy Diana's castle to try to save the Specialists, but this is just a theory...)
 * Lampshaded/Parodied in Stroker and Hoop with Hoop learning ninja skills for plot-related reasons in one episode, but never using them again. It's then brought up in another episode. Turns out you have to actually continue practicing to maintain ninja skills. Who knew?
 * In Futurama, Richard Nixon's head uses a robot body in one episode, and Beck's head controls a small set of robot arms in another. If heads in jars can control robot bodies, why don't most of them do it?
 * Heck, the day Nixon was elected, he got himself a gargantuan robot body complete with integrated rocket launchers. Where did it go?
 * Star Wars: The Clone Wars. Cad Bane steals a Jedi holocron. Anakin steals it back, but an explosion causes him to drop it. It lands between him and Bane? Hmm, should I use the Force to summon it to my hand, far to fast for Bane to react to, as I have done with my lightsaber on countless occasions? Nah, he's a bounty hunter, he's too cool to lose.
 * It's funny in that in the same arc (or perhaps even the same episode) Bane threatens to throw Ahsoka out the airlock if Anakin doesnt open the Holocron. He does so, Bane gets what he wants and still tosses Ahsoka out the airlock. Anakin force-pushes the airlock's laser door switch, shutting it off and saving his padawon. Wait, why didnt he just do it before? especially since Cad Bane didn't even move.
 * This is nowhere near the only situation where the Jedi forget about telekinesis. Ahsoka had an episode where someone stole her lightsaber and she engaged in a long chase to try and get it back. Why didn't she just summon it? She used telekinesis twice in the very same episode for different purposes and had dozens of situations where she could've just taken it. There were episodes where grenades were used against the Jedi - a perfect moment to use telekinesis. This never happens. There was an episode where a Mad Scientist drops a vial of deadly virus to the ground - and Obi-wan(I think) lunges after it, desperately trying to catch it, instead of just summoning it to himself. That's just the tip of the iceberg. The number of situations where the Jedi don't use telekinesis in this show is my biggest headscratchers about it. And yes, it does seem like every bounty hunter has a mysterious aura that makes Jedi into idiots.
 * Birdman can only recharge his solar powers in sunlight. In the episode 20 "The Wings of Fear" he develops "Solar Energy Storage Bands", which provide him with solar energy to replenish his powers when he's out of the sunlight. After this episode they're never mentioned again, even though they would have been incredibly useful.
 * Challenge of the Superfriends is notorious for this trope. Lex Luthor invents teleporters, time machines, cloaking devices, a gizmo that sucks the Green Lantern Ring off its wearer's finger, etc., etc. ... uses them once ... and then then never uses them again, even in situations where one of them would save the Legion of Doom's bacon.
 * One particularly damning example has Luthor forget a piece of Phlebotinum only moments after acquiring it. When he time travels to alter several of the Super Friends' origins, Luthor switches places with Hal Jordan and becomes the recipient of Abin Sur's Green Lantern ring. He dons his own Green Lantern suit and uses the ring to fly back to the Hall of Doom, and then promptly puts his purple jump suit back on and makes no other attempt to use the ring. This after having been defeated by Green Lantern's power too many times to count!
 * A rare villain example in The Nightmare Before Christmas: Oogie Boogie has the ability to Airbend suck in everything like a gigantic vacuum, which is how he . He never thought to use this in his battle against Jack Skellington, though this is somewhat justified, as Oogie was trying to get away from Jack. Still could have ate him, though.
 * Star Trek: The Animated Series. The "life support belts" were clearly introduced to save on animation costs, but they're certainly a handy device. They never show up in the rest of Trek.
 * Teenage Mutant Ninja Turtles 2003: In the Fast Forward season, the Turtles and Splinter are stranded a hundred years in the future with no way of returning to their own time... not one mention is made of their time-traveling friend Renet, who is supposed to keep an eye on the time stream and would almost certainly have noticed if the Turtles were suddenly in a different time period than they were supposed to. Possibly justified if the Turtles and Splinter were supposed to spend some time in the future.
 * Season 1 of My Little Pony: Friendship Is Magic is full of this. The Elements of Harmony (and Princess Luna) are completely forgotten about after the second episode, Twilight Sparkle never teleported after episode four, Pinkie's "Pinkie Sense" is never mentioned again, and while the Sonic Rainboom pops up later it is only in a series of flashbacks. Fortunately, season two turned all of these and more into Chekhov's Boomerangs.