->''"The issue in an ongoing series is once you've [used time travel] and it wasn't a fluke, it's like you've shown that one of your characters got Franchise/{{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."''
-->-- AltText for '''''Webcomic/DinosaurComics''', [[http://www.qwantz.com/index.php?comic=1307 9/18/08]]''

A situation, most common in SpeculativeFiction, 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 trope does not necessarily denote bad writing. It can be (as noted in the ''[[Webcomic/TheOrderOfTheStick Order of the Stick]]'' example) essential to the story. If the protagonists have some piece of phlebotinum that makes them nearly or actually invincible, [[DramaPreservingHandicap many plots have to be thrown out the window.]] Some would call this unwillingness to [[StatusQuoIsGod 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 SharedUniverse this is even more the case.

Creator/LarryNiven is scornful 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 NoPlansNoPrototypeNoBackup but Niven would maintain that this smacks of lazy writing. At the very least, the precedent that such a machine is ''physically possible'' in the setting must be maintained, which would make it likely that older, HigherTechSpecies will possess it even if it never became prevalent in the protagonists' society.

Note that this trope 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 ForgotAboutHisPowers. If the device is remembered, but there's some contrived excuse as to why it isn't available or won't work, that's HoldingBackThePhlebotinum or ItOnlyWorksOnce. Not to be confused with WeHaveForgottenThePhlebotinum. If they (finally!) remember to use it in the end, it's a ForgottenSuperweapon. When it's ''not'' forgotten and ''is'' used in a later episode because a writer wants to [[ContinuityNod acknowledge continuity]], it is ChekhovsBoomerang. Compare also with NeverRecycleYourSchemes in the case of villains.



[[folder:Anime and Manga]]
* ''Manga/ACertainScientificRailgun'': There are a number of things that only come up once and are discarded once Touma or another hero defeats them.
** The Level Upper program, for example, is a noise file that increases a listener's power level before putting them in a coma, in which state they can be used as an organic computer for the controller of the network to make use of ''all'' their powers--this would be absolutely perfect for the Sisters, but it's never brought up. [[spoiler: Though in the case of the Sisters, Level Upper is related to, and derived from, the Misaka Network, which is utilised in a similar manner to create Fuse=KAZAKIRI, an artificial Angel as strong as a Level 5. Also, Level Upper, as it presently existed, resulted in comas, which is bad, and an uncontrollable menace, which is worse. The Misaka Network is the better version, and Level Upper is shown in the prequel sections of Railgun in order to lead into that.]]
** "Capacity Down" is a BrownNote in that causes [[spoiler:immense physical and mental distress in espers, leading them to collapse and unable to use their powers]]. This is used once in ''Railgun'' by [[spoiler:a non-esper villain against the esper heroes]], and then once again in ''Index'' during the Academy City "Battle Royale"... and never mentioned again despite how effective it is, including being one of the very few things in the series known to actually affect [[InvincibleHero Accelerator]].
** Early in ''New Testament'', we're introduced to several machines that artificially reproduce the abilities of the Level 5 espers. While not quite as powerful as the genuine article, they are nonetheless extremely effective and able to be mass-produced. They're around for maybe two volumes and then forgotten.
* In ''Anime/CowboyBebop'', 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 ''Anime/DigimonAdventure'', 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.
* This is not uncommon in ''Anime/{{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. One the early [[MonsterOfTheWeek 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.
* In ''Manga/FullmetalAlchemist'', Dr. Marcoh reveals he has developed a way to [[spoiler: destroy Philosopher's Stones, and uses it to great effect against Envy.]]Yet neither Marcoh himself, nor any of the alchemists he could have taught this skill, ever uses it again.
* ''Anime/MazingerZ'': Often it was played straight. Many times Dr. Hell came up with a [[{{Robeast}} Mechanical Beast]] equipped 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 earthquakes 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. [[ChekhovsBoomerang However, sometimes Dr. Hell reused and improved some strategies or weapons]], or deceived the enemy in believing he was using the same trick.
* In ''Anime/MobileSuitGundamWing'', the main Gundams all have shields with offensive capabilities. Wing Gundam's shield can be used as a stabbing weapon; Wing Zero has the same, except it actually extends to be more effective; Deathscythe (and its upgrade. Deathscythe Hell) has a Buster Shield, which opens up and produces a blade and is then launched at an enemy, Heavyarms' shield is integrated into its Beam Gatling; Sandrock's shield can be combined with its weapons and jetpack to create a superweapon and can be used as a blinding flare, and Shenlong can throw its shield to destroy a mobile suit. Yet other than the Heavyarms, these offensive capabilities were each used precisely once by the suit over the course of the series.
* ''Anime/SailorMoon'':
** In the first episode 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 [[WeirdnessMagnet Naru]] being attacked by the MonsterOfTheWeek 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/SailorMoon 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.
* ''Manga/SgtFrog'': The Kero Ball and Angol Mois's Lucifer Spear gets used less and less each season. This gets {{lampshaded}} in the third movie - both items are lost at different points, but recovered from the wreckage during the end credits.
** The 7th season of the anime attempted to reestablish the basic characters and setting of the show. As a result, both of these elements come to the forefront again. There are 3 episodes with the Keroball in the center of everything just in the first half of the season. That's more than in all of the first season!
*** The Lucifer Spear may be condemned to this [[TooSoon after the earthquake of March 2011.]]
* In ''Anime/SuperDimensionFortressMacross'', we see that the HumongousMecha 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''.
** Thankfully, the head cannons see more consistent use in later ''Anime/{{Macross}}'' series.
* ''Anime/TekkamanBlade'' 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.
* ''Anime/TengenToppaGurrenLagann'': The Gurren Lagann is equipped with a powerful DeflectorShield, which stops CombatPragmatist 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 CompilationMovie, however.)
* ''Anime/YuGiOh5Ds'':
** Yusei's Turbo Warrior would have been immune to the effects of the Meklord Emperors and likely would have been useful, but Yusei conveniently forgets that he has Turbo Warrior whenever a Meklord Emperor hits the field.
** An even better option that he also had was Dragon Knight Draco-Equiste, a Fusion Monster, meaning that the Meklords couldn't equip it even if its effects were negated. Draco-Equiste only ends up being used in 1 duel.
* ''Manga/YuYuHakusho''
** In early episodes Hiei has telepathic abilities and has the ability to transform into a more powerful demon form, which eventually disappeared. 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 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. Many of the techniques he can use are dependent on which plant seeds he has on his person at the time, so it's possible he only has a limited number of different seed types on him at any given time.
** Kuwabara 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.
** And to round out the quartet, Yusuke masters his teacher's secret move, the ultimate fighting technique, the Spirit Wave. He successfully uses it for the first time to beat Jin, then never uses it in battle again. He does use it offscreen once, to heal himself, but considering he has such an incredibly versatile and powerful technique at his disposal, you'd think it would come in handy more often.

[[folder:Comic Books]]
* Franchise/{{Superman}}:
** Silver Age's Superman had a 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.
** UsefulNotes/{{The Silver Age|OfComicBooks}} had ''tons'' of ForgottenPhlebotinum. 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, [[CriticalResearchFailure stops being cold]]. 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. [[ItOnlyWorksOnce 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, Creator/DCComics'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 ''[[Comicbook/TheAvengers Avengers]]'' stories (the relaunch with Creator/KurtBusiek). Justice, sidelined with a broken leg, goes on an ArchiveBinge and realizes that the best way to defeat an Adamantium robot on a homicidal rampage is with Antarctic (type B) Vibranium (AKA Anti-Metal), a metal that somehow destroys any other metal within range when 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 ComicBook/{{Ultron}}'s rampages, some of the finest minds ([[ComicBook/IronMan Stark]], [[ComicBook/AntMan 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, thanks to SupermanStaysOutOfGotham. Comicbook/{{Batman}} operates in a much lower-tech universe than the rest of Franchise/TheDCU, despite hanging out with Superman in the Justice League half the time. This becomes wall-banger material when you consider Barbara "Comicbook/{{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.
* ''Comicbook/XMen'':
** Comicbook/{{Rogue}} can't have a relationship with anyone due to her powers. Comicbook/{{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, but are never mentioned in relation to this issue. [[WordOfGod At least one writer]] has admitted that there is no in-universe reason someone like Forge couldn't whip up a gadget to temporarily nullify Rogue's powers whenever she wanted, but that would make her character "less interesting".
** Comicbook/{{Cyclops}}, during a plan to infiltrate ComicBook/{{Magneto}}'s band by disguising himself as Erik the Red, creates a pair of gloves that somehow allow him to fire his optic blasts out of his hands instead of just his eyes, which he uses to devastating effect for the duration. While the identity of Erik the Red itself is used by several other characters, these gloves are never used by Cyclops again despite the incredible effectiveness it showed, the much greater versatility it could have afforded him, and the fact that it would have finally allow him to have another option during the numerous times he's had his optic blasts blocked for one reason or another.
* ''Comicbook/SpiderMan'':
** 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 ''Comicbook/OneMoreDay'' 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, ''[[IdiotPlot 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|IsUseless}}, and Comicbook/DoctorStrange - 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 [[DealWithTheDevil letting Mephistopheles wipe out the entire history of his marriage in exchange for Aunt May's life]].
* At the end of Volume 2 of ''ComicBook/{{Runaways}}'', the team gets their hands on a device that allows them to travel through time. It is never mentioned again, even though it might come in handy after Xavin is taken prisoner by the Light Brigade and carried off into space in Volume 3.

[[folder:Fairy Tales]]
* ''Literature/ArabianNights'': In the tale of the three princes who each go to seek a marvel, Prince Ahmed finds a magic apple that restores health to anybody who smells it, even if they are at the point of death, and presents it to his father the Sultan. This tale has a sequel, in which the Sultan's advisors poison his mind against Prince Ahmed and persuade him to send Ahmed on a series of impossible quests; one of these is for a MacGuffin reputed to cure all diseases -- and not one person, not even Prince Ahmed who gave it to him, thinks to mention that he's already got one.
* In "Literature/TheTwoBrothers", collected by Creator/TheBrothersGrimm, the twin brothers accidentally acquire a magical ability to make gold appear from nowhere, which leads to them being run out of town at the instigation of the man who'd been attempting to acquire the ability for himself. In the course of their many subsequent adventures, the ability is never mentioned again.

[[folder:Fan Fic]]
* ''Fanfic/CalvinAndHobbesTheSeries'' has the MTM (whose name stands for Mini Time Machine), whose original intent as a smaller TimeMachine is forgotten in favor of his DoAnythingRobot nature. This is eventually lampshaded in "Nocturnals".
* {{Averted}} in ''Fanfic/AVoiceInTheWilderness''. The crew of the crippled USS ''Bajor'' '''does''' remember that they have shuttlecraft which are independently warp-capable, and make plans to evacuate with them. Unfortunately they can only get about 150 crew of over a thousand off that way.

[[folder:Films -- Animated]]
* A rare villain example in ''WesternAnimation/TheNightmareBeforeChristmas'': Oogie Boogie has the ability to suck in everything like a gigantic vacuum, which is how he [[spoiler:recaptured Santa and Sally]]. He never thought to use this in his battle against Jack Skellington.

[[folder:Films -- Live-Action]]
* In ''Film/StarTrekFirstContact'', 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 slingshotting around a star at warp 10, but it's never mentioned again. And if the Borg posses the technology to TimeTravel at will, why not use it to undo all their failures? In particular, they could have averted their HopelessWar with Species 8472.
* ''Film/StarTrekNemesis'':
** At the end, 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 then die with the enemy ship himself. Everyone seemed to forget ''the shuttles have their own independent transporters''. Also, the shuttlecraft themselves; they could just send Data over carrying a spacesuit and have him jump through the forcefield with Picard back to the shuttle.
** Then there's the Captain's Yacht, a large auxiliary 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 B-4's parts are located).
* In the 2009 ''Film/StarTrek'' reboot, the Spock from an alternate future introduces "transwarp transporting", which works across interstellar distances. He shows this to the younger version of Scotty, and uses it to send him and Kirk to the ''Enterprise'', which is currently traveling at warp speed. In ''Film/StarTrekIntoDarkness'', Harrison uses it to beam from Earth to the Klingon homeworld Qo'noS. This demonstrates that knowledge of the technology ''did'' leak out, but is now exclusively used by the villains, with the heroes seemingly no longer remembering how to do it, even though they were the first to learn how it works.
** In ''Into Darkness,'' the fact that [[spoiler: an augment's blood can resurrect the dead]] is a pretty significant discovery to have lost for two hundred years. [[StatusQuoIsGod Dimes to donuts]] [[StoryBreakerPower we won't see it]] [[ReedRichardsIsUseless in the next movie, either.]]
*** Indeed, ''Film/StarTrekBeyond'' ignores pretty much everything about ''Film/StarTrekIntoDarkness.''
* The "throwing 'S' shield" in ''Film/SupermanII''. 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 [[Film/SupermanIII the evil Clark]] or [[Film/SupermanIVTheQuestForPeace Nuclear Man]], even if it was a cheap-looking effect.
* In ''Film/AvengersAgeOfUltron'', Helen Cho has built a "regeneration cradle" that can heal grievous injuries by creating synthetic tissue and bonding it to the victim's cells. Apparently, nobody thought to use this on [[spoiler: Comicbook/WarMachine]] after a serious fall cost him the use of his legs in ''Film/CaptainAmericaCivilWar''. Heck, the movie even has a scene where doctors are shown examining him after the accident, and Helen isn't even ''mentioned''. Justifiable, considering that [[spoiler:there seems to only be one cradle, which was severely damaged in ''Ultron'', when Vision awakened and exploded out of it violently enough to knock Thor across the room]].

* In ''Literature/InheritanceCycle'' 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''.
* The ''Literature/GreyGriffins'' books forget their phlebotinum all the frigging time. All the time. Other times they [[HoldingBackThePhlebotinum 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 [[RandomEventsPlot randomness of the plot]].
* A very subtle version of this happens in Creator/LEModesittJr'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.
* ''Literature/HarryPotter'':
** Averted in ''Literature/HarryPotterAndThePrisonerOfAzkaban''. The [[TimeTravel Time Turner]] is, with only one exception, solely used by [[{{Nerd}} Hermione]] to [[MundaneUtility take multiple classes at the same time]]. However, the perception of this as forgotten phlebotinum is a common fandom mistake: the Time Turner follows the StableTimeLoop model of TimeTravel (you can't actually change the past, only participate in it from a different perspective) and only works in half-hour increments, which isn't that useful.
** Actually, in the play ''Literature/HarryPotterAndTheCursedChild'', it is revealed that the [[TimeTravel Time Turner]] *can* alter the past, as the villain is nearly successful at using one to bring back Lord Voldemort.
** The same book also introduces the magical Marauder's Map, which shows the location and name of every person inside the Hogwarts grounds, and was created by James Potter and his friends. Despite being one of the most useful items in the series, and created by a bunch of teenagers, no similar map is ever seen to be used by anyone else.
** Sirius' two-way mirror, which is 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 [[spoiler:saved his life]].
** In ''Literature/HarryPotterAndTheHalfBloodPrince'', the potion 'Felix felices' is introduced- an incredibly powerful ''good luck potion''. It's very rare, and has negative long term effects, but you've got to be a little bit surprised that neither side thought to brew up some for any of the really big/dangerous stuff.
** In ''Literature/HarryPotterAndTheDeathlyHallows'', Harry and his friends escape from the [[spoiler: Malfoy]] 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 Creator/LarryNiven 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.
** Larry Niven formulated "Niven's Law" after realizing that the "Literature/KnownSpace" series featured no less than three indestructible elements -- the Slaver stasis field, General Products spaceship hulls, and "Ringworldium" -- Ringworld flooring material. He correctly concluded that these boxed him in by eliminating many possible storylines.
* In the Franchise/StarTrekNovelVerse, the phase-cloak seems to go through this a lot. After its introduction (and successful use) in an episode of ''Series/StarTrekTheNextGeneration'', it's largely ignored by the books (as well as later TV series). A short story in a ''Literature/StarTrekNewFrontier'' anthology eventually suggested the prototype was destroyed soon after the episode. By the time of ''Literature/StarTrekTheGenesisWave'', 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 ''Literature/StarTrekTitan'' comes round no-one's using it. Finally, in the ''Literature/StarTrekTyphonPact'' 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 Alliance Rune in ''Literature/TheMortalInstruments''. After ''City of Glass'' it's never used again, even when Shadowhunters and Downworlders are going into battle ''right next to each other''.
* Kvothe from ''Literature/TheNameOfTheWind'' learns a mental technique called Heart of Stone, which [[ExactlyWhatItSaysOnTheTin eliminates the user's emotional response]] and lets them "sit at the funeral of their sister and feel nothing". He then doesn't use it ever in the following ''three years'' he spends trying to cope with emotional trauma while living on the streets.

[[folder:Live-Action TV]]
* ''Series/RedDwarf'' is a major offender. By the end of the seventh season, the crew had access to, among other things: An inter-dimensional transporter, a virus which brings dreams to life, a portal leading to Red Dwarf, pre-radiation incident, a batch of developing fluid which allows one to enter any photo and alter time from that point, a matter transporting paddle, a DNA reconstituter, a triplicator, a virus which bestows incredible luck, and nanomachines which seem to be able to create almost anything (up to and including ''planets and sentient life forms''). None of these items are ever used or even referenced more than once (Except the luck virus, which becomes a ChekhovsBoomerang in Season 8). But worst of all, in a show where the entire plot conceit is a crew stranded in deep space, trying to get home, the Dwarfers find a ''fully functional time drive'' which allows them to travel anywhere in the universe, at any point in time. And yes, they can and do use it to go to Earth. More than once. Somehow, [[JustEatGilligan this doesn't end the show]].
* ''Series/StarTrekTheOriginalSeries'' was notorious for this.
** In "Plato's Stepchildren", the Enterprise crew discovers kironide, a [[{{Aesoptinum}} drug that gives people telekinetic powers]]. 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 original series did this many other times with Scalosian water ("Wink of an Eye"), and spores that can regenerate lost body parts, restore the human body to perfect health and give immunity to radiation ("This Side of Paradise").
** 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, like a handy-dandy anti-Borg weapon. But in ''The Next Generation'' and ''Deep Space Nine'' terraforming is a long and arduous process that yields modest results. Ultimately it's a case of NoPlansNoPrototypeNoBackup; every copy of the schematics is lost and everyone who knows how to make it is dead. Given that the Klingons considered it a treaty-violating potential planet-killing superweapon, there was probably political pressure not to rebuild it.
** The episode "The Return of Archons" shows that hand phasers have a wide-beam stun setting that can stun a roomful of people all at once. It shoots a flat beam arc at least ninety degrees wide and has a range of at least twenty meters. Despite the obvious usefulness of such a feature, it is used again exactly ''once'' in an episode of ''Voyager''.
*** Similarly, "A Piece of the Action" showed the ship's phasers could stun people on the ground and could even exclude the building in the middle of the stun area. Never seen again.
* ''Series/StarTrekTheNextGeneration'' also indulged in this far too often.
** Consider the "dimensional inverter" used in the episode "The High Ground", 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 NegativeSpaceWedgie that blocks the beam).
** We also have 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 re-materialize a previously saved version of a crew member.
** 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 and primary weapon systems. 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 twice in the first season of the show, and once in the season 4 premiere, but after that it was forgotten and only sometimes referred to, just to drop the idea afterwards (it was, however, used in one of the movies). The ''Star Trek: The Next Generation Technical Manual'' explains in a footnote that the writers never forgot about the Saucer Separation capability, and originally intended to use it as a fairly standard maneuver whenever the ship expected to go into trouble. However, what they discovered is that depicting the separation on screen and showing crew moving to the other set just took too much time during the episode to show fully, and not doing so was too jarring of a sudden transition. Hence, they used it sparingly (if at all) for pacing reasons.
** "Hero Worship" has Geordi connecting the shields to the warp drive, more than doubling their strength. Never mind the other hundred times when this would have been useful.
** The transporters were remembered. The shuttles were (usually) remembered. What was ''not'' usually remembered was that the shuttles have independent transporter units of their own, separate from the ship's (they didn't in TOS, but TNG established it).
** "Who Watches the Watchers" has Troi and Riker implanted with subcutaneous communicators, allowing two-way communication between themselves and the ''Enterprise'', which only the ground team can hear. Naturally, this technology was only ever used ''[[HoldingBackThePhlebotinum four]]'' more times over the course of the entire franchise, despite existing as early as the 22nd Century according to ''Series/StarTrekEnterprise''.
** In "[[Recap/StarTrekTheNextGenerationS6E7Rascals Rascals]]", a transporter accident turns some crew members into young children. To be sure, this was inconvenient at the time, but the [[FountainOfYouth staggering medical implications]] are never explored. The entire plot of ''Film/StarTrekInsurrection'' need never have happened if they'd just figured out how to zap people back to their twenties every time they got north of, say, fifty. Considering that they were able to reverse-engineer the accident well enough to return the crew members to their original age in the space of one episode, perfecting the treatment shouldn't have been too difficult.
*** This actually came up before. In ''StarTrekTheAnimatedSeries'' episode, "The Counter Clock Incident", the crew goes to a universe where time runs backwards and are reduced to children. Fortunately, the ship's original captain and science officer are on board and are de-aged to their prime and resolve the issue. The crew is restored via the transporter's memory.
*** A previous TNG episode had Dr. Pulaski get a mutated virus that aged her. They used a stray hair from her brush and a transporter to restore her to a condition before she contracted the illness. In other words, if you keep a healthy DNA sample, you can restore someone to any age and cure any disease.
** How about Dr. Soong's having spent decades trying to hook Data up with a positronic brain capable of sentient thought, while Geordi did the same thing with one poorly-worded request in the holodeck? Oh, they ''tell'' Moriarty that they'll look into how and why it happened so they can try to figure out a way to let him leave the holodeck... and then promptly forget all about it until he shows up again seasons later. He's understandably unhappy about it.
*** Seeing as sapient holograms are in service [[Series/StarTrekVoyager just a few years later]], it looks like someone did look into it. [[UnfortunateImplications And they're given about as many legal rights as a wad of used Kleenex.]]
** The Exocomps from ''[[Recap/StarTrekTheNextGenerationS6E9TheQualityOfLife The Quality of Life]]'' were non-humanoid robots made of off-the-shelf parts that rapidly improved themselves to the point of sentience. Despite being hugely effective and friendly (one of them performs a HeroicSacrifice to save the heroes), they're never mentioned again.
* ''Series/StarTrekDeepSpaceNine'':
** They 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 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 the easily replicable TR-116 rifle, a projectile weapon capable of shooting through walls thanks to a recent modification the episode's villain had made to it (a combination of x-ray goggles and micro-transporter).
* In ''Series/StarTrekVoyager'', 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 TechnoBabble here] to create portals, without any unpleasant StarTrekShake-inducing side-effects that we saw. The crew could have done whatever it was that he 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 AppliedPhlebotinum gave out.
** 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 ignoring the lizard-fication, "Threshold" mentions that the experimental shuttle's computers were jam-packed with detailed and helpful navigation aids and maps for the entire sector--which are never mentioned again.
*** This might just be because everyone involved was so embarrassed by the episode they declared it non-canon. Future episodes pretended it never happened.
** In one episode Seven of Nine designs Borg-tech shield enhancements, which appear on the outer hull in standard Borg green. These useful modifications vanish after this episode, never to be seen again.
** Through contact with a vengeful alien, the crew gained access to the quantum slipstream drive, which could have literally taken them home inside a hour. After the first attempt to replicate the technology was deemed too dangerous to use again, they improved it. This version worked for three minutes, then would have crashed the ship if not for TimeTravel fixing. It's discussed, at length, that the technology is stable for those three minutes. No one ever considers just running the drive for two minutes at a time, and they never use the technology again.
*** Various novels do bring the technology back however. Seems it took Starfleet's finest minds a good decade to study Voyager's preliminary tests and iron out all the kinks. A bit too much, and too great a risk to try again, for 1 ship and 1 crew while trying to survive on its own.
** 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. Only that one time...
** Throughout the series, Voyager manages to cut a collective ''30-50 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 '''26 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.
** Several Borg-focused episodes had them assimilating low-tech worlds just to swell their numbers. What makes it this trope is that cloning technology has been frequently shown in the franchise to be widespread and very cheap; if all they need is bodies they can just make some.
* ''Series/BuffyTheVampireSlayer'':
** One episode had the minion of the First Evil falsely claim he had kidnapped a Potential Slayer. Nobody thought of using the "detect a Potential" 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 [[spoiler:causing the spirit of the First Slayer to try to kill everyone involved in their dreams]], explaining its absence from later seasons.
** At the end of Season 5, Buffy uses a hammer that allows her to ''pulverize'' Glorificus, a literal PhysicalGod that had [[NoSell 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.
** Similar to the flamethrower instance from ''Series/{{Angel}}'' mentioned below, in one second season episode Buffy kills a random mook vampire by shoving a burning torch/brazier against its chest, which causes it to go up in flames like it was made of straw soaked in gasoline. Given how the series occasionally spiced up vampire-killing up with decapitation or holy water, it's a small wonder fire didn't come up more.
** Anya's power center necklace. When she turned demon again, it seemed she never needed it. You'd think, when Buffy decided she had no choice but to kill Anya to keep her from massacring more fraternities, someone might have pointed out that turning Anya back into a human would have limited her carnage potential.
** Discussion [[http://io9.com/on-the-ethics-of-vampire-slaying-in-buffy-the-vampire-s-1602090535/all here]] regarding the lack of willingness to use an established spell that returns a vampire's soul. There are consequences and complications, but the fact is that they are never discussed and the protagonists happily go around slaughtering vampires as if this ritual doesn't exist.
*** In the series it's use is presented as downright evil. The soul is pulled from the afterlife to cohabitate it's corpse with the thing that killed him, with the added bonus of carrying the memories and guilt of everything the vampire did in your skin. It was originally designed as a tool of vengeance and was recreated by the heroes once out of desperation when it was either that or the world died (poor Angel had had already had the damage done so it was more of a straight resurrection); in any other situation it's useless.
* ''Series/StargateSG1'' usually avoids this, with plenty of {{Chekhovs Boomerang}}s 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 [[ClassyCatBurglar 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.
* ''Series/StargateAtlantis'':
** By the end of the series, 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 ''Series/StargateUniverse'' (no sending a scientist back to, say, research useful ways to get help), but the [[spoiler:Lucian Alliance can somehow sneak past it]]. WordOfGod 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 ''Atlantis'' movie had it been greenlit), which addresses only some of these omissions.
** Justified with the Aterro device, an Ancient superweapon that destroys Wraith ships when they jump to hyperspace, but also causes stargates to explode when dialed. While the Avenger Program from SG-1 ''could'' have crippled the gate network and allowed the device to work safely for brief periods to help end the war with the Wraith, by the time that the Atlantis team figured out what had happened, Todd had already hijacked the ''Daedalus'' and set a course to destroy the device.
* ''Series/{{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.
** In the 8th episode "They've Got A Secret", the living ship Moya decides that her passengers pose a threat to her unborn baby. Using just one D.R.D. (a little robotic repair drone), Moya proceeds to glue Aeryn Sun to the deck - instantly neutralizing her in a non-lethal manner. In the following 80 episodes and the two hour mini-series, not one of the crew ever thinks of using this same tactic on the endless stream of hostile and dangerous beings that board the ship despite the fact that they have hundreds of D.R.D.s at their command.
* ''Series/DoctorWho'', both Classic and New Series, did this a ''lot''.
** At the end of ''The Christmas Invasion'' [[spoiler: 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, [[spoiler: the same technology (or, at least, similar special effects) seems to now be incorporated into the ''Valiant'', as seen in "The Poison Sky" so even if the main weapon was destroyed, it's not all gone to waste. At least, until "The Stolen Earth", when spoiler:the ''Valiant'' is overwhelmed and destroyed by the Daleks off-screen]].
** The TARDIS has had many features used over the decades that were completely forgotten soon afterwards: it can drift back to its owner if separated from them in time ("Revenge of the Cybermen"), it has a Space-Time Visualiser ("The Space Museum", "The Chase" and "The Moonbase"), a Hostile Action Displacement System ("The Krotons", "Cold War") and other features. Considering that the TARDIS was a museum piece even before the Doctor stole it over 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 [[TimeyWimeyBall apparently]] very bad.
** At the end of "Partners in Crime" the Doctor discards into a rubbish bin a sonic pen which opened a deadlock seal when used with his own screwdriver, one of the few locks his sonic screwdriver can't deal with.[[note]]Justified since the deadlock seal in question belonged to the pen's owner, so was probably the ''only'' deadlock seal that the pen could open.[[/note]]
** A machine that created a candy-bar-shaped equivalent to FoodPills appears once in the 1963 season and is never seen again.
** The Chula, mentioned in "The Empty Child"/"The Doctor Dances", [[spoiler: 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? One possible justification is that the Chula are implied to be [[ProudWarriorRaceGuy warriors]], with Jack's ship and the pod containing the nanogenes being a stolen Chula Wsrship and ambulance for patching up their soldiers, respectively. Such a trip might not only be too dangerous, but the Chula might not ''want'' to share this technology with anyone.
** In Fourth Doctor story "The Hand of Fear", the Doctor states that the TARDIS' temporal grace disallows violence within the TARDIS. Subsequently (and previous) stories ignored this. The Eleventh Doctor later admitted that it was a "clever lie".)
** Another Fourth Doctor story, "The Pyramids of Mars", The Doctor says the TARDIS' controls are isomorphic, meaning only he can operate it, which keeps Sutekh from having him killed. Given the number of times others have operated the TARDIS, this was either another lie or security setting.
* ''Franchise/PowerRangers'':
** Ever since Creator/{{Disney}} took over the franchise, teams have been getting [[StockSuperpowers 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/MightyMorphinPowerRangers'', this happened far more frequently, with {{MacGuffin}}s 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 MonsterOfTheWeek 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. Honorable mention also goes to the Thunder Slingers. Tommy and Jason spend an entire episode working together to get them, and then after one battle they're never used again.
** The SoLastSeason nature of the franchise causes a few instances of this in the first few series. Notably, both the Zeo Crystal and the Zeo Zord fleet from ''Series/PowerRangersZeo'' survived the end of that particular show, but nobody ever thinks to use these again after the Rangers lose their Turbo powers and Zords at the end of ''Series/PowerRangersTurbo''.
* Claire's blood in ''Series/{{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). Her 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 ''Series/KnightRider'' 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 [[InvertedTrope 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.
* The 1950s ''Series/TheAdventuresOfSuperman'':
** In an episode, 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 [[HeroInsurance property damage]]. Like the splitting power, gets forgotten from then on.
* ''Series/WizardsOfWaverlyPlace''
** In the movie, 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 anything as long as you can make up a rhyme for it. Although, the wizards lessons pretty much disappear after that one. Presumably, all the spells afterward ''were'' the improv spell, with a few exceptions.
* In one episode of ''Series/{{Angel}}'', Wesley uses a flamethrower against a bunch of mooks. 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 ''Series/{{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.
* ''Series/{{Supernatural}}''
** The Colt. Sam and Dean spend the first part of season 5 trying to recover it, only to discover that [[spoiler:it doesn't work on Lucifer]]. They never use it again, despite how it would still work on lots of the other things they fight (Lucifer says he's one of only four beings in existence it can't kill, Michael and God presumably among them). They do use it when they [[TimeTravel travel back in time]] to kill a phoenix in season 6, though, so it's not ''completely'' forgotten. Forgetting it is justified in at least some cases since there are knives that can kill angels and demons as well, but what about the Leviathans?
** In "Holy Terror", Sam is possessed by an angel, and Dean needs a way to get it out. Kevin comes up with a ritual that will temporarily give Sam control, allowing him to expel the angel. This doesn't work, but not due to any problem with the spell -The angel merely found out about it beforehand and sabotaged the ritual. One episode later, in "Road Trip", Dean and Castiel capture the angel, still in Sam's body, and need a way to expel him. The only idea they can come up with is a [[DealWithTheDevil deal with Crowley]], that he enters Sam's mind and explains the situation to him in exchange for his own freedom. Except, now that the angel can't interfere, there was nothing stopping the original -and much safer- plan from working. [[IdiotBall So you've got to wonder why they didn't just do that instead]].
* ''Series/BattlestarGalactica2003'':
** In the middle of season 2, Roslin's cancer takes a turn for the worse, and she's [[DeusExMachina saved at the last minute by the unborn Hera's blood]]. Now it's likely that Roslin is not the only one in the fleet with cancer (indeed, the season 4 episode "Faith" involves another character with terminal cancer). Yet no one even suggests the possibility of using Hera's blood to cure other cancer patients (or to try it on people with other kinds of terminal illnesses, for that matter). Even more ridiculous is when Roslin's cancer comes back in the season 3 finale, the question of using Hera's blood to cure her again is brought up only once (and ignored) by a reporter. This is also [[TheyWastedAPerfectlyGoodPlot wasted plot potential]], since they could have done an episode about the ethics of regularly harvesting a baby's blood for medical purposes.
** The Blackbird fighter. 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.
* ''Series/TheBigBangTheory'' 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-optional social convention''". Not only does it work, but he immediately accepts it without argument. This has been used a grand total of once in the show's history.
* Every once in awhile this will happen in ''Franchise/KamenRider'' due to the Heisei Series being more toyetic.
** ''Series/KamenRiderX'': In Episode 2, Jin Keisuke uses a two-barrel blowdart gun that manages to make short work of GOD minions AND stop the Kid Of The Week from being lynched. In spite of it looking like the coolest thing ever, it is never seen again.
** ''Series/KamenRiderKabuto'' had the ZECT camera which could denote Worms from ordinary humans. That would've been useful later, but instead, characters had to find out who the worm was the hard way. It was completely forgotten after a few episodes.
* ''Series/OnceUponATime'':
** The [[BigBad Evil Queen]] in has the ability, at least in the Enchanted Forest, to [[AndShowItToYou pull a person's heart out]] yet leave them not only alive, but subject to her will. It's implied or explicitly stated that she's done this many times, yet she has to send Snow White into the woods with the Woodsman to get him to cut her heart out mundanely, rather than doing the job herself and turning her hated step-daughter into a slave and puppet ruler.
** [[TheHero Emma]] has the self-proclaimed superpower of being able to tell if someone is lying, yet when she is questioning [[spoiler:Greg Mendell about whether he saw Rumplestlitskin use magic]], although she has every reason to need to know, she can't tell that [[spoiler:he]] is lying through [[spoiler:his]] teeth. Although Snow did [[LampshadeHanging lampshade]] the fact that her ability is inconsistent, particularly when she's emotionally compromised, implying that Emma's claims of her "superpower" are somewhat exaggerated and/or really worked best during her bailbondperson job, when she was was confident and in control of the situation. She is significantly less in control of things once the magic is revealed, and she's the new kid on the block trying to figure everything out.
** TrueLovesKiss cures all curses, yet Snow White and Prince Charming occasionally go looking for curses that have befallen one or the other.
* The spirit board from ''Series/{{Charmed}}'', which was used for all of three episodes before disappearing in season three until it finally resurfaced for one last use in season 8.
* In ''Series/TimeTrax'', in some episodes the fugitives, after the requisite dose of TXP, were sent back immediately after S.E.L.M.A. emitted a "transmission tone", but in others, the paralyzed fugitive's body had to be hidden until a personal ad was placed in a previously agreed newspaper, so TRAX could retrieve the fugitive. (That without taking into consideration the oft-repeated assertion that [[TimeyWimeyBall the changes in the past wouldn't affect the future because Darien Lambert and the fugitives were sent to a parallel timeline]].)
* ''Series/AgentsOfSHIELD'': In the pilot, Fitz and Simmons create a special version of the [[StunGun I.C.E.R.]] that fires out of a sniper rifle. The weapon proves exceptionally effective, allowing Ward to subdue the superpowered VillainOfTheWeek without ever having to get close enough for the villain to use his powers against him. In fact, he never even realized Ward was there! Unfortunately, nobody ever seems to remember this when they need to non-lethally subdue threats in future episodes. While the I.C.E.R. itself becomes part of S.H.I.E.L.D.'s regular arsenal, they're all handguns, which have considerably less range and leave the user much more vulnerable.
* ''Series/BlakesSeven''.
** "Project Avalon" states that the Federation MindProbe is an infallible method of interrogation. That doesn't stop ColdBloodedTorture being used in later seasons, despite being less efficient.
** In the TwoPartEpisode that introduces Orac, the MagicalComputer can extrapolate the future and destroy an enemy vessel by hacking into and detonating the missiles it carries. Neither of these extremely useful abilities are ever mentioned again.
** In "Cygnus Alpha", Avon and Jenna discover the Liberator is carrying a vast fortune in jewelry, more than in the entire Federation banking system according to Avon, who [[LeaveYourQuestTest urges Jenna to take the money and run]], saying that Blake will just use it for his HopelessWar. But we never see Blake using it to finance LaResistance (except for a few crystals in "Shadow"); in fact several future episodes are based on TheCaper in order to steal for the revolution (or [[IfIWereARichMan make our anti-heroes rich]]).

* The ability of various ''Toys/{{Bionicle}}'' characters to [[FusionDance 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 BuiltWithLego). 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 [[MerchandiseDriven the toys said so]]).

[[folder:Video Games]]
* ''VideoGame/Borderlands2'' has a standard-fare New-U ([[TradeSnark TM]]) Respawn system. For the most part, it's [[GameplayAndStorySegregation ignored]]; all characters are [[KilledOffForReal killed off for real]] within the game's plot... with the sole exception of the "[[ExactlyWhatItSaysOnTheTin Kill Yourself]]" sidequest, in which the player must use the New-U to collect their reward. The game's developers regretted this so much that DLC New-U machines would sometimes say "This respawn is not canon" when resurrecting the player.
* The [[WaveMotionGun 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 [[KillItWithFire flash-packs]] from ''Wing Commander IV'' isn't mentioned anywhere at all in later ''VideoGame/WingCommander'' games, as if the tech has vanished.
* ''VideoGame/WorldOfWarcraft'' 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.
* Crops up in ''VideoGame/CityOfHeroes''. Interestingly, your character is often the ForgottenPhlebotinum; for example, [=NPCs=] often caution you to "be careful, these guys are dangerous!", even after your character has defeated monsters, giant robots, and demigods.
* One of the most notorious parts of ''VideoGame/MegaManClassic'' lore: what happens to the weapons he obtains from the Robot Masters? In every game, he receives several invaluable weapons and tools, but by the time Dr. Wily threatens the world for what feels like the hundredth time, Mega Man has discarded them. Among other amazing weapons, you'd think he'd keep something like the Ice Slasher[[note]]A ''VideoGame/MegaMan1'' weapon capable of freezing enemies, even suspending them in ''mid-air''.[[/note]], Atomic Fire[[note]]A ''VideoGame/MegaMan2'' weapon, and likely the most powerful (and energy-consuming) fire weapon in the Classic series when fully charged.[[/note]], Metal Blade[[note]]Also from ''Mega Man 2'', and arguably one of the most useful weapons in the entire series. Could be considered a GameBreaker, as it can be used over 100 times before running out of energy, fired in any direction and is effective against numerous bosses and enemies.[[/note]], or the Flash Stopper[[note]]A ''VideoGame/MegaMan4'' weapon that emits light so bright most robots' circuits seize up briefly. It's similar to ''Mega Man 2'''s Time Stopper, but while that actually [[ExactlyWhatItSaysOnTheTin stopped time]], Mega Man couldn't shoot while it was in effect, unlike the Flash Stopper.[[/note]] on hand for when Wily predictably reappears, but no. Especially groan-worthy when you realize that he sometimes has to obtain these weapons for the second, third, fourth, or ''(rarely) FIFTH time'' in later games (such as the arcade and Game Boy titles). Some adaptations (such as the [[ComicBook/MegaMan Archie Comics series]]) try to HandWave this.
** Also extends to other games in the franchise, such as the [[VideoGame/MegaManX X series]]. Here, it's even worse as Mega Man X loses entire suits of his own powerful upgraded armor. But could be justified given the [[ReluctantWarrior personality]] [[TechnicalPacifist of the protagonist]] and that it was shown he usually [[VdeoGame/MegaManX6 disposes of the armors]] after the crisis.
** Surprisingly [[AvertedTrope averted]] in ''VideoGame/MegaManLegends'', where the BagOfSpilling causing Mega Man to lose all his good gear from the first game is [[JustifiedTrope justified]] at the beginning of the second game: [[WrenchWench Roll]] had to sell off all his gear to afford to repair their ship back to top condition.
* In ''VideoGame/{{Starlancer}}'', a big part of the storyline is the development of portable warp drives for fighters and capital ships that allow them to zip around the Solar System with ease and without detection. Cut to ''VideoGame/{{Freelancer}}'', taking place 800-900 years later, and that technology is nowhere to be found. Yes, there are cruise engines (basically, really powerful afterburners) and starlanes, but those are limited, and ships using them can be detected and intercepted. Heck, even the intro shows the Coalition fleet making a DynamicEntry by warping in and immediately opening fire on the surprised Alliance ships.
* In ''VideoGame/MetalGearSolid4GunsOfThePatriots'', the [[WarForFunAndProfit War Economy]] has made oil and gas "as valuable as diamonds". Guess [[VideoGame/MetalGear2SolidSnake OILIX]][[note]]Synthesized petroleum that's as good as the real thing.[[/note]] never really caught on, then.
* The ''VideoGame/StarTrekOnline'' mission "House Pegh" has the eponymous Klingon [[InformedAttribute black-ops team]] in possession of a wide-area cloaking device capable of hiding ships in formation from Iconian sensors. [[SarcasmMode Surely this is a brilliant advance that will change the course of the war!]] (Nope, never mentioned again, which amazingly is one of the ''lesser'' reasons the mission is considered an IdiotPlot.)

* [[InvokedTrope invoked]] in Webcomic/ThePrincessPlanet, Christi has a magic wand that can do anything, but has only ever used it twice because she prefers the challenge of [[GuileHero outwitting her opponents]].
* Early in ''Webcomic/TheOrderOfTheStick'', 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.
** [[JustifiedTrope Though a careful reading of the D&D rules establishes that the spell wouldn't actually have worked on Xykon, and it's possible that Durkon figured this out in the meantime.]]
** 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 ''Webcomic/SluggyFreelance'', an early StoryArc 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 TimeTravel having more DeusExMachina potential than anything else. Though considering how much trouble they get into dealing with {{Alternate Dimension}}s and how big a mess they caused with the last time-travel jaunt, even [[MadScientist Riff]] would hesitate to use it.
* A particular device in ''Webcomic/SpecialSchool'' is ''designed'' to make people (except certain psychics) to forget all about it.

[[folder:Web Original]]
* As pointed out in ''[[WebVideo/YuGiOhTheAbridgedSeries Yu-Gi-Oh!: The Abridged Movie]]'', Kaiba never used the cards given to him by Pegasus ever again, even in episodes set later, and 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.)
* In the first episode of ''WebVideo/ChadVader'', 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.
* ''Literature/{{Phaeton}}'' has Trayen, who often gets caught up in battle and forgets just what he's really capable of. And Teliha isn't very good at remembering her spells either.
* Chuck Sonnenburg of Website/SFDebris loves to call out ''Franchise/StarTrek'' for forgetting that the ship has shuttlecraft, and was stunned when ''Series/{{Firefly}}'' '''didn't''' (from his review of [[http://sfdebris.com/videos/firefly/firefly7.asp "Out of Gas"]]).
--> '''Chuck:''' They cancel ''this'', and let ''[[Series/StarTrekEnterprise Enterprise]]'' run for four seasons?

[[folder:Western Animation]]
* Events surrounding the second season finale of ''WesternAnimation/AvatarTheLastAirbender'' left many [[http://mad-sniper.deviantart.com/art/Forget-Something-43181126 wondering]] why Katara, who had theorized that her [[ChekhovsGun 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 TragicHero Jet a few episodes beforehand. [[WordOfGod The DVD commentary]] has them admit that they forgot it, [[AuthorsSavingThrow but said]] [[HoldingBackThePhlebotinum it wouldn't have worked anyway]].
* In ''WesternAnimation/{{Ben 10}}'', Ben's EvilCounterpart 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 [[Series/{{Heroes}} Sylar]] and [[Manga/InuYasha Naraku]].
* ''WesternAnimation/Ben10AlienForce'':
** Kevin's powers are even more limited, as he can only absorb the properties of solid matter. This 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 [[WithGreatPowerComesGreatInsanity makes him go insane]], which usually makes it a bad idea.
** The [=DNAliens=] are all [[TheVirus 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 SeasonFinale.
* Incredibly, ''WesternAnimation/Ben10UltimateAlien'' appears to have forgotten about the Ultimate forms, which ''give the new show its title'' in one of its first episodes.
* ''WesternAnimation/{{Birdman}}''. The title hero can only recharge his solar powers in sunlight. This means that in almost every episode (exceptions: "The Menace of Dr. Millenium", "The Deadly Duplicator" and "Versus the Speed Demon") he runs out of power, is captured by the villain and he has to re-expose himself to the sun's rays to regain his powers. In 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.
* In the ''WesternAnimation/DungeonsAndDragons'' episode "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. [[FailureIsTheOnlyOption Naturally, they are forced to turn around]], at 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. Even considering the source, if there was ever an episode to throw on the FanonDisContinuity pile ''anyway''...
** The episode "In Search Of the Dungeon Master" introduces a "Know Tree," a talking tree who claims to know "everything there is to know," including how to get the kids home. However, when Dungeon Master is kidnapped, the tree instantly becomes aware of it and gets so spooked that he clams up and stops talking. Still, we never see the kids return to try asking him ''again,'' once things have calmed down. Heck, it's implied that he isn't even the only Know Tree around.
* ''WesternAnimation/TheFairlyOddParents''. 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.
* In ''WesternAnimation/{{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?
** The day Nixon was elected, he got himself a gargantuan robot body complete with integrated rocket launchers. [[WhatHappenedToTheMouse Where did it go?]]
* ''WesternAnimation/GIJoeARealAmericanHero'' was particularly bad about this. C.O.B.R.A. would develop a new superweapon, and the new superweapon would be destroyed by the G.I. Joe team. Just imagine if C.O.B.R.A. were to use more than one of the same weapon at the same time, or if it were to use several different superweapons at the same time.
* In the original ''WesternAnimation/MyLittlePony'' the Rainbow of Light was the ponies' best weapon and said to be one of the most powerful magic artifacts in Ponyland. The number of times it was used can be counted on one hand; against Tirek, against Catrina, against the Witches of the Volcano of Gloom and the Smooze, and to clear away rain clouds so they could pick cherries. After that, it was never seen nor mentioned again, save for once when Megan remembered they had it and planned to use it, only to drop it where she couldn't get it back and had it returned to her after the threat had been defeated.
* ''WesternAnimation/MyLittlePonyFriendshipIsMagic'': Season 1 is full of this. The Elements of Harmony ([[WhatHappenedToTheMouse 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. Season two turned ''all'' of these and more into {{Chekhovs Boomerang}}s... for, in most cases, exactly one episode each, after which they all went back to being forgotten if RuleOfDrama requires it. By the end of season 2 the teleport spell was casual use. Some of these were due to an instruction from Hasbro to avoid ContinuityLockout, since they wanted to be able to broadcast and syndicate the episodes in any order. That doesn't explain late-Season-1-Twilight's aversion to teleportation, though.
* ''WesternAnimation/TheSimpsons'':
** The Season 2 episode "Simpson and Delilah" reveals a new drug, Dimoxinil, which is shown to immediately cure baldness and grow a thick mane of hair just overnight. Homer uses it, becomes promoted and heavily respected at work, but loses the hair and his respect after Bart accidentally spills the Dimoxinil. After this episode, it is never seen or mentioned again, despite the fact that such a product would change the medical landscape due to actually curing all baldness. Homer is seen caring about his baldness in future episodes, despite the fact that baldness should be a thing of the past in that universe now. Somewhat justified, in that Dimoxinil was extremely expensive ($1000 for a 6 month supply), but you would think such a thing would at least be TALKED about.
** The Season 3 episode "Brother, Can You Spare Two Dimes?" saw Homer's half-brother Herbert invent [[AppliedPhlebotinum a device]] that translated baby-talk into intelligible English phrases. During the episode, this actually allows Maggie to stop being [[TheVoiceless the voiceless]] and actually interact with the rest of the cast. The device is never mentioned again, which is surprising from a show that frequently [[LampshadeHanging lampshades]] its own reliance on StatusQuoIsGod.
** Season 6's "Grampa vs. Sexual Inadequacy" had Grandpa Simpson make a aphrodisiac tonic which helps Homer and Marge's sex-life, which they start selling to the rest of Springfield. That tonic sure would've helped in later episodes where Homer and Marge's sex-life was struggling, e.g. season 9's "Natural Born Kissers".
* ''WesternAnimation/SpaceGhostCoastToCoast''. 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.
* A major problem with ''WesternAnimation/StarTrekTheAnimatedSeries'' 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 was that it was cheaper to animate a glowing outline than it was to draw spacesuits on everyone.
* Justified in ''WesternAnimation/StrokerAndHoop'' 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?
* ''Challenge of the WesternAnimation/SuperFriends'' is notorious for this trope.
** ComicBook/LexLuthor [[CutLexLuthorACheck invents]] teleporters, time machines, cloaking devices, a gizmo that sucks the GreenLanternRing 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. [[MakeWrongWhatOnceWentRight 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 Franchise/GreenLantern 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! (On the other hand he's on a team with an ex-Green Lantern who got busted abusing his powers. Maybe he's playing it safe.)
* ''WesternAnimation/TeenageMutantNinjaTurtles2003'': 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. Then again, it could be the Turtles and Splinter ''were'' supposed to spend some time in the future.
* ''Franchise/{{Transformers}}''
** Waspinator's EyeBeams that he shoots at Cheetor with in the ''WesternAnimation/BeastWars'' pilot. He never uses them again in combat, even in the Season 2 opener, when Cheetor knocks his gun out of his hand and those EyeBeams would have been a nice alternative to running away. He did use them at one other time, when he and Terrorsaur (who was also using EyeBeams) were trying to cut into Tigatron's stasis pod, but as a general rule 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.
** ''Franchise/TransformersGeneration1'' had both sides constantly creating weapons that would be a GameBreaker in the hands of non-idiots. Instead of being used for what they [[MisappliedPhlebotinum 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''' [[NewPowersAsThePlotDemands one-shot powers displayed by individual Autobots]] that would never be used again. (Most iconically, the PureEnergy [[EpicFlail flail]] and [[AnAxeToGrind axe]] used by Megs and Prime, respectively, in the series premiere only and never again.) They also suffered from NoPlansNoPrototypeNoBackup: 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.
* ''WesternAnimation/WinxClub'':
** 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. (However, 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 [[spoiler:Nabu]] 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 SoLastSeason, 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...)
* ''WesternAnimation/XiaolinShowdown''
** The Shard of Lightning 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, but never use it again.
** The Golden Tiger Claws, which 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 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.
** The Emperor Scorpion, a Shen Gong Wu that can control ''any'' other Shen Gong Wu. After being used to [[spoiler:defeat four Mala Mala Jongs (giant demons made of Shen Gong Wu) at once]], 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 "The Black Vipers", Jack is seen flying away after being deceived into losing a bag full of Shen Gong Wu. You can see the Glove of Jisaku on his hand as he flies away. The Glove of Jisaku has the power to attract other objects - even other Shen Gong Wu. He even used it to steal a bunch of Wu at once in an earlier episode. Yet he doesn't use it here.

[[folder:Real Life]]
* There are some inventions and compounds from the ancient times that we know that existed, but we can't replicate, even with modern techonogy, either because recipe or plans were lost over time, or because the required resources are used up. [[https://www.youtube.com/watch?v=e8u1RJ7obFk This video]] shows seven of them: heat-ray weapon made by Archimedes, Roman concrete, Silphium, Uunartoq discs (or Viking compasses), flexible glass, Damascus steel and Greek fire.
* The mortar used for the Great Wall of China - which has lasted for hundreds of years - also used to fall into this trope [[spoiler: until recently, when scientists discovered that the secret was...sticky rice!]]