Forgotten Phlebotinum

"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 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."

— Alt Text for Dinosaur Comics, 9/18/08

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

This trope does not necessarily denote bad writing; it even can be essential to the story. After all, real people make mistakes all the time and don't always do everything optimally. If the protagonists have some piece of phlebotinum that makes them nearly or actually invincible, many plots have to be thrown out the window. Some would call this unwillingness to change the Status Quo and then adapt to the new order of things "lazy", but when one is working on a regular series, changing the status quo, interesting though it can be dramatically, is not something to be done lightly. When it's a Shared Universe this is even more the case.

Larry Niven 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, there was a Technology Erasure Event, or there really were No Plans, No Prototype, No Backup — 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, Higher-Tech Species 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 Forgot About His Powers. If it is remembered, but there's some excuse as to why it isn't used or won't work again, that's Holding Back the Phlebotinum or It Only Works Once. If they acknowledge it but don't have it on their persons, it's We Have Forgotten the Phlebotinum. If they (finally!) remember to use it in the end, it's a Forgotten Superweapon. When it's not forgotten and is used in a later episode because a writer wants to acknowledge continuity, it is Chekhov's Boomerang. Compare also with Never Recycle Your Schemes in the case of villains.

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Examples:

Anime & Manga

• A Certain Scientific Railgun: There are a few things that only come up in the spinoff once or twice before being discarded.
  • 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. Despite its obvious use to the more ruthless elements of Academy City, it has only been brought up again once, when Gensei Kihara makes use of a small esper network during the Daihaseisai arc. The smaller size of Gensei's network also appears to avoid one of the bigger issues with the original, that is, the creation of the AIM Burst.
  • "Capacity Down" is a Brown Note in that causes immense physical and mental distress in espers, leading them to collapse and unable to use their powers. This is used once in Railgun by a non-esper villain against the esper heroes. Academy City has AIM Jammers that can similarly be used to supress espers, but they are limited to being static security measures for esper prisons and other important places, while Capacity Down's mobility advantage is never used beyond the arc that introduced it.
• In Cowboy Bebop, the crew seems to conveniently forget that they have a nigh-omnipotent hacker on board who could take over other ships at will in her initial appearance but never, ever does that again, even though that would not exactly be the least effective way to catch their bounties. It was particularly bad in the episode where the villains are immobilizing ships through a computer virus. Gee, if only they had someone on board who could counter that...
• In Digimon Adventure, there were several instances in which Gomamon's Marching Fishes technique could have come in handy, especially since he was seen in the first episode carrying the entire group down a river on the fishes.
• Digimon Frontier: In one episode, Takuya learns that being a legendary warrior also gives him the power to control/predict the weather which he can use to increase his attack strength. After that one episode, it never comes up again.
• This is not uncommon in Doraemon. There are several predicaments that Doraemon and co. face that one of his gadgets that have been mentioned in previous episodes could have easily get them out but for some reason Doraemon seems to have to use the gadget that was introduced in episode they were in. One the early gadgets of the week (chapter 54, "Lies Become Truths") was a beak-like toy which one could wear, and anything uttered while using it will be spontaneously proven as fact. Nobita lied that his father can shatter a huge rock with his bare hand, and then he can do it with ease. Quite frankly this should have made 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 Fullmetal Alchemist, Dr. Marcoh reveals he has developed a way to destroy Philosopher's Stones, and uses it to great effect against Envy. Yet Marcoh never uses it again.
• JoJo's Bizarre Adventure:
  • Battle Tendency: It's shown that Germany had the technology to turn people into extremely powerful cyborgs, even those who have suffered deadly wounds like being blown up by a grenade. This is rarely referenced afterward, save for two characters' mechanical hands. When one of those characters loses their legs, the idea of them ever being replaced with cybernetics is never brought up.
  • Stardust Crusaders: Polnareff's Silver Chariot had forgone its high defense and high-speed afterimages after he was freed from DIO's control.
• Mazinger Z: Often it was played straight. Many times Dr. Hell came up with a 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. However, sometimes Dr. Hell reused and improved some strategies or weapons, or deceived the enemy in believing he was using the same trick.
• In Mobile Suit Gundam Wing, 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.
• In The Nightmare Before Christmas: Zero's Journey, Lock, Shock, and Barrel don't bring their walking bathtub, despite that their ulterior motive to go to Christmas Town was to steal candy and toys while searching for Zero.
• Sailor Moon:
  • 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 Naru being attacked by the Monster of the Week and goes to save her. This power is never shown again in later episodes, even though it would probably have come in handy. Similarly, in the first chapter of the manga, the costume included a mask in which she could see the monster attacking Naru by looking in the goggles. The goggles were quickly phased out in the manga, last seen in one transformation sequence where Usagi discarded them as she transformed, though why this happened was never explained. Considering that the monsters of the week/chapter were usually close by anyway, this power wasn't really that necessary in retrospect...
  • Another ability her hairclips had was turning Sailor Moon's crying into a sonic attack. This was only used twice, in the first episode and the Super S episode with the toothpaste monster.
  • 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.
  • Sailor Moon S could also be called Sailor Moon FP. Much of the conflict wouldn't have occurred if the Senshi simply remembered the skills and devices they have that would've been useful in a number of situations.
    • In the fifth episode, Protect the Pure Heart! A Three-Way Battle, Ami uses her mini supercomputer to track Unazuki's Pure Heart Crystal after it had been stolen from her. Ami doesn't use her supercomputer again in this season, in spite of how useful it could have been in tracking the Talismans.
    • In one episode, Rei divines using the Sacred Fire to try to uncover the answer behind her strange premonitions. Why she couldn't have done this to find out more about the Talismans is never stated.
    • After Chibi-Usa's Pure Heart Crystal is stolen by a possessed Hotaru, the evil entity controlling her was revealed to be Mistress 9, not Sailor Saturn, and the other Senshi realize that Hotaru had been possessed by a Daimon the whole time. It would have been logical for Sailor Moon to use the Silver Crystal to free Hotaru from Mistress 9's possession, as she had used it in the previous seasons to return the carriers of the Rainbow Crystals to their human forms, freed Mamoru from Metallia's control, purge the evil from the Spectre Sisters and turn them into normal humans, and purged the evil from Black Lady and turned her back into Chibi-Usa. Purging evil from a person or place has been a power of Sailor Moon's since the beginning and it's a power that doesn't drain her of her energy, so it would stand to reason that this would be able to save the world and defeat the enemy without sacrificing Hotaru. Instead, the Silver Crystal isn't even mentioned as a possible option in defeating the Death Busters or saving Hotaru, with its last mention being the second episode of the season where Usagi worried it had lost its power after her transformation came undone. It was revitalized by her and Mamoru's love and allowed her to transform again after her brooch became the Cosmic Heart Compact, but it's never mentioned for the healing powers it is known for, resulting in a lasting dilemma over whether Hotaru can be saved or not.
    • Even more frustrating, after Sailor Saturn awakens, she rejects Sailor Moon's assistance because without the Holy Grail, she couldn't become Super Sailor Moon which she claims would've been useful to her. This, again, ignores the Silver Crystal which was stated to have the capacity to destroy an entire planet (the same capability as Saturn's power) or heal it, and was much more powerful than the Holy Grail.
• Sailor Moon Crystal: Mamoru collects the remains of the Four Kings of Heaven with which he's able to summon their spirits for counsel. He only does it once.
• Sgt. Frog: 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 after the earthquake of March 2011.
• In Super Dimension Fortress Macross, we see that the Humongous Mecha piloted by humans have head-mounted cannons - that got used exactly four times during the entire show. Only twice as a weapon - both of the other times they were used as cutting tools. Thankfully, the head cannons see more consistent use in later Macross series.
• Tekkaman Blade gives us the Hi-Coat Voltekka, an upgrade to Blade's Voltekka. It's used once in it's introduction episode to defeat Evil and then again three episodes to shoot down a nuke. Then the time skip happens and it's not seen again.
• Tengen Toppa Gurren Lagann: The Gurren Lagann is equipped with a powerful Deflector Shield, which stops Combat Pragmatist enemies from attacking it in the middle of its formation, as well as stopping any powerful attacks thrown at it. It only appears in episode 3 and is promptly forgotten for the rest of the series, where it could have been very useful. (It reappears in the Compilation Movie, however.)
• Yu-Gi-Oh! 5Ds:
  • 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.
• YuYu Hakusho
  • 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.

Comic Books

• 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.
  • The Silver Age had tons. For example, there is Action Comics #252, an issue otherwise better known for being Supergirl's first appearance. In the lead-in story, though, Superman is being menaced by kryptonite, and he escapes by melting it with his heat vision, at which point he learns the liquid kryptonite is no longer harmful. (In a real head against wall moment, he even says that it's because when items change their state, they lose other properties, like how ice, when it melts into water, stops being cold. Superman forgets that liquid kryptonite is harmless to him thereafter, and in fact, liquid and gaseous kryptonite are shown being harmful to him in later stories. So maybe he found the only chunk of kryptonite in the universe that would be harmless to him if it were liquid.
  • In those days, Superman also collected all manner of exotic gadgets in his Fortress of Solitude, in addition to all the Kryptonian gizmos in the Bottle City of Kandor. Generally, DC Comics's Superman continuity cop (and world's biggest Superman fanboy) E. Nelson Bridwell was the only writer who consistently remembered what a fantastic array of machines Superman had access to.
• Shows up in one of the many Avengers stories (the relaunch with Kurt Busiek). Justice, sidelined with a broken leg, goes on an Archive Binge and realizes that the best way to defeat an Adamantium robot on a homicidal rampage is with Antarctic (type B) Vibranium (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 Ultron's rampages, some of the finest minds (Stark, Pym et al) in the world never linked the "really tough metal" and "destroys metal on contact" dots together.
• X-Men:
  • Rogue can't have a relationship with anyone due to her powers. Gambit's mutant powers used to allow him to touch her without an issue, which has since been forgotten. In addition, there have been numerous items that temporarily disabled mutant powers, many of which have been captured by the X-Men, but are never mentioned in relation to this issue. 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".
  • Cyclops, during a plan to infiltrate 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.
• Spider-Man:
  • Peter makes a gas mask for himself that is completely concealed by his Spider-Man suit; he uses it once and never again, even though he is regularly hit with gas attacks several times during the early run of the comic.
  • The loathed One More Day storyline. Aunt May is dying (well, she's only been in her mid-80's for a few decades now, but she was actually injured). Subverting this trope, Peter scours half the mainstay Marvel cast looking for someone that can heal her. Playing this trope straight, nobody can. There's very, VERY thinly implied instances where it's the fact that she's already so old and frail that conventional medicine can't heal her, but considering the fact that those who he approaches include the X-Men (who had no fewer than 3 people at the time whose powers could explicitly heal any wound), Reed Richards, and Doctor Strange - who is both the Sorcerer Supreme and a former neurosurgeon, there's no reason that SOMEONE couldn't have helped him before he ended up letting Mephistopheles wipe out the entire history of his marriage in exchange for Aunt May's life.
• At the end of Volume 2 of 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.

Fairy Tales

• Arabian Nights: 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 "The Two Brothers", collected by The Brothers Grimm, 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.

Fan Works

• Calvin & Hobbes: The Series has the MTM (whose name stands for Mini Time Machine), whose original intent as a smaller Time Machine is forgotten in favor of his Do-Anything Robot nature. This is eventually lampshaded in "Nocturnals".
• Paradoxus: The five time-travelers might need reminding about the Memory Stones. The paradoxes preventing them to achieve their goals may have been still in place, but it would've been far easier and war budget-friendly to use the Memory Stones presented in the seventh season of Winx Club to get to the past. Nevertheless, it's justified since the fic started to be written at a time the seventh season wasn't even rumored, and thusly, it would have been a pain for the authors to change such a key Plot Point in favor of humoring the new canon.
• Averted in A Voice in the Wilderness. 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.

Films — Animated

• A rare villain example in The Nightmare Before Christmas: Oogie Boogie has the ability to suck in everything like a gigantic vacuum, which is how he recaptured Santa and Sally. He never thought to use this in his battle against Jack Skellington.

Films — Live-Action

• Star Trek:
  • In Star Trek: First Contact, the invading Borg are able to create a "temporal vortex" to travel back in time to the 21st century. At the end of the movie, the Enterprise is able to easily recreate this effect to travel back to their own time. This method of time travel seems easier and much safer than the other established method of slingshotting around a star at warp 10, but it's never mentioned again. And if the Borg posses the technology to Time Travel at will, why not use it to undo all their failures? In particular, they could have averted their Hopeless War with Species 8472.
  • Star Trek: Nemesis:
    • 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.
    • 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).
    • Writers constantly forget that phasers can be programmed to fire on their own ("The Game") and that an overloading phaser makes a passable hand grenade. Also that it's possible to beam a photon torpedo aboard an enemy ship with a transporter. Any one of these could have been used to destroy the Scimitar without requiring Data to make a Heroic Sacrifice.
  • In Star Trek (2009), 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 Star Trek Into Darkness, 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.
  • Star Trek Into Darkness: It is revealed late in the film that an augment's blood can resurrect the dead, and is subsequently used to heal Kirk from fatal radiation poisoning (after a point in which everything but his higher brain functions have failed). This is a pretty significant discovery to have lost for two hundred years. At the end of the movie, Starfleet just refreezes Harrison/Khan and locks him and the other augments away.
• The "throwing 'S' shield" in Superman II. During a fight which occurs just before the climax, Supes rips off a copy of the 'S' emblem on his chest and uses it to temporarily incapacitate Non (one of the escaped Kryptonians). It's never explained how Clark does this, and he never uses it again in the following films. It sure would have come in handy against the evil Clark or Nuclear Man, even if it was a cheap-looking effect.
• Marvel Cinematic Universe:
  • In Iron Man, Tony Stark uses a small missile to easily obliterate the military tank that has just knocked him out of the sky. A small magazine of these would be incredibly useful in many situations in future movies, but apparently, he forgets to include more than one per suit.
  • In Avengers: Age of Ultron, 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 War Machine after a serious fall cost him the use of his legs in Captain America: Civil War. Heck, the movie even has a scene where doctors are shown examining him after the accident, and Helen isn't even mentioned.
  • Iron Man 3 prominently features Extremis, which can heal almost any injury short of decapitation or a headshot. It has the minor drawback that it might make you spontaneously explode, but Tony fixes that issue at the end of the movie, and uses it to remove the shrapnel from his chest. It is never referenced again, even when it could be used to heal Rhodey in Civil War, and he doesn't seem to have any kind of healing factor in any of the other movies.
• Star Wars: In the epilogue of The Empire Strikes Back, Luke Skywalker gets a like-OEM prosthetic hand installed to replace the one Darth Vader removed in their duel. This is apparently the level of medical care that is available to plucky ragtag rebel groups within a week of the Galactic Empire kicking their asses at Hoth. Which has the effect of making Padmé Amidala's prophesied Death by Childbirth in Revenge of the Sith as motivation for Anakin seem like she never visited a doctor the entire time she was pregnant: by precedent it would be surprising if anybody in her social class ever even had a bad cold that century.

Literature

• In Animorphs, Elfangor forgot he could morph to heal himself from his crash-induced injuries. This leads to his death.
• In Inheritance Cycle Eragon learns that Brom's ring contains a massive store of magical energy, enough to rip castles apart. He keeps outright, explicitly, forgetting that he has it.
• The Grey Griffins books forget their phlebotinum all the frigging time. All the time. Other times they hold it back. Max can sense portals and enter them... wait, now he can't, except when he suddenly needs to warp into one much later. Max has a pet "spriggan" that he cares deeply about. Where'd it go, and how come neither Max nor the book cares? You get the idea. Contributes to the randomness of the plot.
• A very subtle version of this happens in L.E. Modesitt Jr.'s Spellsong Sorceress cycle. In the first chapters of the first book, a spell is cast that teleports the main character in from Earth. It's implied that although the lady casting this spell isn't a very strong sorceress, she can still send people to locations halfway across the continent with a bit of help. This use of magic is never mentioned again, despite the fact that it would be tremendously useful in a variety of circumstances.
• Harry Potter:
  • Averted in Harry Potter and the Prisoner of Azkaban. The Time Turner is, with only one exception, solely used by Hermione to 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 Stable Time Loop model of Time Travel (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.
    • It would still be immensely useful for spying, though - had Umbridge or the Inquisitorial Squad possessed one in Order of the Phoenix, Dumbledore's Army probably wouldn't have lasted a day. Indeed, it's probably for this reason that Rowling had the entire stock destroyed during the climactic battle in Phoenix, just as the series was accruing more and more Spy Fiction elements.
  • 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 saved his life.
  • In Harry Potter and the Half-Blood Prince, the potion 'Felix felices' is introduced- an incredibly powerful good luck potion. It's very rare, difficult to make, 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, such as the Battle for Hogwarts.
• Science Fiction author Larry Niven coined "Niven's Law," which states that once a technology is introduced into a setting, it must continue to be present in all later stories in that same setting.
  • Larry Niven formulated "Niven's Law" after realizing that the "Known Space" 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 Star Trek Novel 'Verse, the phase-cloak seems to go through this a lot. After its introduction (and successful use) in an episode of Star Trek: The Next Generation, it's largely ignored by the books (as well as later TV series). A short story in a Star Trek: New Frontier anthology eventually suggested the prototype was destroyed soon after the episode. By the time of Star Trek: The Genesis Wave, the Romulans are making use of the technology again, or something very much like it, but then it drops off a second time, and when Star Trek: Titan comes round no-one's using it. Finally, in the Star Trek: Typhon Pact series, we're explicitly told the Romulans have finally perfected it.
  • At least in The Next Generation, it was mentioned that the Federation had negotiated away its right to use cloaking technology in a treaty with the Romulans, making the Federations research into the phase-cloak illegal (i.e., a treaty violation that could lead to war with the Romulans). The episode showing the Romulans were working on it themselves showed that it was giving them trouble, and the illegal Federation project Riker had been a part of had not ended well either.
• The Alliance Rune in The Mortal Instruments. 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 The Name of the Wind learns a mental technique called Heart of Stone, which 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.
• At no point in Mostly Harmless or And Another Thing... does Arthur Dent show any sign that he knows how to fly, a skill he learned in Life, the Universe and Everything and which was fairly significant in So Long, and Thanks for All the Fish.

Live-Action TV

• The 1950s The Adventures of Superman:
  • 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 property damage. Like the splitting power, gets forgotten from then on. (The same thing happens on Lois & Clark four decades later.)
• Agents of S.H.I.E.L.D.: In the pilot, Fitz and Simmons create a special version of the I.C.E.R. that fires out of a sniper rifle. The weapon proves exceptionally effective, allowing Ward to subdue the superpowered Villain of the Week 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.
• In the Angel episode "Dad", 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.
• Battlestar Galactica (2003):
  • In the middle of season 2, Roslin's cancer takes a turn for the worse, and she's 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 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.
• The Big Bang Theory has a rare non-F&SF example: Sheldon can be persuaded to do some things he finds ridiculous or inexplicable by telling him that the thing is a "non-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.
• Blake's 7:
  • "Project Avalon" states that the Federation Mind Probe is an infallible method of interrogation. That doesn't stop Cold-Blooded Torture being used in later seasons, despite being less efficient.
  • In the Two-Part Episode that introduces Orac, the Magical Computer 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 urges Jenna to take the money and run, saying that Blake will just use it for his Hopeless War. But we never see Blake using it to finance La Résistance (except for a few crystals in "Shadow"); in fact several future episodes are based on The Caper in order to steal for the revolution (or make our anti-heroes rich).
• Buffy the Vampire Slayer
  • Justified with Adjoining Spell used to defeat the demon cyborg Adam in "Primeval". "Restless" had the First Slayer try to kill everyone who used the spell in their dreams, explaining its absence from later seasons.
  • 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.
  • In "The Gift", Buffy uses a hammer that allows her to pulverize Glorificus, a literal Physical God that had shrugged off absolutely everything that was thrown at her before. Said hammer is never seen or mentioned again, despite how useful it would have been against, say, the Turok-Han or Caleb.
  • Similar to the flamethrower instance from Angel mentioned above, in "When She Was Bad", 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.
  • A mundane technology version is the rocket launcher used against the Judge in "Innocence". Buffy is shown to have it as far on as "Him", but she also never uses it against Caleb or his minions.
• The spirit board from Charmed (1998), which was used for all of three episodes before disappearing in season three until it finally resurfaced for one last use in season 8.
  • Additionally, their original Power of Three Spell ("The Power of Three... Will Set Us Free..." chanted ad infinitum) is treated in-show as something of a magical Nuclear Option. If the Charmed Ones started to chant it, any evil being in the immediate area was vanquished without trouble. Naturally, it would be rather boring to conclude every episode the same way (and allegedly the actresses didn't care for it that much,) so early on, things were changed so the sisters had to come up with more elaborate spells and potions to vanquish demons. Fair enough, but the spell's popularity meant it was frequently brought back in particularly Darkest Hour moments, often season finales when they were up against their biggest Big Bad of the season, including one literally called The Source of All Evil. This begged the question that if the spell was that powerful, why would they ever need to use anything else.
• Doctor Who, both Classic and New Series, does this a lot.
  • 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 Visualizer ("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 apparently very bad.
  • A machine that created a candy-bar-shaped equivalent to Food Pills appears once in the 1963 season and is never seen again.
  • "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 a security setting.
  • In "The Hand of Fear", the Fourth 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".
  • The Chula, mentioned in "The Empty Child"/"The Doctor Dances", 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 warriors, with Jack's ship and the pod containing the nanogenes being a stolen Chula warship 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.
  • At the end of "The Christmas Invasion" the vast alien starship that had been menacing the whole planet is utterly destroyed by a colossal laser fired from beneath London. This fantastic weapon devised by Torchwood London from captured alien technology for the defense of the kingdom from extraterrestrial perils is never again mentioned, despite London being menaced by aliens so frequently that its citizens get into the habit of evacuating over Christmas so as not to be there when the monsters turn up. However, the same technology (or, at least, similar special effects) seems to now be incorporated into 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.
  • 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 that was probably the only deadlock seal that the pen could open.
  • "Orphan 55" has an inexplicable absence of the TARDIS' Translator Microbes when Graham and Yaz are unable to read a sign written in Cyrillic, requiring the Doctor to translate for them — even though the translation has worked on both writing and Russian before. The only even slightly plausible explanation, not counting Negative Continuity, is that the team's current distance from the TARDIS (since they were teleported directly to the planet the episode is set on) made it impossible, but the fact that everyone else at Tranquillity Spa thus apparently speaks 21st century English puts a big hole in this theory.
• In Eureka, the cryo sleep chamber that was used to put Fargo's grandfather in suspended animation could have been used many times to buy time during emergencies where people are mutating or dying of some horrible disease.
• 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.
• Season 2 of H₂O: Just Add Water introduces a magical potion that can grant wishes. Although it goes slightly haywire during the episode, the characters know exactly what went wrong in making it, and could presumably remake it again without the consequences. This appears in exactly one episode and is never mentioned again, even when it would solve almost every problem the characters have.
• Henry Danger: The episode "Birthday Girl Down" shows Ray using a Truth Fingers gadget to massage Henry's head so he can analyze his brainwaves to see if he is telling the truth or lying over being the one who knocked Debbie Putch off the roof; the results are he's telling the truth. "Sick and Wired" has Ray become convinced Henry is lying about his cold to avoid coming to work, but he never thinks of using the Truth Fingers on him so he learns the truth.
• Claire's blood in Heroes. It can heal people. It works on anything, and nobody even mentions it in situations where it might be useful (for instance, on Nathan at the end of season 3). 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.
• Kamen Rider:
  • Kamen Rider X: 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.
  • Kamen Rider Kabuto: Early episodes had ZECT employ thermal cameras which could tell Worms apart from ordinary humans because their Super-Speed-granting metabolism also raised their body heat. This took away any ability to tell a mystery story, so the cameras were completely forgotten after a few episodes, and some late-game episodes feature ZECT trying to engineer much more convoluted ways of telling Worms apart from humans. The ZECT Mizer from the same series is one of the most infamously Merchandise-Driven weapons in the franchise's history, a handheld Attack Drone spawner which all of the Riders allegedly have, summons dozens of homing explosives when used, and is used exactly three times.
  • Kamen Rider Wizard: Shortly after getting his Super Mode, which makes him invulnerable, the title character immediately goes to confront the Big Bad with it. The villain promptly demonstrates the ability to wave his hand and take all of Wizard's magic away, and just as promptly forgets that he can do this and never does it again.
  • Kamen Rider Build: Build's Super Mode, Build Genius, is described in its debut as having been made using the combined essence of all 60 Fullbottles, each of which grants its own superpower when used for a lesser transformation. Genius can use all of the powers...but across the entire last quarter of the series, it only uses Diamond's ability to make a shield, and only once.
• Happened so often in Knight Rider that it became one of the jokes of the series. Aside from the common stunts, Bonnie/April would mention off-the-cuff that they'd added some cool new feature to KITT... which just happened to be exceedingly useful for that episode's problem. Then, it would never be heard about again despite the gadget being a solution to a later problem. Only a very few added features went on to be regularly featured without being implied to have always been there (which invoked the reverse of this trope, why hadn't they been using it?) A few gadgets were explicitly mentioned as being failures and being removed in order to avoid this trope when they were a little too powerful, such as the laser and device that let KITT drive on water.
• Misfits has a fourth season character (Jess) with the power of x-ray vision. In one of the episodes, the heroes are hiding in a flat with a crazy killer stalking them. One of them looks through the peephole and nearly loses an eye when the killer stabs through it. They have conveniently forgotten that Jess can see through walls. Oh, and Jess was the one looking through the peephole.
• Once Upon a Time:
  • The Evil Queen in has the ability, at least in the Enchanted Forest, to 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.
    • Moreover the Evil Queen's magic mirror is able to locate Snow White anywhere at any time as shown in multiple episodes (such as True North, Ariel and Souls of the Departed), yet there are a few instances where Regina is desperately looking for Snow White; in Page 23 Regina needs a special item (an arrow of Cupid's to find her) and in The Evil Queen she kills an entire village because its inhabitants refuse to tell Regina Snow White's whereabouts.
  • Emma has the self-proclaimed superpower of being able to tell if someone is lying, yet when she is questioning Greg Mendell about whether he saw Rumplestlitskin use magic, although she has every reason to need to know, she can't tell that he is lying through his teeth. Although Snow did 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.
  • True Love's Kiss cures all curses, yet Snow White and Prince Charming occasionally go looking for curses that have befallen one or the other.
  • The Storybrooke Sheriff's Department has got a surveillance camera recording everything inside the room (its recordings are even viewed in season 3 finale to investigate upon Zelena's death, yet no one thinks of watching the recordings after a Round Table knight is mysteriously killed inside the department in Siege Perilous because that would make the heroes learn Arthur's true colours too early and advance the plot too fast.
• Power Rangers:
  • Ever since Disney took over the franchise, teams have been getting single special abilities while untransformed. Except in Ninja Storm and Jungle Fury, where these powers were highly plot important, the Rangers would generally completely forget they had these powers for a dozen episodes at a time.
  • In the original Mighty Morphin' Power Rangers, this happened far more frequently, with MacGuffins being introduced regularly and never being mentioned again. The worst was the Sword of Power, summoned by a brainwashed Tommy as part of a ploy by Lord Zedd to steal it. After regaining his mind, Tommy goes to great lengths to get it back, taking on the Monster of the Week single-handedly. He retrieves it... and it's never seen again. For that matter, it wasn't clear why it was so desirable in the first place. 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 So Last Season 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 Power Rangers Zeo 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 Power Rangers Turbo.
• Red Dwarf 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 Chekhov's Boomerang 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, this doesn't end the show.
• Stargate:
  • Stargate SG-1 usually avoids this, with plenty of Chekhovs Boomerangs shown years apart, but it still has its examples.
    • Kull Warrior Armor. That stuff shrugs off claymore explosions, and is light enough to wear, yet while Vala is able to get her hands on a suit and capture a starship — a United States starship, no less, so you'd think they learn — with it, the US military doesn't even seem interested in it. Plus it looks really badass.
    • Season 3, "Past and Present": They discover a drug that reverses the effects of aging. Next episode, it's forgotten.
  • Stargate Atlantis:
    • 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 Stargate Universe (no sending a scientist back to, say, research useful ways to get help), but the Lucian Alliance can somehow sneak past it. Word of God is that Atlantis had a fail-safe that required it be returned to the Pegasus Galaxy a few weeks later (this was to have been the plot of the 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.
• Star Trek:
  • Star Trek II: The Wrath of Khan introduces 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 Star Trek: The Next Generation and Star Trek: Deep Space Nine, terraforming is a long and arduous process that yields modest results. Ultimately, it's a case of No Plans, No Prototype, No Backup; 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.
  • Star Trek: The Original Series is notorious for this.
    • The episode "The Return of the 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 Star Trek: Voyager.
    • Spores that can regenerate lost body parts, restore the human body to perfect health and give immunity to radiation ("This Side of Paradise").
    • In an example which works in reverse, in The Devil In The Dark, McCoy uses a silicon-based cement to heal the Horta, said to be used to build emergency shelters. Too bad they didn't bother to beam any of that stuff down to Sulu and company as they froze on the surface of the planet in The Enemy Within.
    • "A Piece of the Action" shows that the ship's phasers can stun people on the ground and can even exclude the building in the middle of the stun area. Never seen again.
    • The subcutaneous transponder ("Patterns of Force"), which gives the ship the ability to lock onto and beam up the landing party if they're out of contact. Its actual purpose in the plot is 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're separated from their communicators, the thing is never seen before or since. You'd think it would be standard issue.
    • In "By Any Other Name", the warp drive is modified by the Kelvans to reach Warp 11 with ease, but this technology is apparently forgotten.
    • In "Plato's Stepchildren", the Enterprise crew discovers kironide, a 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?
    • Scalosian water ("Wink of an Eye") gives Super-Speed (though with a serious risk of dying from injuries that would normally be trivially minor).
  • Star Trek: The Next Generation indulges in this far too often.
    • 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.^note Plus, without the saucer component, the stardrive section looks very silly.
    • 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).
    • The episode "Lonely Among Us" shows how the transporter can bring the dead back to life! While the person's consciousness has been converted into energy by the being that has possessed him, the episode seems to imply that they can always re-materialize a previously saved version of a crew member.
    • How about Dr. Soong's having spent decades trying to hook Data up with a positronic brain capable of sentient thought, while Geordi does the same thing with one poorly worded request in the holodeck in "Elementary, Dear Data"? 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 in "Ship in a Bottle". He's understandably unhappy about it.^note Seeing as sapient holograms are in service just a few years later, it looks like someone did look into it. And they're given about as many legal rights as a wad of used Kleenex.
    • "Unnatural Selection" has Dr. Pulaski get a mutated virus that ages her. They use 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.
    • "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 four more times over the course of the entire franchise, despite existing as early as the 22nd Century according to Star Trek: Enterprise.
    • Consider the "dimensional inverter" used in the episode "The High Ground", that can transport things straight through even a Galaxy-class starship's shields (or any other shields) without trouble, but has a cumulative and lethal side effect on people who use it repeatedly. While that's obviously a sane reason not to use it in normal service, it does nothing to explain why they don't use it for, oh, one-way trips by inanimate objects straight through enemy starship shields... objects like armed anti-matter warheads, for example. (Or as a Plan B for when crew members are in danger on the planet but can't be beamed up due to an attack on the ship that requires them to keep their shields up or a Negative Space Wedgie that blocks the beam.)
    • "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.
    • In "Rascals", a transporter accident turns some crew members into young children. To be sure, this was inconvenient at the time, but the staggering medical implications are never explored. The entire plot of Star Trek: Insurrection 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.
    • The Exocomps from "The Quality of Life" are non-humanoid robots made of off-the-shelf parts that rapidly improve themselves to the point of sentience. Despite being hugely effective and friendly (one of them performs a Heroic Sacrifice to save the heroes), they're never mentioned again. Decades later, one of them shows up in Star Trek: Lower Decks as a junior officer (with daddy issues), the Federation having officially recognized them as a sapient race in the meantime.
    • "Second Chances" reveals that using two beams due to interference led to the creation of a second Will Riker, completely by accident. Both are physically identical and have the same memories up until that point. Not only is it capable of cloning a person despite there being no additional mass, but there's the question of who is actually materialized, the original person or a copy?
  • Star Trek: Deep Space Nine:
    • 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. Apparently the special effects team found it more cumbersome than the franchise-standard viewscreen communications.^note This was eventually retconned in Star Trek: Discovery as having existed even before TOS days. It was removed from the Enterprise both because it interfered with her older systems and because Pike just plain didn't like how it made everyone look like ghosts. It's possible the rest of Starfleet soon gave up on the tech and went back to the good old screens.
    • The easily replicable TR-116 rifle in "Field of Fire", 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).
    • Traditional military equipment in general, for everyone, during a major galactic war. Everyone uses very slow firing energy weapons that require being charged. No-one has squad level machine gun style weaponry, no-one uses grenades or explosives, the force fields that everyone has on their ships aren't modified for mobile local use in combat, and most of the military forces are only equipped an energy rifle or pistol, perhaps a handheld scanner and if you are Klingon, a bladed melee weapon. The military forces on all sides are generally lacking any form of armour or personal equipment that a soldier or sailor in modern times would carry, they don't wear helmets in combat to protect themselves from falling debris or shrapnel and rarely take any obvious food & water with themselves.
    • "The Siege of AR-558" is based around a Starfleet unit trapped on a planetary facility being attacked by Jem'Hadar infantry. The Starfleet unit is equipped with nothing but small arms and the Jem'Hadar have no artillery or air support and thus have to make frontal assaults on a fixed position. The episode does reveals that the Jem'Hadar do one trick up their sleeve. Cloaked anti-personnel mines, which are literally all over the Federation base. Which begs the question of: why don't they just program the mines to kill everyone in the base by going off at the same time? Those mines also don't show up at any other point, when they would be extremely useful as a hidden surprise upon having to relocate.
  • In Star Trek: Voyager, the crew conveniently forgets several gadgets that could have gotten them home, or at least closer to it:
    • 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.
    • In "Threshold", 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.
    • Q Jr., depowered, retains enough Q knowledge to use the Delta Flyer's [insert Technobabble here] to create portals, without any unpleasant Screen Shake-inducing side-effects that we see. The crew could have done whatever it was that he did and gotten home via a series of portals, or at least — as is often the case with Voyager's non-deadly shortcuts — shaved a decade or two off their trip before the Applied Phlebotinum gave out.
    • 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.
    • In "Mortal Coil", Seven of Nine brings 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...
    • The show often gives a reason why 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 (to the point that petty criminals from low-tech worlds used it to fake their death); if all they need is bodies, they can just make some.
    • In Star Trek II: The Wrath of Khan and the Star Trek: The Next Generation episode "The Wounded", it's explained that each Starfleet ship has its own unique prefix code, which allow another Starfleet ship to access their computer and make commands remotely, thus preventing an enemy who doesn't know it from maintaining control. Curiously, in "Message in a Bottle", when the USS Prometheus gets stolen by Romulan operatives, none of the Starfleet ships sent out to stop the Prometheus think about using it.
    • Averted in "Hope And Fear", as Seven wants to keep working on the Quantum Slipstream Drive found on the "USS Dauntless". This leads to the events of "Timeless". It also gets a namecheck 900 years later in Star Trek: Discovery's "That Hope Is You: Part I", where the technology is reliable, but the fuel source is so scarce as to be practically unattainable. So far the drive's only reliable use is in the not-precisely-canon relaunch novels.)
  • Star Trek: Discovery:
    • Season one introduces as a core plot element an experimental jumpdrive that proves to have Story-Breaker Power if you bother to think about it for more than five minutes: for example, it's accused of trivializing Star Trek: Voyager's entire Myth Arc since a spore drive-equipped ship could have gotten Janeway et al. home in a day had Starfleet not declared the entire project Over-the-Top Secret in the season 2 finale. They use it fairly intelligently in season 1, but in season 2 especially they ignore its existence in several instances, most egregiously in the season 2 finale where the protagonists fight a Big Badass Battle Sequence they could have easily avoided by jumping Discovery someplace beyond the feasible range of standard warp drives instead of to Xahea.
    • Season three reveals that in the far future, the Federation collapsed because of "the Burn", when all the galaxy's dilithium mysteriously exploded. Nobody apparently remembered that it's perfectly possible to power a warp drive without dilithium, a component that is specifically part of matter/antimatter reactors: Zefram Cochrane didn't have access to it and used a fission reactor on the Phoenix in Star Trek: First Contact, and Romulan ships are Powered by a Black Hole in TNG and DS9.
• Supernatural
  • The Colt. Sam and Dean spend the first part of season 5 trying to recover it, only to discover that 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 five beings in existence it can't kill, Michael and God presumably among them). They do use it when they 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?
    • Season 12 actually answers this. Crowley got his hands on it shortly after the aforementioned unsuccessful attempt on Lucifer, and gave it to Ramiel. Ramiel then held onto it for the next 7 years (why he didn't destroy it is unclear), before the brothers finally got it back again. Sadly it gets permanently destroyed shortly afterward.
  • 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 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. So you've got to wonder why they didn't just do that instead.
  • Infamously, when a demon was first introduced early in season 1, the word "Christo" would make a possessed person flinch. This was promptly forgotten for the rest of the series depite demons being one of the most reccuring threats. "Christo" was only brought back briefly in a season 14 episode.
• In Time Trax, 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 the changes in the past wouldn't affect the future because Darien Lambert and the fugitives were sent to a parallel timeline.)
• The Walking Dead (2010):
  • In several episodes, dead people's intestines smeared on you are enough to enable you to pass by walkers unmolested, which everyone seems to forget immediately afterwards. Word of God is that it's simply not very reliable, and the disgusting odor of fresh guts makes it supremely unpleasant. Gabriel was also sickened by pathogens from the guts in a later episode.
  • When Michonne is first introduced, she has two walkers on chains like pets. She says that removing their arms and lower jaw makes them non-aggressive and keeps other walkers away. Shouldn't everyone use that? It's not like there are a shortage of walkers.
• Wizards of Waverly Place
  • 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.

Newspaper Comics

• Calvin and Hobbes: Calvin's various cardboard inventions usually only make one or two appearances, even in instances when they would be extremely useful for the situation at hand. In particular, his Cerebral Enhance-O-Tron, which gives him Super-Intelligence, was only used once, where the effects wore off before Calvin could even do anything with his newfound genius, and then never appeared again despite the fact such an invention would be incredibly useful for a Book Dumb person like Calvin. Potentially justified because they might only work in Calvin's imagination, explaining their inability to be useful.

Toys

• BIONICLE
  • The ability to form Kaita (made up of three individuals) or Nui (six) combinations has been all but forgotten and was only used a handful of times early on in the series. Nui fusions last showed up in the 2004 books and willful fusions in general were last referenced in backstories of side characters from 2006. The reason is simply that LEGO stopped designing Kaita and Nui combiners after 2004 and if the toys couldn't combine, the writers didn't make the in-story characters fuse either. Some official combinations were never used in the story at all. There was some lore justification: most characters lacked the training or know-how to fuse with each other, and a Toa Nui was straight-up said to be impossible as it would have been too powerful — Krahka could mimic a fascimile of a Toa Nui but the power overwhelmed her. Very early lore might have also justified why fusions were rare, as the original two Toa Kaita could only be formed in specifical underground chambers after collecting a set of MacGuffins, however this limitation was ignored in later canon.
  • Another seemingly forgotten "power" is the ability for a character to rebuild itself (since they're Built with LEGO). Granted, this ability apparently requires the character to have an amount of secret knowledge, have pieces lying around and having strong enough muscles to support a new body, but still... the ability exists and has been used to make the characters stronger, but only on one occasion (because the toys said so).
  • Franchise co-creator Bob Thompson was fond of kooky ideas like having an oddball character called Kapura travel super fast by moving super slow. Since none of the other writers understood this ability, it was never utilized beyond a game mechanic in a 2001 online game.
  • Certain characters can form lasting psychic links with each other, like Gali does with Takua to let him see and record what she witnesses underground. While this ability seemingly has limitations when the plot calls for it, it was only brought up a handful of times and never used when it would have made sense, like when characters split up to explore in a dangerous area.
  • In the original storyline, Makuta creates evil doppelgangers of the six Toa Mata by manifesting their innate darkness into physical form. These Shadow Toa are strong enough to nearly kill the real ones and have a will of their own. The idea that Toa can be corrupted is one of the strongest running themes of the franchise, with different examples showing up in almost every arc (brainwashed via a corrupted mask or krana, poisoned, mutated, drained of their light or gone rogue by their own choosing) the "conjured" type of Shadow Toa seen in the first arc are a One-Scene Wonder, never attempted later.
  • One of the main plot points of the 2004 arc is the Toa Metru team depleting their Elemental Powers after their clash with the Morbuzakh and Krahka, explaining why they don't use them until the end of the story. Yet Toa can refill their powers by absorbing their own elements from their surrounding, which they never do until after their powers have already replenished by themselves. While it might be hard to come by an open fire or ice, they had access to air, earth, rocks and water. The one justification could be that the Metru were very much novices at being Toa.

Video Games

• Borderlands 2 has a standard-fare New-U (TM) Respawn system. For the most part, it's ignored; all characters are Killed Off for Real within the game's plot... with the sole exception of the "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.
• Mother 3: The Egg of Light, which is set up as an incredibly powerful artifact against the Pigmasks. Once Lucas the team retrieves it, it's never used at all. Leder later explains what it is, but even afterwards they never do anything with it.
• Wing Commander: The Phase Transit Cannon from Wing Commander II is never mentioned again outside of a brief note in the manual for the Kilrathi Saga compilation mentioning that it was discontinued due to technical problems, and the flash-packs from Wing Commander IV isn't mentioned anywhere at all in later games, as if the tech has vanished.
• World of Warcraft could be said to have this. When questing it is not uncommon to be given an incredibly powerful item to help with the quest, for example a crystal that can fire a beam to shrink down giants, making them much easier to fight, to never be used again. This has caused many an Obvious Rule Patch over the life of the game to keep players from abandoning the quests to keep the items.
• Crops up in City of Heroes. Interestingly, your character is often the Forgotten Phlebotinum; 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 Mega Man (Classic) 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 Mega Man weapon capable of freezing enemies, even suspending them in mid-air., Metal Blade^note AMega Man 2 weapon, it can be used over 100 times before running out of energy, fired in any direction and is effective against numerous bosses and enemies., or the Flash Stopper^note A Mega Man 4 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 stopped time, Mega Man couldn't shoot while it was in effect, unlike the Flash Stopper. 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 Archie Comics series) try to Hand Wave this.
  • Also extends to other games in the franchise, such as the 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 personality of the protagonist and that it was shown he usually disposes of the armors after the crisis.
  • Surprisingly averted in Mega Man Legends, where the Bag of Spilling causing Mega Man to lose all his good gear from the first game is justified at the beginning of the second game: Roll had to sell off all his gear to afford to repair their ship back to top condition.
  • Happens again in Mega Man Battle Network. The current theory is that software updates render the battlechips used in the series from the previous update useless, which is why Lan starts with the same folder full of junk every game.
• In 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 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 Dynamic Entry by warping in and immediately opening fire on the surprised Alliance ships.
• In Metal Gear Solid 4: Guns of the Patriots, the War Economy has made oil and gas "as valuable as diamonds". Guess OILIX^note Synthesized petroleum that's as good as the real thing. never really caught on, then.
• The Star Trek Online mission "House Pegh" has the eponymous Klingon black-ops team in possession of a wide-area cloaking device capable of hiding ships in formation from Iconian sensors. Surely this is a brilliant advance that will change the course of the war! (Nope, never mentioned again.)

Web Comics

• The Order of the Stick:
  • When preparing to face Xykon for the first time, Durkon enchants Roy's sword with an anti-undead spell, which could 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 they face Xykon, but nobody thinks to mention it. Although since the actual spell in the game doesn't do what Durkon says it does they might simply have realized the mistake off-panel.
  • 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?"
• invoked in The Princess Planet, Christi has a magic wand that can do anything, but has only ever used it twice because she prefers the challenge of outwitting her opponents.
• In Sluggy Freelance, an early Story Arc had Riff and Dr. Schlock work together to build a time machine. After the machine is destroyed by a potato chip (it was balloon based), neither of them ever tries building one again, despite Time Travel having more Deus ex Machina potential than anything else. Though considering how much trouble they get into dealing with Alternate Universess and how big a mess they caused with the last time-travel jaunt, even Riff would hesitate to use it.
• A particular device in Special School is designed to make people (except certain psychics) to forget all about it.
• In Yokoka's Quest, Yang gives Yokoka some Lava Powder, which is a medicine to cure fevers - she then forgets to use it when she gets a fever in the next chapter, and it is ruined ^note (the inventory listing is updated to Soggy Powder) in a fight a few chapters later.

Web Original

• As pointed out in 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. This is because the movie wasn't part of the original storyline, neither anime or manga.
• In the first episode of Chad Vader, he is shown to have the ability to force choke people, and he uses it on a guy who annoys him. In later episodes, his nemesis repeatedly humiliates him, and he just fumes impotently.
• 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 SF Debris loves to call out Star Trek for forgetting that the ship has shuttlecraft, and was stunned when Firefly didn't (from his review of "Out of Gas").
  Chuck: They cancel this, and let Enterprise run for four seasons?

Western Animation

• Avatar: The Last Airbender: Events surrounding the second season finale left many wondering why Katara, who had theorized that her vial of water from the sacred oasis retained healing properties, didn't make so much as an attempt to whip it out and use on the mortally wounded Tragic Hero Jet a few episodes beforehand. The DVD commentary has the co-creators admit that they forgot it, but said it wouldn't have worked anyway.
• Ben 10:
  • Ben 10: Ben's Evil Counterpart Kevin 11's original power was the ability to absorb energy, useful for shorting out/controlling machinery and creating instant lightning blasts. As a side effect, he was also able to use it to absorb alien superpowers via physical contact. After using his ability to steal Ben's 10 superpowers, however, Kevin seems to completely forget about his original ability, even though he names himself "Kevin 11" specifically because he has 1 more power than Ben. He fails to use energy control in situations it would have been extremely helpful (i.e. when being held captive by robots), and also fails to absorb any more alien superpowers despite apparently spending a few months roaming the galaxy doing nothing except beating random aliens up. The alternate future episode "Ken 10" shows how useful this would have been, as Future Kevin finally uses his power-stealing ability to become a formidable combination of Sylar and Naraku.
  • Ben 10: Alien Force:
    • 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. It's eventually explained that Kevin could still absorb energy, but chose not to because it makes him go insane.
    • The DNAliens are all infected humans, but Ben finds out the Omnitrix is capable of curing the people and is able to help Gwen's brother this way. He then proceeds to forget that he can do this until the Season Finale.
  • Ben 10: Ultimate Alien: Ben himself appears to have forgotten about the Ultimate forms, which give the new show its title in one of its first episodes.
• Birdman (1967). The title hero can only recharge his solar powers in sunlight. This means that in almost every episode (except "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.
• Dungeons & Dragons (1983):
  • "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. However, the kids never return to try asking him again once things have calmed down. For that matter, it's implied that he isn't even the only Know Tree around.
  • In "Beauty and the Bogbeast", a magical river is introduced. At a particular time every year, it can take the heroes anywhere they want to go — Earth included. 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.
• The Fairly OddParents!.
  • Sometimes Timmy Turner wishes for superpowers. Sometimes he doesn't unwish them. These actually show up later and affect the plot. Same with magical items, handwaved by saying that they were neglected or that Cosmo was screwing with them.
  • In one episode, Timmy discovers that mixing Cosmo's sweat into lemonade results in a drink that grants rule free wishes. You'd think after Cosmo had a chance to rest (he was run dry making it) he'd use it to solve more problems, but it never comes up.
• Family Guy: "Blind Ambition" has Peter go blind, and ends with him back to normal thanks to an eye transplant. Fast forward to "LASIK Instinct", where Lois goes blind from botched LASIK surgery. Peter mentions his own brush with blindness in the former episode, but no one mentions the eye transplant. Fortunately, her eyes end up healing on their own.
• In Futurama, Richard Nixon's head uses a robot body in one episode, and Beck's head controls a small set of robot arms in another. If heads in jars can control robot bodies, why don't most of them do it?
  • The day Nixon was elected, he got himself a gargantuan robot body complete with integrated rocket launchers. Where did it go?
• G.I. Joe: A Real American Hero 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.
• He-Man and the Masters of the Universe (1983): Skeletor had robot knights that he used as foot soldiers alongside his Evil Warriors. After a while, he stopped using them.
• Jackie Chan Adventures:
  • The Book of Ages, introduced at the end of the second season. An all-powerful book that naturally records the events of history in real-time, it also has the ability to manipulate history if written in. Used by Shendu to take over the world and later by the J Team to defeat him, this ridiculously useful artifact is completely abandoned, and never even mentioned again post the season two finale. While it could be argued that this was done because the good guys saw it as being too dangerous to use haphazardly, the more likely reason is that the writers realized that by having the characters remember this weapon, then all subsequent threats could be rendered null and void in a manner of minutes.
  • In the episode "Armor of the Gods" Jackie finds the Armor of the Eight Immortals, a magical suit of armor that allowed him to go toe-to-toe with the earth demon Dai Gui. After that, the armor was never used again.
  • The Macguffin of the episode "J2 Revisited" was the Arcanum of Chi, a magical orb capable of increasing the power of any chi spell tenfold. You'd think something like this would've come in handy in the battle with Drago in the following episodes...
• Miraculous Ladybug does this in the New York Special with the Horse Miraculous that allows the Holder to teleport, which Ladybug should have since she is the Guardian of the Miracle Box by this point in time. So instead of wasting time fighting Technolizer, she could've gone back to Paris and fixed the damage in a city without a Hero instead of pinning all the damage onto Cat Noir.
• My Little Pony:
  • My Little Pony 'n Friends:
    • The Rainbow of Light is introduced in My Little Pony: Rescue at Midnight Castle as a powerful Fantastic Nuke and said to be one of the most powerful magic artifacts in Ponyland. The number of times it's 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 can pick cherries. After that, it's never seen nor mentioned again, save for the episode "Quest of the Princess Ponies" when Megan and the ponies remember they have it and planned to use it against Lavan, only to drop it in the Jewel Desert where she couldn't get it back and had it returned to her after Lavan had been defeated. Also, not once was the Rainbow let out the locket.
    • Also in Rescue at Midnight Castle, the Sea Ponies give Megan a seashell that she can use to call them. In the show proper, she only uses it once.
    • In "The End of Flutter Valley, Part 3", it turns out that Wishful the bushwoolie can make things happen just by wishing them, something he never does again afterwards in either the episode or the series — although, granted, it doesn't actually work the one time he tries it.
  • My Little Pony: Friendship Is Magic:
    • Season 1 is full of this. The Elements of Harmony (and Princess Luna) are completely forgotten about after the second episode, Twilight Sparkle shows an ability to teleport in episode four and never uses it again, Pinkie's "Pinkie Sense" is never mentioned after its debut episode, and while the Sonic Rainboom pops up later it is only in a series of flashbacks. Season two turns all of these into Chekhov's Boomerangs... for, in most cases, exactly one episode each, after which they all go back to being forgotten if Rule of Drama requires it, although after the end of season 2 the teleport spell sees casual use. Some of these were due to an instruction from Hasbro to avoid Continuity Lockout, 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.
    • One wonders why Twilight Sparkle doesn't ask for more of that "Remember Past Events Potion" from Zecora to help Spike figure out his origins and the identity of his parents, considering there's been two episodes dedicated to the simple fact he's desperate to know where he came from. Never mind that it could probably also be pretty darned helpful to understand the motivations of antagonists in a series where sympathetic backstory = easy redemption or unraveling the mysteries of the past.
    • Season 2 introduces us to the threat posed by Changelings, who remained a serious threat all the way into Season 6. During a time travel adventure, Twilight and Spike both wind up in an Alternate Timeline where the Changelings had overrun Equestria long ago and a small resistance living within the Everfree Forest is all that's left to oppose Queen Chrysalis. This resistance is led by Zecora, who managed to develop a salve specifically-designed to force any disguised Changeling back into their original form when applied to their skin, which the resistance decorates their bodies with to affirm they aren't changelings. This salve and its potential uses go unremarked upon in the main timeline, with the characters not even investigating if their Zecora knows how to make it.
• Oscar's Orchestra: Eric's Bermuda Triangle powers will never be used except in designated Time Travel Episodes, and even then their use is still kept to a minimum- usually one jump to go to whatever time period they're going to and then another jump to bring them back to the present.
  • Also, if an episode's plot involves some invention of Rebecca's, expect it to never appear again once the episode concludes. No matter what.
• Happens semi-regularly in Rick and Morty with Rick's portal gun. In some episodes, the portal gun is malfunctioning, missing, or confiscated to cause drama for the heroes. Many times, though, they'll drive Rick's spaceship-car to wherever they're going instead of portaling there, which takes significantly longer and sometimes creates mishaps along the way, and then have to escape quickly once they inevitably make a mess or otherwise get into trouble wherever they've gone. Much of the drama of their escape attempts involves trying to get to the ship or some other escape sources so they can leave and narrowly getting out safely once they do, whereas, if they'd had Rick's portal gun, it would have been a non-issue. Since Rick is a Death Seeker adrenaline junkie who's Allergic to Routine, this may in some cases be a self-imposed Drama-Preserving Handicap to make his adventures more exciting.
• The Simpsons:
  • "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's 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 the setting now. It was supposed to have been lost to Homer because of its expense ($1000 for a 6 month supply, which Homer almost got in trouble for paying with his insurance), but given how immediately it improved his career, you'd think he'd invest in more (especially when later episodes show him as wasting larger amount of money on less useful things).
  • "Brother, Can You Spare Two Dimes?" sees Homer's half-brother Herbert invent a device that translates baby-talk into intelligible English phrases. During the episode, this actually allows Maggie to stop being 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 lampshades its own reliance on Status Quo Is God.
  • "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".
• Space Ghost Coast to Coast: "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.
• Star Trek: The Animated Series introduces enormously useful life support belts, which surround their wearers with a glowing forcefield within which breathable air is provided, and which never appear in any later Trek works. Meta-wise, this was because it was cheaper to animate a glowing outline than it was to draw spacesuits on everyone.
• Stroker and Hoop: Justified with Hoop learning ninja skills for plot-related reasons in one episode, but never using them again. It's brought up in another episode, where it turns out out you have to actually continue practicing to maintain ninja skills. Who knew?
• Super Friends: Challenge of the Super Friends is notorious for this trope.
  • Lex Luthor invents teleporters, time machines, cloaking devices, a gizmo that sucks the Green Lantern Ring off its wearer's finger, etc., etc. ... uses them once ... and then then never uses them again, even in situations where one of them would save the Legion of Doom's bacon.
  • One particularly example has Luthor forget a piece of Phlebotinum only moments after acquiring it. When he time travels to alter several of the Super Friends' origins, Luthor switches places with Hal Jordan and becomes the recipient of Abin Sur's Green Lantern ring. He dons his own Green Lantern suit and uses the ring to fly back to the Hall of Doom, and then promptly puts his purple jump suit back on and makes no other attempt to use the ring. This after having been defeated by Green Lantern's power too many times to count! (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.)
• Teenage Mutant Ninja Turtles (2003): In the Fast Forward season, the Turtles and Splinter are stranded a hundred years in the future with no way of returning to their own time... not one mention is made of their time-traveling friend Renet, who is supposed to keep an eye on the time stream and would almost certainly have noticed if the Turtles were suddenly in a different time period than they were supposed to. Then again, it could be the Turtles and Splinter were supposed to spend some time in the future.
• Transformers
  • Waspinator's Eye Beams that he shoots at Cheetor with in the Beast Wars 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 Eye Beams 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 Eye Beams) 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.
  • Transformers: Generation 1:
    • Both sides were constantly creating weapons that would be a Game-Breaker in the hands of non-idiots. Instead of being used for what they could be, they'd be used to create/stop the problem of the day, and then never be seen or heard from again. They also suffered from No Plans, No Prototype, No Backup: If the ultimate weapon whipped up in the days since the previous episode gets smashed at the end, just making another is apparently never an option.
    • Also, the many, many, many one-shot powers displayed by individual Autobots that would never be used again. (Most iconically, the Pure Energy flail and axe used by Megs and Prime, respectively, in the series premiere only and never again.)
    • In a rather impressive case of this, in the first two episodes of the show, the Autobots can all fly under their own power. This is even shown off in the opening titles. However, by the third episode, Optimus needs to borrow a jetpack to fly, and after some inconsistent appearances in the first season, the ability mostly vanishes except for a number of specific characters (i.e. the Dinobots). It's hard to even count the number of times that being able to fly would have resolved any given threat, and it's typically suggested that everyone being able to fly would make altmodes rather irrelevant.
• Winx Club:
  • "Layla's Choice": Layla giving up her chance to heal her eyesight for the sake of the dying Queen Ligea is regarded as a sacrifice big enough to earn her the mythic Enchantix. The thing is, she could've waited until the next sunset to use the Coral Gem and heal her blind eyes then.
  • The Charmix from season 2. Sure, it was So Last Season, but there's nothing else preventing the fairies who hadn't earned their Enchantix from using it in season 3, especially 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.
• Xiaolin Showdown
  • 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 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.

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. 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 problem for several of them is we know several ways to reproduce it, the question is which (if any) they actually used. For instance, Greek fire is described so many different ways in different sources that a leading theory is that it was a general term for incendiary weapons, not a single recipe.
  • Roman concrete could be an example of an inverted As You Know, as the unknown ingredient was simply seawater. At the time, everyone knew that you use seawater to make concrete, so no-one ever bothered to write it down.
  • Damascus steel is more of a curiosity than a lost technology, because Technology Marches On and there are many types of modern steel alloys that are far superior to damascus steel. The interesting part is how exactly they made such good steel without technology. They somehow accidentally found a way to add carbon nanotubes to the alloy, but we haven't been able to reproduce how.
• The mortar used for the Great Wall of China - which has lasted for hundreds of years - also used to fall into this trope until recently, when scientists discovered that the secret was...sticky rice!

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