Some superhero comics authors seem to get bored of the same old powers. They add new ones to the same characters whenever they feel that a new power would open up a new story, or a new danger needs a new response, or what the hell, whenever they feel like it. It's bad enough writing in a new hero from nowhere just because you want to include a new power, but a lot of writers are worse than that. They tack new powers onto existing heroes.
Sometimes a retcon, a power upgrade or some bit of Phlebotinum is employed to explain the new power, but often the character just does something they've never done before and when their friends say, "I didn't know you could do that!", they come back with either "I've never needed to, till now," or worse, "Neither did I!"
This is sometimes employed as a form of Deus ex Machina - having written themselves into a corner with a villain or situation that's too overwhelming for our heroes to handle with the tools they've been given, the writer decides to have the hero instantaneously learn the one ability he needs to save the day. Frequently, without any form of Foreshadowing to suggest that he can do that. It gets worse if he conveniently forgets this ability when it would come in handy in a later situation.
Not all New Super Powers fall into this category, just the ones resulting from an Ass Pull. Generally speaking, this trope is far, far more forgiveable early in the story- a character who has only recently been empowered is fully justified in not knowing what he can do. Likewise, "neither did I until now" in an experienced character can be reasonable, if it's happening in some circumstance or special condition that the character has never encountered before. An inherent power of Science Heroes - other than their Two Fists, they're supposed to be building bizarre devices to deal with bizarre circumstances. It only gets annoying if the writer can't come up with a reason the guy would have that precise device on his person at that precise moment - other than him just being crazy.
Commonly used to bring a character Back from the Dead.
Giving a character a Green Lantern Ring avoids this. Compare Magic A Is Magic A, So Last Season and finally Strong as They Need to Be.
Suddenly Always Knew That is the same type of retcon as this, but instead of "Neither did I", the character will explain that You Didn't Ask.
If the plot was crafted to fit the powers (as opposed to the powers changing to meet the needs of the plot) you have a Plot Tailored to the Party or a job for Aquaman. See also Adaptive Ability, where your power is the acquisition of new powers/immunities. When the new ability is something overly narrow or silly, this often leads to Flight, Strength, Heart, as was common in The Silver Age of Comic Books.
Is one of the common traits of a Mary Sue/Marty Stu.
Compare Eleventh Hour Superpower. Contrast with The Worf Effect, where the plot demands the character's powers and abilities are suddenly rendered impotent.
The Sharingan frequently gets new abilities. Generally, these tie into the eyes in some fashion (copying moves, mild precognition, trapping someone in an illusion by looking at them, shooting black fire from the user's eyes, hypnotic abilities), but powers such as Susanoo (some sort of giant spirit robot), Izanagi (extra lives), Izanami (warping time and space), and transforming into the Rinnegan don't have anything to do with that, with many of them (especially the last three) coming out of nowhere.
This has extended to Sasuke's allies. Karin and Jugo reveal miraculous healing powers when Sasuke is wounded after their battle with Killer Bee, though they did not use these abilities when Sasuke was bedridden from injuries after fighting with Deidara. Karin also gains the ability to use chains capable of restraining a giant wooden statue when absolutely no other ninja is capable of doing so during the war arc, and is handwaved as it being because she's an Uzumaki.
In Chapter 562, in order to get all the five kages to fight a revived Uchiha Madara, we learned Genma's team are able to use a watered down version of Tobirama's Hiraishin ("Flying Thunder God") technique and they quickly use it to transport the Raikage.
And apparently Minato didn't invent the Hiraishin, but the Second Hokage Senju Tobirama did, which has never once been hinted at.
Sakura revealed during the war arc that she learned Tsunade's Souzou Saisei three years ago and has been charging it this entire time. This despite the fact she'd recently lamented how incredibly weak she was, as little as a few days ago within the story itself. Not to mention that she stopped using it and hung back watching the fight almost immediately afterward.
Minato has apparently learned Sage mode AND get the Kyuubi cloak, two abilities he's not only never been hinted at having, but he only gained the Kyuubi chakra moments before his death.
JoJo's Bizarre Adventure has two especially egregious examples of this trope, each used to finish off the Big Bad of a story arc. The first example occurs in Part 3: the villain Dio Brando is virtually unstoppable because his Stand has the ability to stop time, so how do the heroes stop him? Jotaro Kujo's Stand suddenly gains the power to stop time, which also lets him move in Dio's time stop, despite the fact that its only powers so far were Super Strength and Super Speed. Of course, this is explained by saying that Jotaro's stand has always had a smaller version of Dio's time stop power, and what had been seen as a Super Speed attack (his trademark "HORA HORA!") was actually him stopping time and then punching his foes' faces repeatedly (Jotaro never realized himself what was really happening, because he managed to stop time just briefly, and so it all happened very quickly before time went back to its regular flowing). Also possibly justified in that Dio is using the body of Jotaro's great great grandfather and his Stand, The World, may actually have come from Jonathan Joestar; thus it makes some sense that Jotaro's Stand would have similar powers. Part 5's was pretty bad, though: Giorno Giovanna stabs himself with a Stand Arrow, evolving his Gold Experience into Gold Experience Requiem, and giving it the power to nullify any action an opponent takes.
Oh, it happens to the villains too. Part 4 had Kira getting the ability to reset time back to the time a kid woke up in the morning so that he could find out who got killed trying to figure out who Kira was because the kid was under the effect of Kira's just gotten the night before power. Part 6 had the main villain of that getting the power to alter the universe's gravity, causing time to accelerate to the universe's end so that he could reset time to the way he wants it to be.
Speaking of part 4, this is the whole schtick behind Koichi's Stand, Echoes. It starts out as an egg, which hatches when he needs it to, allowing him to imprint sounds on a person, making it echo over and over in their heads. When that isn't enough, it evolves into a form that can imprint words onto objects, granting the object the property of that word. For instance, making a solid, pointy rock become bouncy by imprinting "bounce" on it. And later on it evolves again to allow him to increase gravity on a target to the point where it becomes helplessly pinned to the ground. Koichi's pretty much a Swiss Army Knife among Stand users.
At the beginning of Part 5, it's showed that Koichi is now able to summon the earlier versions of Echoes when he uses Echoes 2' flying power and greater range to look for Giorgio.
Parts I and II only avoided this trope by making ripple powers a sort of Green Lantern Ring.
Yes! Precure 5's Cure Aqua suddenly picked up the ability to turn her "Aqua Ribbon" baton into a sword. The reasoning behind this was that this allowed an awesome swordfight. The sword returned during The Movie for exactly the same reason.
Cure Beauty of Smile Precure is in the same boat, having being shown to create an ice sword once during her fight against Joker, then doesn't do it again until their movie.
In Mermaid Melody Pichi Pichi Pitch, the characters are frequently granted new karaoke songs (the weapon of choice in the series) whenever the ones in the previous episode didn't work. This becomes somewhat ridiculous, considering these upgrades are manifested by the goddess the protagonists are attempting to summon, and when they actually summon her, all she does is tell them to sing...
Oh yeah, and that amnesia kiss? Lucia gets that too. Randomly.
In Persona 4: The Animation, Yuu tends to gain a new ability, such as Persona Change or Fusion, whenever a boss turns out to be too much to handle.
This is justified in the fact that Yuu is going through Social Links, resulting in new Personas. He summons Pyro Jack because he's befriended Yosuke, Ara Mitama because of Chie, etc.
Teddie conveniently recalling that Kintoki-Douji can cast Energy Shower, which (just as conveniently) cures Enervation ("old" status).
Again, justified. In the game, Teddie starts with that ability.
Dragon Ball Z: For most of Dragon Ball, whenever Akira Toriyama wanted to introduce a new technique, or to show off a better version of an established one, it would usually belong to Tien and to a lesser extent, Piccolo. Tien introduced solar flare, Aura Flare's, self replication, finger lasers, telepath, telekinesis, flight, and the ability to copy techniques. Piccolo introduced regeneration, size shifting, arm stretching, and mouth blasts.
The innate ability to sense energy is meant to be quite rare in DBZ, hence why Frieza's entire army (and even Frieza himself) utilize scouter devices. Vegeta relied on a scouter when we first meet him - although he knew not to trust them entirely - yet by the Namek Saga has randomly gained the ability to sense energy all on his own.
Even Goku does this from time to time. He reads Krillin's mind when he arrives on Namek (and then never uses it again), gains Instant Transmission from the inhabitants of a planet he crashed on, and figures out how to turn into a Super Saiyan 3. And he learned all three of these techniques conveniently off-screen.
The first example was given some Lampshade Hanging, with Krillin asking "Where did you get such an ability from?!", and Goku replying he wasn't even sure that would work. It was also reused during the Android/Cell saga.
As for the Ki sensing, it wasn't that Frieza and his minions couldn't do it, but they didn't know that it even could be done. Vegeta mentioned that after he found out it was possible, he easily taught himself the ability.
Almost exactly the same happened with Goten. He achieved the Super Saiyan level without knowing that it was a hard thing to do, but being 7-8 years old, he hasn't learnt how to fly by himself.
This goes without mentioning the continuous "Super Saiyanjin Transformations". In the beginning, it was believed that only one saiyan can make said transformation every 1,000 years. Not even a full year after Goku BECOMES that saiyan, two more saiyans show up with said ability. Granite, one of them was from the future, but only by about 15 years. Later, another character gains this ability. Seven years later, two children discover this ability completely by accident. And apparently in the distant future, two more kids also discover this ability. This, of course, goes without mentioning the fact that "Super Saiyan" isn't even a big transformation anymore. The entire thing was dwarfed in power once "Super Saiyan 2nd Grade" was introduced. Then there was "3rd Grade", then "Super Saiyan 2", which completely dwarfed all of said powers combined. Once "Super Saiyan 3" was introduced, viewers weren't even surprised and even expected it. This transformation wasn't even a big deal during the saga it was introduced, as it was completely ignored once methods such as "Fusion" and "Ultimate Power" were discovered.
Happens about once per story arc all over the Sailor Moon franchise, when it isn't leaning on the Eleventh Hour Superpower instead. Also combines with So Last Season in that suddenly gaining new abilities usually means the characters cease using their old ones.
Subverted with Sailor Moon herself, in that the power of the Silver Crystal is explicitly stated and demonstrated early in the series and her attaining greater and greater power with it is literally the main story arc of the series. Though she usually gains this as part of an Eleventh Hour Superpower.
The rest of the cast just tend to get powerups because the plot usually needs them to get boosts around the same time Usagi does. Most egregious in the Makaiju arc in the Sailor Moon R anime, in which the four Guardian Senshi each get a Character Focus episode that concludes with them using a new ability to highlight their emotional connection to the episode's storyline. For the most part, they also never use these powers again outside their Character Focus episode.
Yammy is introduced as the 10th Espada. After the protagonist and his allies have defeated Espada #4-9, it's revealed that he's only the 10th Espada while in unreleased form. When he does finally release his power, he morphs into the 0th Espada. He mocks his opponents with Who said the Espada were numbered 1-10?, a question the fanbase tends to answer with "Arrancar #11, actually". That said, there are occasional hints prior to his reveal that he does have a more powerful form and that he's been eating souls to prepare for a power increase. However, no-one expected his resureccion to break the numerical ranking of the Espada (despite it already having been revealed Ulquiorra had a unique second resureccion form). It's also shown that his in-universe reputation for being so stupid he's incapable of using his power remains true right to the end: he may be the 0th Espada, but he is not capable of using that power as effectively as other Espada are able.
The Hougyoku's original power was to dissolve the barrier between Hollow and Shinigami, known as hollowfication. This is the explanation for the creation of the modern Arrancar and also the Vizards. Later on, Aizen claims it actually alters reality based on the wishes of the user, but only if it's possible to achieve those wishes without the Hougyoku. However, it was hinted all along that the original explanation of its power was wrong. If it was supposed to cause hollowfication, why was its original use to try and reverse hollowfication instead? Also, Aizen's own knowledge has turned out to be inaccurate as well, so the second explanation may be as inaccurate as the first explanation.
In the InvasionArc. the villain's zanpakutou is this. Kageroza's zanpakutou has the power to manipulate space/time. Then it's revealed to have the power to teleport and duplicate. Then it has the power to resurrect "dead" reigai. Then, when Ichigo is about to defeat the villain, his zanpakutou is revealed to have a cloning ability. The apparent damsel of the arc, Nozomi's zanpakutou has the power to drain reiatsu. Then it's revealed to be able absorb the attacks of anything thrown at it, combine it with her own power and throw it back as a more powerful attack. Then it's revealed that Kageroza's and Nozomi's power are two halves of the same original zanpakutou and, when recombined, the zanpakutou gains all the powers in combination which suddenly results in the power to destroy the entirety of Soul Society with a single activation command. And, somehow, the original zanpakutou never had any of these abilities at all until the owner, fed up with being treated as weak, turned evil and decided to obtain more power by splitting himself in two and recombining himself (how this makes him more powerful and gives him powers he never previously possessed is never explained).
While being attacked by Juha Bach in the Thousand Year Blood War arc, Ichigo spontaneously develops the Quincy ability that prevents him being Impaled with Extreme Prejudice. However, this is immediately explained in-story in a manner that does tie-in to a quincy power restoration method that was used on Uryuu hundreds of chapters previously. It had also been theorised in the fandom for many years that Ichigo's mother would end up being revealed as a quincy.
Soul Eater has quite a few of these, both in manga and the anime — most blatantly in the last four episodes of the anime, where Black Star, Kid and Maka suddenly acquired Deus ex Machina-like superpowers in that order after being beat to the floor by the villains.
Many creatures in the Pokémon anime would learn new attacks when the plot required it or evolved at just the right time.
Ryoko of Tenchi Muyo!! certainly falls into this category when she gets immobilized from the neck down and shoots a bunch of lasers from her hair.
In Code Geass, Suzaku's "live" command, which initially took over his body and forced him to survive by any means against his will, is significantly repurposed in Turn 22. Now Suzaku can consciously activate its effects in order to become a better fighter. The only way this makes sense if Suzaku can re-interpert what "live" is supposed to mean.
Part of it might be that it forces Suzaku to act in whatever way is most likely to not get him killed. For example, fighting both Kallen at her best and with a really powerful new machine and Bismarck Waldstein, the world's strongest Knight, cause his Geass to activate and take some other action, rather than running away, because they outclass him so much that exposing his back to them will give them the opportunity to kill him. In both cases, this results in him exposing himself to a seemingly greater danger - a very large explosion, and a rock slide, respectively - because he has a better chance of surviving those, and he does. During his second fight with Bismarck Waldstein, Suzaku makes his command kick in late enough to force him to win, which presumably works because he knows the Lancelot Albion has a better machine and the "live" effect allows him to not hold back, which cancels out any intimidation factor caused by Bismark's ability.
The sudden ability to remember what happened under the command's effects, however, is completely unexplained, and has no other examples during the original TV series.
The possible implied explanation is that all the times the command activated before Turn 22, he was trying to either kill himself or let himself be killed. Now that he's not trying to die, the Geass doesn't react as strongly. We've seen that long term Geass effects be bent or broken completely.
In Transformers Victory, Deathsaurus uses Transformer-eating insects to try and kill Star Saber. They turn out to be vulnerable to cold, and then Victory Leo decides to reveal he has a freeze power.
In Yu-Gi-Oh!, the Millennium Items seem to have their standard powers, plus random other powers used in one or two situations, then forgotten for the rest of the show, even in situations where they would have been useful. For instance, in the manga Yugi holds his puzzle asking it to show him where Jounouchi is, and it does it.
There's also the Millennium Ring, which near the end of the Pegasus arc showed the power to manifest the effect of Duel Monsters cards as real, allowing Bakura to use Chain Energy to bind Pegasus' goons and summon the Man-Eater Bug to attack him. This power is never brought up again.
Being Merchandise-Driven, this often happens with brand new cards just revealed, and often these cards are A) highly situational to the point in any real deck they'd be dead weight, and B) never seen again after their one usage. On occasion the new card that is used is a real life card that they just didn't use in the show before (such as Skilled Dark Magician), other times the card is completely made up with powers verging on Game Breaker levels (such as the entire Orichalcos archetype).
The Winged Dragon of Ra is clearly this, revealing a new secret ability every single time it gets played. A full list of its abilities: its ATK and DEF are equal to the ATK and DEF of the monsters tributed to summon it, you can pay all but 1 Life Point to increase its attack by the same amount, you can pay 1000 Life Points to destroy all monster on the opponent's field, and you can tribute other monsters to add their ATK to Ra's ATK. And that doesn't count the requirement one needs to have Ancient Egyptian heritage and has to recite an ancient chant in order to summon it, and the immunity to card effects it shares with the other god cards.
"Conveniently, my Millennium Puzzle allows me to put souls back into their original bodies!".
"I activate a spell that allows our monsters to trade places! Which would be completely useless in any other situation."
"I have placed a part of my soul inside the Millennium Puzzle, because apparently I can do that."
"Now I shall use Mega-Ultra Chicken's secret ability that I just this second made up to convert my Life Points into Attack Points, merging me with the beast itself!"
Yu-Gi-Oh! GX season 4 has Trueman, who exhibits a new power in nearly every appearance, ranging from teleportation to cloning to possession to shapeshifting to ripping through the dimensional fabric to thought manipulation.
In Yu-Gi-Oh! 5Dsthe Crimson Dragon did it. The Crimson Dragon's abilities are never really explained at any point, so the Signers can showcase new ones at their leisure. Most notably the Savior/Majestic Dragon, which Yusei and Jack get during their duels against Dark Signers. It doesn't actually exist in their deck, it just appears on top whenever they need it.
Yu-Gi-Oh! ZEXAL has the Shining Draw, which literally creates a card that will have the exact effects needed to beat his opponent, no matter how convoluted, up to a monster that lets him attack outside of his battle phase, an ability seen on no other card in the game.
Deconstructed in the second episode of Yu-Gi-Oh! ARC-V. Here, the new monsters in the series, Pendulum Monsters, are capable of being created by the main protagonist's pendant. However, he doesn't fully understand how they work right off the bat, and even believes that they only come out when he's in a pinch, and when he reveals how he got them to a crowd of people, he's accused by them of cheating in his duels.
Parodied in Fullmetal Alchemist where Ed, separated from Al and feeling rather desperate, tries to "CONVENIENTLY AWAKEN TELEPATHIC POWERS!" to contact him. It doesn't work.
Though later revealed in the manga omake, it did work. He just didn't reach Al. He reached Pinako.
Detective Conan: Conan can do anything. ANYTHING. He can drive a car, scuba dive, shoot a gun, drive a boat, drive a Jetski, fly a small plane, and FLY A HELICOPTER (which is completely different from flying a plane). Why can he do these things? Because he "learned in Hawaii!"
Haru, the lead in Rave Master, actually has the power to acquire new powers when he's in a sticky situation thanks to the Rave of Wisdom. The technical explanation is that he has a sword with ten (preset) forms, and he learns a new one whenever he happens to need it most. This manages to avoid being an Ass Pull at several points because he either obtained some powerful artifact to grant him another sword, he had to get some reforging done before he could access one form, and another form was used by a villain before he figured out how it worked.
One Piece: Always present to some extentnote Plot Plot No Mi, as the fandom calls it, though normally the powers are natural outgrowths of previously existing abilities or brand new items with some degree of How Do I Shot Web?. However, in the Enies Lobby Arc, it's practically endemic, with every single character pulling bizarre new abilities and weapons out of nowhere. Oda has said he doesn't like to do training arcs like in other Shonen series. Instead, he'll have a character pull out a new ability and then go into a flashback about how they trained for that ability off-screen.
Luffy learned how to use Gear Two and Gear Three. He supposedly perfected both techniques in the roughly week and a half after being beaten to near-death in the previous encounter. Luffy also learned how to flash-step via mimicry mid-battle.
As Sogeking, Usopp unveiled a brand new super-slingshot with tech-boosted firepower, range, and accuracy. It's not clear when exactly this weapon was built, as Usopp had been holding the Distress Ball for at least a day and a half before arriving at Enies Lobby, and the weapon definitely would've prevented much of that distress.
Nami showed off the upgraded Clima Tact, which Usopp also put together at a vague point in the timeline.
SANJI learned BLAZE KICK! He had more time than the others to figure this out, to some extent, as he hadn't been in the same dire battle as the other Strawhats from several hours prior. This ability in particular seemed almost tailor-made for the situation, as he was fighting an enemy that was especially resistant to normal blows, but not fire.
Zoro activated a brand-new nine-sword-style goddess form. Unlike the above, there was basically no attempt to explain this, and the form's properties are still very unclear.
Earlier, Zoro had outright banked on this while fighting the literally Made of Iron Mr. 1. Zoro doesn't know how to cut steel yet, but is sure he'll figure it out with his life on the line. He does.
It works more logically after the Time Skip, as all the characters had two years to train, but aren't going to exhibit all of their powers at once. Across Fishman Island and later arcs, the Straw Hats seem to bust out very new powers according to plot demands. The most off-the-wall being that Brook discovered the true basis of his devil fruit, which isn't about bringing him back to life so much as it is about giving him super-soul-powers that not only include the ability to come back from the dead once, but also allow him to astral-project himself, and use ice attacks. He also developed a degree of limited Mind Control through his music.
In the final Season 1 episode of Uchuu Senkan Yamato, Dessler appears out of nowhere in a ship and fires a giant energy blast at the Yamato. Sanada activates a device with reflects the beam back onto this source. This device was never seen used by the humans before (only Sanada seemed to know its existence), and despite its seeming usefulness, is never used or mentioned again.
Star Driver is particularly bad about this. Practically every other episode in the series ends with Takuto suddenly revealing that he has some hidden power that just happens to work perfectly against whatever enemy he's fighting, and using said power to One-Hit KO his enemy. The show justifies it by saying that Takuto is far more experienced than most of his enemies, and he's deliberately holding back most of the time so they don't know how good he really is.
Spoofed in the fifth episode of Tiger & Bunny. When Wild Tiger and Barnaby are in a pinch and almost out of time, their Powered Armor automatically switches to the the brand new Good Luck mode Saito installed and they manage to incapacitate their opponent just before the clock runs out. So what does Good Luck Mode actually do?
Barnaby: So this new mode increases our power?
Saito:Not one bit! It makes you look cool, though.
Also spoofed in the eighth audio drama (a comedic Alternate Universe where Kotetsu and Barnaby are actual Buddy Cops), where the following exchange takes place as they confront the villain.
Barnaby: No good! I'm out of ammunition!
Kotetsu: Me too, Riders!
Barnaby: But you know, I actually have pretty amazing powers! I just normally keep them secret!
Kotetsu: Whaaat!? ... Actually, I got the exact same powers just yesterday!
Super Atragon gives us a battleship that does this: The Ra gains several the following new abilities with no explanation; most are shown exactly once:
For nearly a hundred episodes in YuYu Hakusho the audience has know Kurama to be a plant user. But in the fight against The Elder Toguro, he used a smoke screen? Where'd that come from?
Inazuma Eleven anime develops itself into this. At first, many skills the team learn come from books and manuals, but by the third season, characters repeatly pull out new things whenever the plot demands them. In the movie, the protagonists learn to use super power abilities without any explaination just so they can beat the Ogre, who's argubly stronger than the series' world cup teams.
Haruhi Suzumiya: In the movie that Haruhi filmed for the arts festival, Mikuru's character had the ability to fire a "Mikuru Beam" from her left eye. At the end of the movie, Itsuki's character randomly and conveniently awakens his esper powers just in time to defeat the evil alien. Thanks to Haruhi's subconscious Reality Warper abilities, Mikuru actually did get a "Mikuru Beam" that continuously endangered the lives of the film crew, especially Kyon. Yuki, thankfully, was able to suppress these abilities...at which point Haruhi would give Mikuru a new type of beam that Yuki had to deal with.
Mazinger Z had a number of powerful attacks that were only used once or twice and never seen again. Among them were a "Refined Rocket Punch" which was a tougher arm for Mazinger-Z that fired a much stronger Rocket Punch and the ability to magnetize its fingers.
Mobile Suit Gundam had the "Super Napalm", an odd weapon that was used to destroy the remains of Side 7's Operation V project.
Mobile Suit Zeta Gundam had Kamille pull out a lot of attacks out of his ass thanks to the wonders of the Bio Sensor, including the ability to cause his beam saber to grown many times its normal size to destroy a Mobile Suit and the ability to be supercharged by the souls of the dead so he can ram the title Mobile Suit into the Bigger Bad's Mobile Suit!
HeartCatch Pretty Cure! was incredibly bad with this, with the girls constantly coming up with more attacks than those before them ever had and rarely using them afterwards. Cure Blossom was the worse offender, creating a tornado attack, a spiral energy Kamehame Hadoken attack to counter a foe's similar attack, a Christmas-themed attack with Cure Marine and just randomly throwing out pink energy like ki blasts.
Hilariously, Happiness Charge Pretty Cure runs with it. Megumi, after transforming into Cure Lovely for the first time, is told that she doesn't have set attacks and just make stuff up. She runs with it.
The entire franchise is in LOVE with this trope. Granted, that is how it works. A favorite example of fans is in the Myotismon Saga/Arc where Gabumon and Agumon are magically able to unlock their Mega forms.
Digimon Frontier ESPECIALLY loved this trope. The whole point of the first sixteen or seventeen episodes was to literally get the spirits and evolve to higher levels. Especially displayed when Kouji Minamoto and Takuya Kanbara gain Double Spirit/ Fusion Evolution. With the power coming from an egg. No joke. Watch Episode 26.
Attack on Titan has Eren turn out to be the "Coordinator" at a time when the characters would otherwise be doomed. To be fair, the Female Titan and Beast Titan have shown that some Titans can control other Titans to some extent so it doesn't come completely out of the blue, but it's still a little grating.
Planeswalkers: Old walkers are able to do virtually anything according to the comics and novels, and Post-Mending walkers are capable of quite a bit (shown by them getting printed with new abilities). The players themselves are old walkers: literally capable of casting anything they have in their decks (provided certain limitations). But this is kinda the point of playing.
Superman. This is quite possibly the largest criticism laid at his feet: he started out faster than a speeding bullet, more powerful than a locomotive, able to leap tall buildings in a single bound and invulnerable to anything less than a bomb. Since then he's learned to fly, to blow like a hurricane, to survive nuclear explosions (though just barely), chill things with a puff of breath, shoot lasers from his eyes, and use X-Ray Vision. And that's just the powers that have lasted: during the Silver Age, he gained a new power nearly every month (Super Ventriloquism was bad - being able to travel through time as easily as he could fly was worse). The super-breath, at least, is a logical extension of someone with the kind of lungs he must have...
Interestingly, a lot of this stems from various media adaptations, particularly the Superman Theatrical Cartoon shorts; originally the brothers Fleischer wanted to stick close to a relatively limited powerset, but animating him just "leaping" everywhere was time-consuming and expensive (even with their extravagant-for-the-time budget), so they asked DC "can we just make him fly?" DC said "Sure", he flew in the cartoons which introduced a ton of people to the character who then bought the comic and complained to DC, asking why Superman didn't fly like he did in the cartoon... and, well, we were off to the Super-races.
Superman's Mirror Universe counterpart Ultraman actually has this as his superpower: exposure to Kryptonite, rather than harming him, causes him to develop new abilities (at least in his first appearance. Later on this was changed to Kryptonite being necessary to sustain his full levels of power).
And Red Kryptonite (occasionally, in some continuities) lets the "regular" Superman develop new abilities, albeit temporary ones.
Two Words: Super Weaving. This is actually Van-Zee, Superman's lookalike from Kandor, using super speed to, er, weave really really fast. Why they felt to name it god only knows.
In one strip, Lois is going blind and she wants to see a play based on herself before this happens. But the play is only a script, so Superman uses super-puppetry to make it appear that actors are performing on stage (Lois' vision is blurred so she doesn't notice). He also uses "super-memory" to learn the script, even though he could just read it given that he's offstage.
Other silver age classic powers: super-hypnotism, super-kissing (don't ask, really), and super-mimicry.
A Cracked Magazine Guide for Superheroes encouraged them to simulate super-kissing to improve their love lives. Apparently you use your super-breath to suck all the air out of the room just before you lock lips. The girl passes out from lack of oxygen, and wakes up convinced that she fainted from your kiss.
'Super-hypnotism' - though not called that at the time - was actually acquired at a very early point, certainly by 1940 at the latest (he hypnotises Lois in at least two different stories that year alone.) It's just that it's used so irregularly and the 'super' makes it sound so silly that it seems Silver Age.
The original TV show mostly restrained itself from this, but huffed this trope twice, once to give Superman the ability to phase through walls, and once to let him split himself into multiple two Supermen. Both of these powers vanished after the episode.
The wall-phasing was at least supposed to be an extension of applied super-speed (vibration of such intensity that his molecules could pass through solid objects, a la The Flash) rather than an entirely new ability. Lois and Clark even borrowed the concept, and justified its lack of re-use by suggesting that the process was extremely taxing and even life threatening. The splitting into two Supermen, however, was not quite so validated.
The splitting ability came from his dense molecular structure (at the time, the explanation for his invulnerability) meaning he had enough mass to make up two normal people. The two were significantly weaker than when they were together creating dramatic tension when they couldn't merge.
The basic assumption was that, for any ability a normal man might have, Superman could do it or learn to do it much better. If a man can blow out a candle, then Superman can blow out a forest fire. The problem lay in that the writers didn't consider how ventriloquism or hypnotism really work, so Superman was shown literally throwing his voice, or hypnotizing people almost effortlessly.
The time travel ability is a logical extension of the fact that they'd already established he could fly faster than light; the real question is how he ever broke the light barrier without time traveling. See here for details.
To sum it up, Superman only has 1 super power, the ability to pull any super power he wants out of his Super-ass.
This didn't end with the Silver Age by the way. The modern Superman has been shown to use the psychic martial art of Torquasm-Vo which in one instance allowed him to alter reality.
Spider-Man also has in his rogue's gallery a villain called "The Answer", whose powers are defined as "whatever is necessary in the current situation".
The mutant Lifeguard has essentially the same power. She will develop whatever power will be necessary next to save lives. So, unconscious precognitive adaptation.
Also in the same vein is Darwin, whose body will evolve on the fly to meet the problems in the situation, even though Darwin has no control over what evolves or how it works. Lampshaded during World War Hulk when his power decided the best defense against a rampaging Hulk was...to not be there, as illustrated by his teleporting away.
Which was pretty brilliant, although the power originally created a Gamma Energy Draining power to drain power from the Hulk to weaken him, but the Hulk is one of those sorts who fit the 'generates more energy than the enemy can hope to drain' trope so Darwin was getting nowhere and after being knocked unconscious by the Hulk his power reasoned it had no hope of defending against the Hulk directly and got Darwin several states away where it was relatively safe.
Legion of Super-Heroes (more uses below) took this specific version and applied it even further. Ra's Al Ghul set the Moon on a collision course with the Earth. This gave off "hypertaxis energy", which caused humans to evolve to survive a threat before it happened.
X-Men's Marrow had her heart torn off her body by Storm, but later was revealed to be alive. How? Spare heart.
Speaking of Storm she can slip into this herself (her use of lightning in increasingly improbable ways qualifies), it even bleeds into other adaptations. For instance, did you know that she can apparently use Cerebro in the Black Panther animated series?
Might as well put Magneto in there as well. He started off with the ability to control metal magnetically, then developed the ability to fly with a reasonable enough explanation. Then, as stories became more ambitious, he was suddenly able to control the entire electromagnetic spectrum, which effectively made him invincible. Of course, then there's the Planet X story by Grant Morrison, in which he's powerful enough to (somehow) control gravity and time. (Grand Unified Theory?)
Grant Morrison used this trope by an Ass Pull a Cerebus Retcon in his run on X-Men by introducing "secondary mutations", which would grant entirely new sets of powers to mutants, even years after they first gained their powers. This was his excuse for turning Beast into a cat-person and letting Emma Frost turn into living diamond for no obvious reason besides Rule of Cool.
It was extra fail on the part of Beast since his blue furry ape form was already his secondary mutation, his original mutation was still fairly human looking only with more ape-like build and feet that could work like hands plus the agility and strength. He went blue and furry (and briefly had a super-healing factor at Deadpool or Madcap level) after drinking a mutagen he created, so the ugly cat-like form makes even less sense in light of that.
One Chris Claremont story suddenly gave Storm Super Senses, because she could feel the effect everyone around her had on the local air pressure or something.
Dave Cockrum used to drive John Byrne nuts by giving constantly giving Nightcrawler new powers almost every issue back in the earlier X-Men days. Such as invisibility in shadows, or wallcrawling.
Professor Xavier's less-seen powers include telekinesis and the ability to give other people telepathy.
As originally written pre-Retcon, Phoenix was merely Jean Grey's "ultimate potential as a psi." She'd never shown that she was capable of that level of power before, and later stories brought in outside influences, but originally Jean spontaneously unlocked awesome powers when faced with death.
In the Legion of Super-Heroes, Tyroc had the power to warp reality with his screams. (Of course, this made the "screaming" part just color...no pun intended.) He could do nearly anything, from teleportation to pyrokinesis to... making it rain glue. The character was soon written out; common wisdom is that the writers had no idea what to do with him. In his recent reintroduction he seems to have been Retconned into having more conventional Banshee/Black Canary scream powers.
In the Legion of Super-Heroes supporting characters, Duplicate Boy had the ability to copy any power he wanted, including those he made up. Of course, his abilities were rarely used properly by the writers.
He could copy the powers of anyone he's ever met including multiple powers at the same time much like the Super-Adaptoid. So he was effectively the most powerful being in the 31st Century, which is why they had a 'the rulers of your homeworld deem you must remain here to protect it' restriction on him along with the rest of his team for why he in particular never had an impact against the villains that showed up after his introduction.
The villain Nemesis Kid had the ability to temporarily gain whatever power he needed to fight any single opponent. This one was used just as badly; he was killed in hand-to-hand combat by Queen Projectra — without her using her illusion powers — the only given reason why his ability didn't provide him with invulnerability as well as immunity to illusions was being too intimidated to concentrate on activating his power. One would suppose he would gain invulnerability against physical attacks against any foe capable of throwing a punch... And no, he never fought Duplicate Boy.
Nemesis Kid's powers explicitly only worked on one power at a time. That's why Projectra was able to simply beat him to death: His power was occupied nullifying her illusions.
The Doom Patrol villain "The Quiz" had "every power you haven't thought of". Literally; to fight her, you had to start shouting power names so she couldn't use them.
Gives you a bit of fridge logic as to why declaring 'the power to have every power I haven't thought of' wouldn't eliminate the power and render her powerless since she can't have any power you've thought of and her root power is told to you.
Inverted in an arc of Exiles in which the team arrives on an Earth where the Skrulls have ruled since the 19th century, and several of them are thrown into a gladiator arena to fight other superpowered beings. Mimic, a mutant with the power to copy and hold onto the abilities of up to five other mutants, strikingly showcases "all four" of his various powers as he fights his way to higher tiers of the arena, until he finally comes up against "The Champion", that universe's version of Captain America. The Skrulls are expecting an epic fight, when Mimic ends it in ten seconds by letting loose optic blasts he copied from the X-Men's Cyclops. The reader knows he has this power (if he's been paying attention), but the audience is shocked.
While not powers, per se, Batman seems to always have that one thing in his utility belt that saves the day, despite there never being mention of it before. This was especially true in the Silver Age, on the TV Show (shark-repellent bat-spray), and on the Super Friends ("You're a mouse? I'll put you in the bat belt mouse compartment!"). Fans have come to expect him to have all sorts of basic toys there (as well as a chunk of kryptonite in a lead-lined pouch because you can't be too careful), and the better writers either have him specifically preparing for a fight or have him MacGyver a solution out of things you would expect him to have.
The writers have also shown that Batman, down in the Batcave, has a set of dossier folders on every single hero and villain on the planet, with detailed plans on how to take down each and every one of them if he ever needed to. This even includes the really, really stupid villains for whom the plan ought to be "oh just kick his ass already."
The movies have their share of oddly specific and convenient gadgets, too, such as:
The Bat-Stop-The-Guy-About-To-Drop-Kick-Me-Arm-Apparatus from the 1989 film.
Captain Everything from normalman was the most powerful being on the planet Levram simply because he could defy all laws of physics, exhibiting a new power at every plot twist. Of course, this is just one of the ways in which he's a parody of Superman. He was also a complete moron, who forgot that he could fly while in midflight.
Also from the DCU, Infinity Man had the ill-explained power to, uh (googling it), bend all natural laws. He can modify the atomic structure of things. Good.
Resurrection Man's powers are literally dictated by the plot; anytime he dies, he'll come back immediately possessing some power that would have allowed him to survive what killed him. Drop him off a cliff, now he can fly, shoot him, now he's bulletproof, etc.
New Spider-Man foe The Freak has the same ability.
As does Doomsday, the only monster to ever kill Superman- except he develops new abilities that counter anything that harms him. At one point, he develops bony ear coverings to counter a powerful sonic gun.
Until he is finally undone by the one thing that he evolved that made him weak: Sentience.
Superman: You're different now. You can think for yourself. So think about this. Before, you were a mindless thing. Nothing could hurt you. You couldn't feel pain, much less understand it. But once you have felt it — it changes you — forever. And you'll begin to understand something new. Fear. I've lived with it all my life. You don't want to die again, do you? The agony of what's happened to you affects your speed — your strength... and that little bit of doubt — that you cannot win today — grows.
Although that's mostly Superman trashtalking to use psychological warfare on Doomsday, since the very things he says weakens Doomsday in no way have ever stopped Superman because being sentient in no way keeps Doomsday from overcoming that fear and fighting on anyway. There's no reason why Doomsday can't be emboldened by his ability to always return from the dead, plenty of other characters get by just fine with far less power than Doomsday has beyond the ability to return from the dead.
Doomsday's power could be summarized as, each time he dies and comes back, his overall strength and power increase AND he's made immediately and instantaneously invulnerable to and has the capacity to kill or destroy whatever it was that killed him.
Dial H For Hero is based around a mysterious dial that enables an ordinary person to become a superhero for a short time, by selecting the letters H-E-R-O in order. Each time it is used, the dial causes its possessor to become a superhero with a different name, costume, and powers.
In the children's comic Korgi, the magic korgi spontaneously develops the ability to breathe fire.
Don't forget Ivy suddenly revealing that she has wings a la The Dark Crystal. These sudden powers are perhaps more jarring because the main story has no dialog whatsoever, and the only indication that the korgis are magical comes from the introduction - we're never given any hint as to how this magic manifests.
The DC villain Paragon has the power to mimic the superpowers of any superhero near to him. But he can also add a twist the originator cannot perform, so he thinks he is superior because he can use any power better.
In a non-superheroic example, Thorn from Bone displays more and more ludicrous powers as the plot goes on, everything from simple Psychic Dreams for Everyone to seeing invisible ghost circles to super-strength to flight. This is because her "true" power is power over dreams, and the awakening of the Big Bad is bringing the Waking World and the Dreaming closer together.
The Engineer from The Authority is a repeat offender here. Her "powers" are derived from the "nine pints of liquid machinery" that was developed from a combination of her own research and that of another genius and with which she replaced her blood. It basically means she can create virtually any device she can conceive of on the fly. However, while originally this seemed to be limited to what she could shape out of the actual nanoblood, the scope kept increasing until she could eventually build even very large constructs on-demand, as well create duplicates of herself that shared her abilities. The only limit being how many different things she could mentally multitask at one time.
Seth, the ridiculously powerful metahuman sent to kill and otherwise maim the members of The Authority, might as well be a walking Green Lantern Ring. Having been designed to take down the most powerful superhero team in the world, he is given just about every superpower that his creators can imagine, at one point stating that he has powers "that [his enemies] don't even have names for".
The Mighty Thor was explicitly intended to be the most powerful superhero in the Marvel Universe, and in the early days this seemed to mean "modeled after the Silver AgeSuperman." He whipped out abilities like time travel and even super-ventriloquism on occasion (making his lame-ass early villains even less challenging) before his powers became more clearly defined (and his villains got much more dangerous).
A very '90s miniseries called The Psycho, by James Hudnall and Dan Brereton, is set in a world where people gain superpowers by taking various drugs. At one point the title character develops the ability to breathe water— or maybe he had it from the start; after all, there's no way of knowing until someone's trapped you in a flooded room...
The eponymous Empowered has on at least three occasions demonstrated powers she had no idea her suit possessed: Clinging, surviving in space, and very possibly flight. She's not aware of the third.
The female Green Lantern Arisia, a one-time fling of Hal Jordan's, was thought to have perished. She was found years later (somewhat randomly) on the planet Biot in a pod. We were then told that Arisia's species can go into a deep state of mental and physical hibernation while only appearing dead. All this was done so Geoff Johns could put Arisia into the Green Lantern Corp ongoing. Not the most elegant way of bringing someone back to life.
Hawk and Dove. Holy crap, Hawk and Dove. Geoff Johns likes them so much that one of them will just have whatever powers they need for the plot to work. Army of unstoppable zombies? Well hey, Dove just happens to have an anti zombie laser inside her. Boyfriend dies? Dove can totally hear ghosts all of the sudden. Dove's in trouble? Hawk just happens to have the ability to sense when Dove's using her powers even though he's never had that power before. Sigh.
NICOLE of the Sonic the Hedgehog Archie comic series (and to a lesser extent, the Sonic Sat AM animated series), a small handheld device with utilities ranging from a translator, laser device, a protective forcefield and a scanner that can devise info and history from almost any object or area. In later issues NICOLE was evolved into the powerstation for New Mobotropolis from which she can transport or materialize almost any entity to the heroes' convenience, though at least by this point her multiple powers are becoming less of a surprise.
Spoofed in Tomorrow Stories with Splash Brannigan. "He followed them into the painting! I didn't know four dimensional ink could do that!" "Well duh! It can probably do whatever story purposes require."
At the end of the Buffy Season 8 comics, Buffy gets new powers like flying and super speed due to Twilight.
Herbie The Fat Fury got various superpowers from eating lollipops. These powers could be literally anything, from invulnerability and super-strength to hypnotism, talking to animals, time travel, and knocking out uncooperative indian chiefs.
The Molecule Man, a Fantastic Four villain can control molecules, so he can do just about anything, but he's not the brightest bulb in the shed and not completely evil, so he's often beaten before he can really use his imagination.
Marvel's Doctor Strange makes even Silver Age Superman look downright consistent by comparison. One week he might say it's impossible to change the past, the next week he might casually rewind time by twirling his little finger and prevent the villain from ever being born. At his worst, it was less a case of him getting new powers as the plot demands, but more a case of him being able to do absolutely anything, unless the plot specifically required that he couldn't. This is probably one of the major reasons why the writers eventually stripped him of his Sorcerer Supreme title and most of his power.
In early issues of Marvel's Fantastic Four, the Human Torch would often demonstrate seemingly random new powers all the time. Although all of them were (at least thematically) linked to his ability to control heat and fire, many of them made little sense (some examples include sonar heat waves, creating a lasso made only partially out of fire that could pick up paper objects without burning them, and the ability to surround an object with a coating of fire that could detect and react to someone's human aura in order to guide the object to them.)
In HHHHHHHHHHHHHHHHHHHHHHHHHHHHHH, Harry Potter is given a wide variety of powers never had in canon, including groinsaws, the ability to punch astral vampires in half, the ability to summon holy fuck fire and meteors with his guitar fuck slayer, and the ability to see subatomic particles by squinting.
The Adventures of Kitty Pryde by Melodyrider (a series written as an ongoing companion comic to the Joss Whedon run on Astonishing X-Men) has this chapter where Kitty, Colossus and X-Factor take on a misguided future version of Kitty, who, while not having any new powers, was able to apply her powers in new ways that Kitty hadn't considered before, including phasing through dimensions, sending bioshocks of people who she phases through and phasing through light.
In a Bleach Fanfiction Wiki, Miharu Kurosaki's Zanpakutou is quite possibly the embodiment of this trope, creating anything or having any given effect the wielder (or in this case, the creator of the character) imagines. It's command is even "Improvise".
In Young Offender, 001'sPsychic Powers are constantly evolving, and he often discovers new talents right when they're needed. However, this doesn't help Ivan's mental stability at all; he's particularly terrified that his powers might eventually drive him mad, just as they did his aunt Katarina. His potential also terrifies his own teammates, and to make matters worse? Katarina confirmed that he will develop the same powers that drove her insane, though she believes he might be able to control them if he doesn't give in to the desire to change the future.
Rose Potter in The Girl Who Lived has a tendency to get random superpowers and spells for no reason, solely for the purpose of making her seem "cooler."
Films — Animation
Lampshaded and played for laughs in Who Framed Roger Rabbit?, Roger Rabbit meta-explains his ability to escape his handcuffs easily, when he left them to help stabilize the table as Eddie Valiant was trying to saw them off.
Eddie Valiant: You mean you could've taken your hand out of that cuff at any time?! Roger Rabbit: NO! Not at any time — only when it was funny.
In Jason Goes to Hell: The Final Friday Jason is ambushed by the FBI which leaves his body completely obliterated, forcing his heart to evolve into a small creature that can hypnotize and possess people.
Once Neo realizes he's "The One" in The Matrix, he can pretty much do anything, which is exacerbated played down in the sequels — presumably the writers realised that having a Reality Warper who could kill the bad guys with a thought would kill any kind of dramatic tension. Of course, this leads to problemsof its own...
This trope is also justified at different points in the films, as skills and knowledge the protagonists need can be uploaded directly into their brains.
R2D2 in Star Wars manages to do just about anything when the plot requires, especially in the prequels. Like fly.
The Force itself is this. The Force is apparently able to do anything from seeing the future, to shooting lighting from the hands, to changing creatures into grotesque monstrosities, and so on, whatever's required for the plot of the week. What keeps the Force out of Story Breaker Power territory is that many powers seem to be unique or rare enough that only a few Force Users are actually able to use them, and that the Force is pretty fickle.
Nightmare City, a low budget Italian flick where protagonist Dean Miller has a touch of Commando Concentrate™ (Just add weapons!). A news reporter with no combat training magically knows how to make functional firebombs and handle a sub-machine gun.
The Superman movies were even worse than the comics with this. The movies introduced:
The most infamous new power, from Superman IV: The Quest for Peace: The Quest for Peace, "Rebuild-the-Great-Wall-of-China Vision". This one is especially egregious considering that Superman could have rebuilt it using his existing powers of flight, super speed and heat vision.
Kryptonians suddenly also have the ability to teleport/blink at will, and shoot kinetic beams from their hands in Superman II as well.
Flying around the world backwards to reverse time, though some consider this a visual metaphor taken too literally. Superman could and did travel through time in the comics.
Considering all of the above examples, it comes as a surprise that 1984's Supergirl completely averts this trope. Supergirl has all the powers she's supposed to have, but no "extras" are added.
Horribly abused in Midnight Movie. Try to escape through a window or door? The killer makes them impenetrable. Try to call for help? He disrupts phones. Try to get the attention of someone on the outside? He makes it so no one can see or hear you. All that, combined with him being Made of Iron, being able to teleport, and being able to find people wherever they hide due to literally sensing fear and you've got one of the most unfair Slasher Movie villains in history.
Grandpa Seth in Troll 2 can do pretty much whatever he feels like, by virtue of being dead. Although the ability to stop time he showed at the beginning of the movie would have been very useful later on, to say the least.
Iron Man's previously unmentioned one-use lasers that he used to finish off the Hammer Drones in Iron Man 2. Justified and generally detailed beforehand, though, as Tony never actually stops upgrading his suits, and will readily adapt them as the situation demands.
The hero of Puma Man has a number of random seeming powers such as flight, jumping around really high like an idiot while his sidekick does the fighting for him, teleportation, super strong and sharp claws, superstrength and the ability to go into a deathlike trance. (The Puma Man has the powers "of a Puma Man", rather than them being derived from actual puma abilities. This essentially gave him whatever powers the writers needed to continue the script.)
Parodied in Michael Chabon's The Amazing Adventures of Kavalier and Clay, in which our heroes create a comic strip character, The Escapist, just before the start of World War II. He begins as a detective-escapologist character. By the later years of the war, he's pulling tanks apart with his bare hands.
Happens to nearly every plot-relevant magician in Raymond E. Feist's Riftwar series at some point or other. The meta main character, Pug, seems to experience as much of his development by being forced into new powers by circumstance as by study and learning. Nakor also exhibits this frequently later in the series, though it's implied that he has known his new 'tricks' for a long time and simply did not choose to use them for whatever reason.
Anita Blake is the best example of this ever, having morphed from a simple animator/necromancer in the book series to... frankly, this editor lost track of them all a long time ago. But in pretty much every big confrontation, she gets a new Power of the Month.
In every single book, Anita pulls a new power out of her ass, spends a bit in the hospital from "overdoing it", and from then on can use the power whenever.
Princess Merry in Laural K Hamilton's other main novel line is just like Anita Blake. Starts out as having only a slightly extended lifespan and the ability to craft glamours (illusions) around herself to alter her appearance, making her effectively human in power level. Then gains the power to turn someone inside out by touching them. Then in the second book gains the power to make someone bleed to death in a matter of seconds out of any cut, no matter how small. Then, because it's a Laural K Hamilton novel gains the power to give anyone who has sex with her major mojo. Then gains more random stuff for herself that's typically forgotten by the next book.
In the second book of the Night Watch series, we are introduced to a character called "the Mirror", who is capable of becoming more powerful and acquiring complex magical abilities in order to match whatever situation he is facing at the time. It is justified due to the fact that a Mirror is formed from the magical Twilight for the specific purpose of redressing imbalances in the power structure of the magical Others, and once that goal is accomplished, it ceases to exist.
The resolution of the Telzey Amberdon story "Resident Witch", by James H Schmitz, relies on Telzey's psychic powers including the ability to Body Surf, despite no previous indication that she could do this.
On a series level, not within the series. Apparently certain children of Hephaestus can wield fire and all children of Aphrodite have apparently always been able to speak French, and some can charm-speak people into doing what they want. Justified as it's stated that controlling fire and charmspeaking are ridiculously rare, though some fans still think Aphrodite's children speaking French came from left-field.
There was only one named camper from the Aphrodite cabin in the first series, and she was never in a situation where speaking French would have been remotely helpful. Combining that with the fact that Piper seemed honestly surprised she could speak it, and it's fairly justified.
It was also never really delved into in the first series just what powers were granted to each god's children, and they seem to vary quite a lot among themselves as well.
There's also the convenient twin features that nobody actually knows what his powers are or how to use them intentionally, and his powers always seem to activate instinctively when he really, really needs them. He eventually finds a book the should tell him how his powers work...but cannot read it at the time because he's been depowered (it's established early on that there are magical trip mines in magic books that erase the memory of the contents from the mind of anyone who really shouldn't read that particular book).
The whole Flock in the fourth Maximum Ride book, and Angel throughout the series. All of the flock get this throughout the series, it was just not until the fourth book that Patterson threw up his hands and decided that they were uncontrollably mutating which would cause them to develop random, unplanned powers.
Also more of a case of New Powers Because The Writer Feels Like It, as very few (if any) of them are ever actually used for anything useful.
Daniel in The Dangerous Daysof Daniel X by James Patterson. Almost every chapter he gets a new power. He can create people from thin air, control animals, turn into animals, create different scenes(like turn a messy room into a clean room), has an internal iPod, and is incredibly intelligent. And this is just the first book.
Dwarves in Artemis Fowl get a new ability every book. They can tunnel by eating through earth, fire a devastating barrage of digested rocks/mud/whatever they just dug through, propel themselves underwater and ignore the bends because of intestinal bacteria, have saliva that works as a healing balm, can cling to walls if dehydrated, have glow-in-the-dark spit, which can also solidify to trap enemies, have prehensile beards/antennae (very handy lockpicks/emergency automatic surgical needles).
In The Atlantis Complex, they can now expel 1/3 of their body weight out of their rears as a sort of jet-assisted escape. It's stated that this is a natural reaction to life-or-death situations.
The Dragonriders of Pern books are guilty of this when in the 2001 book The Skies of Pern, the characters suddenly discover the foreshadowed power of telekinesis when the book's protagonist's dragon buddy gets attacked by giant cats (and when they discover that by getting rid of the Red Star and Thread, it opens up Pern for bombardment by meteors).
The Ohmsfords, main characters in some of Terry Brooks' Shannara books, run on this. Fair enough, their power is actually called "The Wishsong", but it means the plot follows a hundred iterations of "Boy he's screwed," ... "but suddenly the song asserts itself and does whatever he needs!"
In The House of Night series, Zoey has this. And later, Stevie Rae and Aphrodite get it as well.
In the Molly Moon series, Molly picks up a new power for each book. She starts out semi-plausible in the first book by learning how to hypnotize people, but from this basis power she begins learning other, increasingly exaggerated, powers: In the second, she learns how to stop time. In the third, she learns how to travel through time. In the fourth, the new power is mind-reading, and in the fifth, it's shapechanging.
In the Star Wars Expanded Universe, Luke Skywalker goes through this to a certain degree. After several books, it became known in-universe that he is ludicrously powerful and can learn pretty much any Force-based skill very quickly. Many of these skills never come up again, or only when that same author writes him again.
Allegiance has the spirit of Obi-Wan guiding Luke through discovering the unlock code for the room he's locked in.
Rebel Force: Luke is captured by someone who's developed a particularly nasty form of brainwashing, involving destroying someone's past and personality to make way for a new, loyal one. He escapes shackles by persuading them to stretch wide enough to let him pull his hands through but is put through the procedure anyway. To all appearances and instrumentation Luke is brainwashed, but thanks to the Force he is in fact completely unaffected. This power also keeps his morale up during the process.
In The Empire Strikes, Luke ends up in a coma and is captured... and later his body fights free of restraints that should have held him when awake, finds his confiscated equipment, and fights off everyone who tries to stop him. While he was in a coma. He came out of the coma completely ignorant of what had transpired.
In Saber Clash while fighting Orman Tagge, a man who had had years more experience in the art of the lightsaber, Luke was initially at a severe disadvantage and almost died... but the Force then let him make a comeback that had him fighting without art or polish but with such skill and control that he was able to cut off Tagge's cyber-vision goggles, leaving him unhurt but a quivering, shocked wreck.
The Last Jedi has Luke calling on the Force to show him where his enemies are and how soon they'll reach him. The artist chose to portray this in a way visually similar to Toph sensing vibrations, albeit more colorfully and rippling out from his head.
In The Truce at Bakura, he can talk to the parasitic lungworms infesting his body, which will soon kill him, and persuade them to stop chewing on his tissues and crawl out of his mouth.
In Luke Skywalker and the Shadows of Mindor, Luke is able to absorb blaster bolts without harm, communicate with meltmassif creatures that do not understand the concept of organic life, 'feed' said creatures so that they are forever free of the Big Bad's control, and in a very trippy mind-battle, when the Big Bad becomes a black hole and swallows him, he becomes a white fountain.
The Courtship of Princess Leia has him casually root through Isolder's memories while the other mentions his past, and when they're falling he manages to slow the falls down.
The Thrawn Trilogy, and Timothy Zahn's work in general, gives him the ability to read someone's presence like a second face, picking up on emotional states and knowing if someone's had an idea, the ability to enhance his senses, plus a kind of short-term Photographic Memory - he can rewind his short-term memories, within an hour or so, and recall with perfect accuracy things he wasn't paying attention to before. There is mention of him learning this from Yoda, at least.
Dark Empire grants him the power to make legions of droids self-destruct, the ability to generate dopplegangers, send two way visual/audio messages across great distances, and in the audio drama, the ability to fix hyperspace anomalies.
In the Jedi Academy Trilogy, a prospective student won't come study with him until he's crossed a lake of lava. Half way across, Luke fights a creature living in the lava, which covers up the stones he's been hopping across. Not a problem! He extends the power he's been using to defy convection and just walks across the surface. He's also able to invade peoples' minds to find if they're Force-Sensitive.
In I, Jedi he has the ability to retrieve lost memories and damp down any of his senses.
The Black Fleet Crisis gives him rather pointless super-architectural powers which would make anyone who's ever worked with stone white with envy. He goes to the beach where his father once had a fortress and finds only widely-scattered rocks.
"The sand around him stirred. The rocks shuddered, shifted, then began to rise from the sea and the sand as though sifted from them by an invisible screen. Swirling through the air as they sought their place, the stones took shape as broken wall and shattered foundation, as arch and gate and dome-the ruins of Darth Vader's fortress retreat. It hung in the air around and above Luke as it had once stood atop the cliff, a dark-faced and forbidding edifice. [...] As he had redeemed and reclaimed his father, he would redeem and reclaim his father's house. Now the stones swirled again in the air, joined by others plucked from the sea and stripped from the face of the cliff. Now broken edge fused against broken edge, and the dark faces of the rock lightened as their mineral structure was reshuffled. Now heavy rock walls and floors thinned to an airy elegance as if they were clay in a potter's press."
He then instantly builds a tower that is perfectly camouflaged with its surroundings, has the gravity act however he wants it to, and makes door and window holes open wherever and whenever he pleases. In lieu of furniture he tells a guest to sit and forms an "air cushion" under them.
The New Rebellion extends the heat-redirection power, letting Luke literally heat the hollow insides of some hostile acidic balloon-things until they burst.
Harry Dresden has a fair amount of this going on, but all of it is rigorously justified. Some of it is off-screen, and he'll remark to various friends or to himself on why he made that Focus or practiced that skill. Demonreach would be an on-screen example, and so would becoming the Winter Knight, though Mab has been predicting that he will do so because he will need to invoke this trope.
The only seemingly-inexplicable examples were Hellfire and Soulfire, but those turned out to be justified.
Justified in Villains Inc. (sequel to Wearing the Cape) when an encounter grants Astra the ability to sense magic, allowing her to make several observations key to the plot. It is suggested that this added power is temporary, however.
In Harry Potter and the Prisoner of Azkaban Lupin repeatedly points out that the Patronus charm is incredibly advanced magic. Lupin highlights that only very powerful wizards can pull it off, and that even fully qualified wizards struggle to master it. By Harry Potter and the Order of the Phoenix, pretty much everyone in the narrative can cast a full corporeal Patronus without any trouble whatsoever, including most of the adolescent members of the DA.
Max Frei books run on this. Throughout the first books, Sir Max discovers a new magic skill in almost every chapter, making him essentially a Creator's Pet.
Live Action TV
Angel: This happens to Cordelia in Season 4, after Jasmine seizes control of Cordy's body.
This happened a couple times on the 1950's Superman TV series. Discussion above in the Comics section.
Spock was a master of this. In various episodes (and movies) of Star Trek, he suddenly demonstrated the abilities of mind-melding, the Vulcan nerve pinch, a light-protective nictating membrane, the ability to go into a deathlike trance at will, and a detachable soul that would allow him to later come back from the dead. Absolutely none of these were telegraphed before he absolutely needed them (as opposed to say, Wesley Crusher being told he had a great destiny by the Traveler long before he pulled the ability to stop time out of his ass.) This, plus his refusal to admit that his parents were the ambassador and his wife or that he had to have sex with his wife or he'd die, make it almost plausible that as of Star Trek V he could have had a long-lost half brother he never told anyone about. Almost.
Klingons get some of this once they cease being Always Chaotic Evil. For instance, in the Next Generation episode "Ethics", a shaky camera accident breaks Worf's spine, paralyzing him. During the experimental operation to replace his spine, something goes wrong, and he goes braindead. For a moment, it looks like disaster; then his other neural system kicks in.
Perhaps he asked for a raise.
Seven of Nine on Voyager had Borg implants/nanoprobes that could allow her to do everything from sense otherwise-invisible aliens to raise the dead when required.
In the first appearance of the Tenth Doctor in Doctor Who, the Doctor gets his hand chopped off in a sword fight. Luckily he remembers that he has enhanced Healing Factor shortly after a regeneration and grows back his hand.
Of course, the Doctor's ability to regenerate is the result of the show's creators needing a way to explain the actor switch from William Hartnell to Patrick Troughton. And as none of the first nine Doctors had their hand chopped off a few hours after regenerating, it's not really a retcon here.
The Doctor's use of this trope pales in comparison to his sonic screwdriver, which has thousands of settings. A list of everything it has done would be as long as this page. The original series wrote it out as it was becoming omnipotent and the writers used it as a get-out-of-trouble-free card too much. The Movie brought it back, and the new series imposed some definite restrictions on its abilities so as to have a reason not to let the Doctor use it to get out of anything. It's still pretty handy, though.
A good rule of thumb is 'Interface with any electronic device from up to thirty feet away', 'open any lock', and 'work as whatever tool is needed to perform the repairs at hand': it's basically the ultimate swiss army knife with built in wifi and lockpicks. It doesn't do wood, though.
In the second episode of the new series, Eccleston's Doctor suddenly has the power to focus his mind and walk between the blades of a giant spinning fan. It would have been nice if he'd remembered he had this power before Jabe died...
The extent of the Doctor's psychic powers in the new series also depend on whatever is necessary to make the episode work. In 'The Girl in the Fireplace', the Doctor shows he can read minds, though in many other episodes he doesn't even when it would be useful (admittedly, moral qualms might stop him from reading minds without consent, but it is strange that he never mentions it). Likewise, in 'The Lodger' the Doctor headbutts Craig to telepathically implant information about who the Doctor was and why people should take his advice, and again, the Doctor has never used this ability before or since, even when it would make sense to do so. Then again, headbutting people can be taken as an act of violence that could get him killed in the wrong stitch, and the Doctor had become frustrated with his subject's disbelief in him, leaving him little choice but to give him the abridged tour of his mind.
The Doctor's previously unseen "mind meld"-like abilities get enhanced even further in 'Journey's End' when he's able to use them to selectively erase parts of Donna's memory.
This ability, however, had already been displayed by the Second Doctor in "The Abominable Snowmen", where he uses it on Victoria. It's made clear that the procedure is not only invasive but also risky and he will never do it unless he absolutely has to.
He also can use his Psychic Powers to tune into a single thought broadcasted by everyone on Earth via the Master's satellites and use it to de-age himself, dissolve his cage, levitate, form a shield bubble, and give him telekinesis.
Another, particularly glaring instance of this is in 'Planet of the Ood', when the Doctor is able to sense the enslaved Oods' pain through their telepathic field, yet when he encountered them before in 'The Impossible Planet' and 'The Satan Pit' he made no mention of sensing their telepathic field or the psychic entity powerful enough to possess multiple Ood and humans simultaneously, which you'd really think he'd be able to sense under the circumstances.
A classic series example: the Fourth Doctor reveals he has a respiratory bypass system in 'Pyramids of Mars'.
Another classic-era example: the idea of the Doctor having two hearts was not revealed until the first episode featuring the Third Doctor. Prior to this, there were several episodes explicitly indicating the Doctor had only one heart.
Ultraman, Ultra Seven and the other Ultra Series heroes are the kings of this. Though they have a set powers base, many develop and use one-shot energy attacks for specific monsters that are never seen again, or, even, completely pointless in the face of a pre-existing energy attack. And each time they would re-appear in another series, they'd only have the very basic forms of Ultraman powers they were known for. However, the worst offender is Ultraman Jack/The Ultraman Who Returned, who has the Ultra Bracelet — a weapon that can shapeshift into whatever is needed at the time: a shield, eye-slugger, blade, sword or Cross-Shaped Lance to stake an alien named Draculas.
This happens to the ghosts from Ghost Whisperer a lot. Sometimes it gets a brief explanation. Usually not.
Played with Clark Kent. Except for invulnerability, Super Speed and Super Strength, all his others powers (X-ray vision, heat vision, super hearing, Super Breath, in that order) come exactly as he needs them, although they are all established powers in the comics.
Played straight for Telekinesis in Crusade, mind reading in Echo and Mind Control in Persuasion.
Chloe Sullivan. She initially develops healing powers to heal Lois Lane and then Super Intelligence which manifests as a machine-like ability to run search algorithms in her head. Although it is later revealed Chloe didn't develop super-intelligence so much as she gained it when Brainiac took up roost in her mind.
Several characters demonstrate the ability to acquire new powers from other powered people. Peter Petrelli copies them, Dad Petrelli takes them, and Sylar rips them out of their heads (killing them, and he gets to use Peter's copying power later).
In general, the whole series operates this trope at a higher level. If the writers need a new power, they don't give it to an existing character, but introduce a new character with the desired ability. One of the benefits of Loads and Loads of Characters is nobody much notices a few more or less.
Ando is a particular victim of this trope. His power goes from power amplification to concussive blasts to tech manipulation to actual electricity.
Power Rangers have been known to dabble in this area, depending on the series. Conner in Dino Thunder for example, is able to access his Battlizer for the first time ever by just...wishing for it I guess... Generally, more technological based teams are better about this, with new weapons and zords being built and tested prior to use.
An interesting subversion occurs in Power Rangers in Space, in which Andros has been carrying his own Battlizer for a good portion of the season, using it only to power up his attacks and control the Delta Megazord. Eventually Carlos asks him why he's never used the highest power setting on the device, to which Andros replies that he worries that it may be too powerful. Later, in a battle in which he is unable to morph, the final setting is activated and he becomes the Red Battlized Ranger for the first time.
Mind you, Andros isn't the one who activated it—it was pressed by a little girl nearby. For all she knew, the setting was too powerful and it could have blown them all up.
Mahou Sentai Magiranger made extensive use of this trope; all one of the Magirangers had to do was demonstrate sufficient courage or learn an important lesson, and they would be gifted with a new spell suited for whatever predicament they've found themselves in. Mind, this was because the spells were being granted to them by the Heavenly Saints, who were always watching over them. This carried over into Power Rangers Mystic Force, and even dropped the justification as there were no higher powers actively giving the spells.
Granted, this was mitigated by two things. First, Yusuke needed to learn how his abilities worked (his initial fight using Dragon Form, before he knew it used a staff, was a No-Holds-Barred Beatdown). And second, in order to get his stronger forms he had to be shocked with a defibrillator.
Also take into account that those forms did exist (or there was at least a rough outline, hard to tell) prior to him wishing from them, how do we know that? Because the belt of all thing had instructions for them! Granted they weren't good instructions (but at least they got the point across), and they were in another language that only just got translated right when he needed them the most, but the fact that they were there before Kuuga first activated them shows that he didn't just 'wish' new forms that didn't exist before into existence. Heck after he got Dragon form the fact that there was still text left to translate basically forshadowed that there was more.
A particularly egregious villain example happens in "Kamen Rider Fourze". When Gamou is about to send Hayami to the Dark Nebula He conviently gains his Supernova power, the Eye of Laplace. While every previous Supernova just turned a Horoscope into a giant monster Libra instead gains the power to see Zodiarts evolutions so he can just look at someone and instantly not only know if they can become a Horoscopes, but find people who are so compatible that just pressing the switch will instantly evolve them. In the 33 episodes before this, only 2 Horoscopes had evolved during the course of the show (5 were evolved before the series began). After this, a new Horoscope was introduced every 2 episodes Without this power, there would be no way for the writers to possibly fit in the remaining 5 Horoscopes in the last 14 episodes.
Kamen Rider Wizard had this in its early episodes. To wit, Wizard uses Rings of Power infused with different magic spells, and he's friends with a magic-ring-maker, but the ring-maker explicitly has no control over which spells go into the rings he makes. Yet some of the new rings he's given are too-conveniently key to beating the Monster of the Week (a ring that emits light when fighting someone who can teleport through shadows); extremely situational yet too-conveniently manages to be helpful soon after he gets it (a ring that puts the one wearing it to sleep); or both (a ring that makes the wearer stink royally, used against a monster with a keen sense of smell). Thankfully, this dropped off when the show ran out of new rings to promote - a few more rings were introduced later on, but most of them were either given appropriate buildup beforehand or just not essential to the plot.
Sookie from True Blood has gained new abilities as well. She can shoot some kind of energy from her hand. She did it accidentally against Maryann and possibly less so against a werewolf who attacked her in season 3.
It's revealed that this power, as well as her telepathy, as the result of her being part-fairy. Her blood also allows vampires to temporarily survive in sunlight.
Ralph Hinkley from The Greatest American Hero got new abilities as the plot required, sometimes completely forgetting he could do them by the next episode. This could be a possible subversion in the times that he wasn't sure how he'd done them in the first place, but doesn't explain how, in one episode, he suffers damage to his lungs (while wearing the suit that gives him the powers) from smoke inhalation, meaning his lungs are not protected by the suit. In another (later) episode, he's able to inhale a room full of tear gas without harm.
In other words, the show justifies this trope, with the reasonable explanation that the main character doesn't know what his suit can do or how it works. What it doesn't justify is continuity-problematic explanations for specific powers that the plot demands (such as that lung-protection thing) or the forgetting powers issue.
Benton Fraser on Due South, which is lampshaded in Paul Gross's commentary on the final episode. By the end of the series, he is an excellent marksman, fluent in at least a dozen languages (most of them obscure Canadian aboriginal dialects), a skillful boxer, capable of putting himself into a trance indistinguishable from death, able to listen to concerts in his head by reading the sheet music and ''able to place the location of a plane by listening-in to the radio telemetry.
Hardison on Leverage manages to do this with random skills as they become necessary to the team. As of season 4 he has: painted the office picture, become a lawyer, played a Stradivarius violin, taught himself how to be a forger, topping it off by landing an airplane. This is in addition to his normal roles as a computer hacker and Techno Wizard.
Thanks to being dosed with Cortexiphan as a kid, Olivia on Fringe can read minds, move things with her mind, heal rapidly, shift between universes, possess other people, set things on fire with her mind or control nanites in someone else's bloodstream and never usually displaying the same power twice. Basically whenever a power is needed on the show, Walter just goes "Thanks to the Cortexiphan you were given, it should be possible for you to [insert required power for episode here]."
In Buffy the Vampire Slayer's season 3 episode "Lovers Walk", Willow and Xander have been kidnapped by Spike. Oz manages to locate them with his highly refined werewolf sense of smell... while in his human form, which had never been shown to possess any supernatural abilities prior to this. It freaks both him and Cordelia out.
In the Spanish series Los Protegidos the villains get new superpowered kids as the plot demands.
This is done very intentionally in Carnivŕle, which revolves around two leads who are introduced with vaguely defined supernatural powers. At the beginning, we're aware that the protagonist Ben Hawkins possesses healing powers, and that the main antagonist BrotherJustin Crowe has the ability to mentally control people. Then as the series goes on, it gradually becomes clear that neither of them have any real limits on their powers, and that their abilities include (but are not limited to) astral projection, weather control, telekinesis, manipulation of illusions, and the ability to turn water into blood. Not surprising, considering they're from the same bloodline that produced Jesus...
In the British series Misfits, they play this in various ways, the most blatant of which is the inclusion of a character whose power is the ability to give/take the powers of others.
Misfits usually justifies this as what powers a person gets is based on their personality and memories and when the power is given to someone else it can reconfigure itself based on the same criteria. For example: a character suffering from regrets over his own actions gets the power to turn back time, but when this power is given to a holocaust survivor he gets the ability to physically jump back in time to WWII. The power dealer guy might not count as he was introduced to create new plot treads, not to solve old ones. A proper example however is Curtis' Disability Immunity that is never mentioned before the episode it becomes useful and is never mentioned again afterwards.
Sam on iCarly eventually gained a skillset that included high skill at dancing, singing, painting, fighting, lock picking, speaking different languages and computer hacking, all without ever practicing because she's a lazy slacker.
Justified (or, more accurately, HandWaved) in The Dark Crystal. At the moment when it would be most convenient, one of the two main characters, who are the last of their kind, exposes wings and starts to fly. They have this matter-of-fact conversation:
Jen: Wings? I don't have wings. Kira: Of course not. You're a boy.
In general, most role players have characters with predetermined skills and abilities. If the player characters run into a monster/enemy/whatever that is nigh unkillable, sometimes the storytellers may bestow the characters with a new power to help them combat the threat. While instilling new powers on the fly can help create a more exciting story, it can also seem like a cheap ass pull when done too much (making the player characters look like they can't do anything until the plot says otherwise) or done badly.
This is such a prevalent trope that most superhero RPGs have some sort of mechanic to represent it. For instance, the RPG Mutants & Masterminds has a Hero Point mechanic that allows you to turn one of your superpowers into another for a single use. While keeping the new power "in theme" with your other abilities is encouraged, it isn't strictly necessary...
There are also the Variable structures, which let you have a pool of points to devote to various powers and that you can reallocate every round, and actual powers such as Nemesis and Adaptation.
And few feats are similarly open-ended. "Jack-of-all-Trades" makes every one of your skills that you don't have points in work as though you had points in it.
Parodied in Paranoia by the aptly-named "Deus Ex Machina Man".
Also, if games use the optional Latent mutant powers rule, the GM is encouraged to throw the players who don't know they have powers into situations where that power would help. While they've had the power the whole time, it certainly seems like this trope for the players.
Of course, like a lot of things in Paranoia, this often becomes a bad thing because all the players are trying to hide the fact they are mutants.
Fantasy Craft, based (loosely) on open source elements of Dungeons & Dragons rules has a feat titled "I can Swim" which allows the player character to place their new skill points at any point before their next level up, instead of doing it right when they gain a new level. This can lead to the same idea, with characters suddenly remembering that they totally always knew advanced mechanics in the same scene that their vehicle or golem breaks down.
This is because Fantasy Craft is developed from another system called Spycraft, which also contained this feat (Spycraft, in turn, was loosely based on the D20 Modern rules, but the difference is that Spycraft is actually a good system, unlike its source of influence).
The original DC Heroes RPG by Mayfair (later republished by Pulsar Games as the generic superhero game Blood of Heroes) actually included this in a number of game mechanics:
The power "Omni-Power" allowed the user to replicate pretty much any power at the same rank as this power by paying a certain fee (the base cost of a power from character creation).
The advantage "Omni-Connection" allowed the character to suddenly pull out a contact of either low level ("My buddy from college is a night watchman there!") or high level ("Wow, Tommy boy did good! He's the CEO!") by paying a fee of 'hero points'
Buying "Omni-Gadgets" allowed the player to create one use, nebulously defined gadgets. Upon pulling it out, he declared what the gadget's power was, used it, and it was 'burned out', simulating the ability to pull out "Bat Shark Repellant" by declaring the gadget was Animal Control, for example.
Later modifications to the rule set included "Omni-Scholar" (pull a specific area of expertise out of your... utility belt), and other New Powers as the Plot Demands type abilities.
Changeling: The Lost includes the Goblin Vow merit, which basically combines this with Dangerous Forbidden Technique, allowing the person to make impromptu deals with various abstract things to gain new (temporary) powers in exchange for either doing something, or refraining from something. Breaking the deal is ill-advised.
Promethean: The Created recommends this as a way of unlocking new Transmutations, or even shifting Refinements entirely - your character is on a constant journey of self-discovery, and odds are they won't know just what they're capable of until they're put in a crisis situation.
Mage: The Awakening forgoes the previous edition's actual time-travel magic (which basically did nothing but annoy the other players by undoing their actions and/or splitting the party and made combat impossibly bookkeeping-based) with this trope. Technically what time-sphere spells do still involves time travel, but what the players see is just the end effect: suddenly, the Mage spent all of his time at university studying quantum physics and practicing at the gun range even though five minutes ago he was a poetry major with no hand-eye coordination. Mind sphere magic can sometimes have a similar effect by pulling previously unknown (mundane) skills out of the group-mind for short periods or stealing them from another character.
This is one of the tropes that Badass is built on. Buying new powers just requires a flimsy exposition sequence between action scenes (a journey of self discovery about being a dinosaur the whole time, a training montage of you learning kung fu, whatever). Or if you've got "Little do you know I am actually a ROBOT!", you can buy new powers in the middle of fight scenes just by declaring that you were secretly a robot (or a ninja, or a mad scientist, or a shark, or whatever) the whole time.
Following the Batman example under 'Comics', GURPS Supers has an advantage for gadgeteer-type superheroes which allows the ill-defined contents of their utility packs to contain just the thing necessary to escape from mortal danger.
Dungeons & Dragons has this in the Chameleon prestige class and the Factotum. The Chameleon, at second level, has a bonus feat he can change daily to whatever he has the requirements for. The Factotum has a pool of Inspiration Points, which he can use for a buttload of stuff, such as arcane spells, sneak attack, ignoring spell resistance, as needed.
FATE system games such as Spirit Of The Century or the Dresden Files RPG encourage gadgeteers and magic-users to have "undefined" gadgets and mystical artifacts (in the case of Spirit) or potions (Dresden Files), which can be activated later to get a necessary effect at a critical moment. After the story in which they're used, or at certain (GM-decided) intervals within the story, they reset back to undefined.
The writers of BIONICLE tried to avoid this trope with their main bad guys, the Makuta. Since at one point, a huge variety of differently colored and shaped Kraata slugs could be bought, they had to come up with 42 different powers for each kind. Since Kraata are basically physical forms of the Makuta's essence, the writers decided to give these powers to them
They played this trope straight with Artakha, Tren Krom also seems to show off unknown powers (and body parts) at times, but in his case it is justified, since he is just this side of a god, and we barely know him. In the case of the Toa Nuva gaining new powers, it is handwaved that they're a special kind of Toa, who have not yet learned all of their abilities.
Also, it seems that Tahu is going to demonstrate this trope in the near future, as Word of God is refusing to state how many Makuta powers Tahu absorbed from the Golden Armour.
Half-Life 2: Episode 1, the Vortigaunts go from electric powers to stealing the essence of Xen creatures to rescuing Gordon and outwitting the G-Man himself.
In Knights of the Old Republic 2, the protagonist finds him/herself in dire straits as he/she is put into a cell full of poisonous gas. Just as all hope seems lost, Kreia contacts you telepathically, and quickly teaches you the Jedi art of Guybrush-caliber breath-holding.
In Batman: Arkham Asylum, Batman already has his entire arsenal of weapons on the island - he just doesn't bother to activate or get most of them until the plot requires it. For instance, Batman always had the components for the Ultra Batclaw (the upgraded three-shot version of the weapon), but he doesn't bother to upgrade it until he needs to; when Poison Ivy's vines destroy portions Arkham Batcave while he's inside it, it becomes the only way to leave. He also has the Cryptographical Sequencer on him from the beginning of the game - but it only works once he gets Warden Sharp's passcodes.
The sequel, Batman: Arkham City, lampshades Batman's apparent habit of going into danger unprepared.
Alfred: I see you've requested another equipment drop, sir. Have you considered a larger belt?
Batman: Tried it. The extra weight slowed me down.
May come up in Persona 3 depending on your dialogue choices. Assuming the protagonist wasn't just hitting buttons randomly (which you can fess up to), or using her women's intuition (which you can ALSO confess to), how DID he/she know which switch controlled the breaks to the train car? Lampshaded in the manga, which revealed the Male MC had a hidden love for trains.
Literally in Psychonauts. Barring three which aren't plot-important, that you get by levelling up, the game basically hands you a new power at the exact time you reach an obstacle that can only be overcome with that particular power. After the first couple of times, they don't even bother giving you some kind of training course to justify it; they just hand you the merit badge and let you get on with it.
In Vampire: The Masquerade – Bloodlines, you spend XP directly to improve your abilities, you receive the benefits of doing so instantaneously, and you can save and accumulate unspent XP indefinitely, so any time you run into a task that is too difficult, you can increase the requisite ability by spending XP on it. Run into a computer you can't hack? Spend some XP on your computer skill and try again. Two seconds later, you know enough about computers to successfully hack this one.
In Dm C: Devil May Cry, at the end of the hostage exchange sequence, the Big Bad Mundas uses his powers to cause a massive chaotic dimensional shift in an attempt to kill the heroes. While Vergil and Kat attempt to escape by car, an earthquake causes them to be thrown into peril. Luckily Dante gains the power to spontaniously shift too-and-fro between dimensions while those in the car suffer from time dilation, allowing him to leap about and rescue them. Tragically he loses this ability the second the scene ends without comment.
In Gundam AGE Cosmic Drive, your gundam gets upgrades even more frequent than in the anime. Even after the most Tear Jerking scene, where Desil kills Yurin, Vargas stills pop up in the middle of the fight, proudly and happily presenting you with a new armor pack.
Aylee from Sluggy Freelance gets this a lot due to periodically undergoing some involuntary Shapeshifting. At various points she's gained the ability to regenerate, fly, breathe fire, extend and retract poison spikes, and emit electro-magnetic pulses. She loses most of her old abilities whenever she assumes a new form, however, so it hasn't made her overpowered.
Aside from the involuntary nature of her shape-shifting, she's also hampered by the time it takes to adapt. She could enter a cocoon and mutate a new form to counter the current threat, but emerge only months after said threat has been dealt with, leaving her in a body she has no idea how to operate or maintain.
Wonderella totally gets like a spillion powers when tied up.
The Monster in the Darkness from The Order of the Stick. The author has stated that he is a pre-existing monster, but we'll have to wait and see how well his abilities synch up with what he is.
It's been suggested that he has Wish as a spell-like ability. And Wish can do practically anything.
El Goonish Shive has a magic system seemingly specifically designed to work this way. Any "Awakened" character can just suddenly develop any power the plot needs, any time it's convenient for the writer.
Axe Cop, having sprung from the imagination of a young child during playtime with his much older brother, tends to have characters randomly gaining powers left and right. Sometimes its explained, and sometimes it's "the secret technique no one knows" or something one of the characters "always had". The adult drawing the strip and crafting it into structure plays such moments for all the laughs they're worth. This truly meets its apex when Axe Cop gains the ability to fly by asking his creator to give it to him.
In Homestuck, the powers of Sprites are never fully defined, allowing them to do pretty much anything as long as it runs on lasers and matches their personal motifs.
Played for Laughs in the backstory of Grrl Power. Maxima, the most powerful super on the planet, has so many powers that she used to just discover new ones every once in a while. Such as when her roommate popped a bag by her ear as a prank, and she blasted a giant hole in the wall, missing him by less than an inch.
Maxima: So...apparently I can do that.
* Mountain Time has Dave, who has shown the ability to duplicate himself (at least enough times to make a basketball team), teleport, shapeshift, fly, create portals to other dimensions out of nothing, pluck out his heart and turn it into macaroni salad, and cause others to shrink and/or grow. He is also adept at carpentry. As these acts are all done at his whim, there's no telling how much he's holding back.
Quite a few of the people in the Whateley Universe are vulnerable to this, Fey and Jade. Chou is something like this, except hers is more Power Creep, Power Seep. A lampshade is hung in Call the Thunder 6.
Jade is an exception. All she can do is "possess" objects using "spirit-selves". After a radiation accident, she can regenerate. However, within the "possessing objects" thing, she has a variety of applications of her power. Her power is closer to a Green Lantern Ring in that respect.
Characters who used a Green Lantern Ring or a Swiss-Army Weapon were often granted more leeway with this than other characters, but even then the players in question had to justify their taking certain of the odder, more "out there" powers.
The only character who was really allowed to get away with this was the Blood Red King, but he was a different kettle of fish altogether.
Italian Spiderman has this in spades. He can teleport, outrun motorbikes, make chickens lay eggs (or cigarette packets), control spiders, summon penguins, fly, and his mustache can be detached and used as an exploding projectile.
In Phaeton Trayen does this all the time, justified in that his power is controlled by the osmosoul.
In Worm, Eidolon explicitly has this as his power. In any given situation, he can focus on the powers that he needs to combat it and end up with up to three major powers. They take time to build to full strength, but when he's prepared he is effectively worth any three high-level heroes.
Ultra Fast Pony uses this as a punchline. Thanks to the abridging process, all of the canon foreshadowing about the powers of the Elements Of Harmony gets left on the cutting room floor. So when it comes time to use those powers:
Twilight: Vaguely established magical friend powers, activate! Night Moon Mare: What the heck is that? Twilight: It's a plot hole. Deal with it!
Conveniently in Vaguely Recalling JoJo, Heirophant Green can create gems for communication (Heirophant Call), bandage wounds (Heirophant Mosaic) and splash water (Splash) on things.
A frequent element used in Danny Phantom where the main hero will often get new powers that'll ultimately help him in the end, the most blatant examples being his ghostly wail and ice powers.
Except for the ice powers. He ditches everything for ice, for some reason. Season 3 did this with more than just Danny, though — Danny got ice powers and temporary weather powers, while Kitty got some bizarre kiss-the-men-away power that seemingly came out of nowhere. Johnny better be careful not to upset her now.
Played for laughs once. The Batman Cold OpenMonster of the Week fired an energy beam at Danny, and he generated a reflective shield instinctively. Once the beam rebounded, he remarked:
As a villainous example, Brother Blood fits as well (in fact, his powers seem remarkably similar to Raven's, apart from the Mind Control). Also overlaps with Power Creep, Power Seep, as he goes from a psychic with a Compelling Voice (in his first appearance) to a near-god who can take all the Titans at once effortlessly and is only stopped by Deus ex Machina (the season finale).
Ironically he would be defeated by Cyborg's own new plot-based power, which was to magically leech parts from Blood until Cyborg regenerated all of his mechanical components, conveniently rendering Blood incapacitated. He even lampshades this at the end, where Beastboy remarks now Cyborg is part magical, with Cyborg retorting that it was just a one-time thing. Oh and the power was said to be of love and friendship.
ThunderCats loved this. Cheetara's psychic powers, Tygra's illusion abilities, almost anything the Sword of Omens did. And that's just the tip of the iceberg.
While The Galaxy Trio had consistent enough powers for Gravity Girl (play with gravity, usually by making things fly) and Meteor Man (grow parts of body, super strength follows), Vapor Man seemed able to do just about anything by attaching "-vapor" to the end. This included, but was not limited to: combustible vapor, freezing vapor, storm vapor (read: lightning), explosion vapor, and steam.
Artha and Beau from Dragon Booster display this a lot. It is explained that Beau has many hidden powers that would manifest themselves with training and experience. This, however, does not explain why the majority of these powers only appear for one episode and then vanish for the rest of the series. Especially jarring in the case of Artha and Beau fusing together at the climax of one episode, as the theme of combining abilities was central to the series.
In an episode of Doug, Doug quickly regrets inviting Skeeter in on creating a story about his superhero alter ego Quailman when Skeeter's own avatar the Silver Skeeter starts pulling powers out of his ass left and right.
His inspiration, the Silver Surfer, is known for doing the exact same thing. Power Cosmic is more or less a ticket to do this.
For that matter, the Omnitrix talking, from the same ep.
The adding of new aliens in the original series almost always worked this way (except for the monster aliens), with their powers just happening to be useful towards the Monster of the Week. It would sometimes be done in different ways such as someone with greater knowledge of the Omnitrix unlocking a specific function or just random unlocking from playing around with it
Same for Ben himself. The writers decide to give him photographic memory so that he can remember some runes that the Big Bad had activated.
Alien Force and Ultimate Alien could get pretty bad about this, such at some point showing Waybig with super speed, Chromastone with the power of flight, and Diamondhead with what's essentially telekinesis.
Averted on Generator Rex. Rex is limited to six weapons (jetpack, giant metallic hands, cannon, sword, giant metallic boots, and hover-cycle): he isn't able to use anymore than those.
So far, the series isn't even one season old. And Rex has already been shown suddenly altering his sword and fist to make them hit harder.
Rex also has other powers on top of his nanite weapons; he can heal other EV Os by shutting down/absorbing their nanites and he can control machines.
Except that he had those abilities in the first place. He has, thus far, never gained any new powers.
As of the season one finale, Rex has at least one new construct he can make- a kind of cable or whip- and it's implied he has several other new ones he hasn't used yet. This is, however, the result of a specific bit of phlebotinum that is unlikely to have the same effect again.
Cathy from Monster Buster Club has so many wacky alien powers, it'd be easier to list the ones she doesn't have. She has a stretchy Mr. Fantastic body, can levitate and perform telekinesis, can glow in the dark at will, spins her forearm around like a drill... and many, many more, all conveniently described on the spot as something Rhapsodians (like her) can all do.
And which is just a bit out of character, seeing that Batman is supposed to be a Badass Normal, not a psychic.
He did explain it, and since it is something he learned from Monks, it makes sense. A black belt in everything is hardly normal. And given that this version is a spiritual brother to the campy 60's series, this may be an Invoked Trope.
Robot Boy does this. When the title character "super-activates", it's as though his circuitry starts running on phlebotnium instead of electricity.
The writers of Futurama admitted that they liked doing this when writing for alien species. Kif's abilities to climb walls and shed his skin were some examples of it.
Although not a superhero, in one episode Steven Hawking breaks up an argument by suddenly shooting lasers out of his eyes.
Hawking: I didn't know I could do that.
An episode of The Powerpuff Girls centers around Blossom discovering that she has ice breath ability, conveniently in time to stop a flaming asteroid from crushing the city. In what might be a deconstruction, she actually notices that her new power is ostracizing her from her sisters and doesn't want to use it to stop the asteroid. She later uses the ice breath in later episodes, although not really more than her other powers.
In another episode that focuses on Buttercup's lack of a special power, the girls do a vast array of powers that has never appeared on the show and is never mentioned again. These powers include cloning, teleportation, size change, shape shifting and bending over backwards in slow motion.
In Jonny Quest: The Real Adventures, Hadji conveniently discovered his "latent" telekinetic abilities in the second season (along with a good many other revamps).
Total Drama Island: Harold basically runs on this trope, he's a geeky Napoleon Dynamite Expy most of the time, but whenever a random (and usually incredibly odd) skill is required for a challenge he suddenly becomes useful again.
The Venture Bros.: "Dude, no one tells me anything!" While 21 doesn't quite develop new powers, it seems that he's informed of the costume's latest capability the second it becomes necessary.
Roger from American Dad! is one of the best examples of this trope you can find, to the point where even Roger is surprised to find out he has certain powers.
Roger (after Stan set him on fire): How did you know I was fireproof, I didn't even know! ... You did know, right?
Transformers does this on occasion. In the the original series Optimus Prime revealed he could mentally control pieces of his body after being disassembled by Megatron. Ironhide sprayed a huge variety of liquids from his sprayer-hand, from glues to liquid nitrogen to firefighting foam to oil to paint, and each liquid would be just what was needed for the situation. Beast Wars had Blackarachnia show off telekinesis after becoming a Transmetal 2 (for one quick scene and never again), and there were so many instances of New Weapons As The Plot Demands (in one episode Cheetor pulls a massive missile launcher bigger than he is out of nowhere, fires it once (missing his target and accidentally hitting Optimus), drops it and forgets about it. The various Japanese Transformers series are even worse about it.
Processor-over-matter from Transformers Animated is not seen or mentioned until the season 2 finale, where Prowl uses an incomplete version of the skill to escape some handcuffs. However, the ability is not immediately forgotten and Prowl spends the rest of the series trying to master it, eventually succeeding.
Near the end of the first season of W.I.T.C.H., Will spontaneously uses the ability to have the Heart of Candracar duplicate itself to fool the bad guys. She never uses this again.
In the second season, all five girls develop secondary powers seemingly out of nowhere—Hay Lin can become invisible (often something related with the element of air), Taranee can read minds, Cornelia gains telekinesis, Irma gets mind control (though this was first demonstrated early in season 1), and Will can talk to electronic appliances. Will also discovers her real element, thanks to Nerissa, and instead of going with 'the Heart' and 'Rebirth', she can use 'Quintessence'. They also, at the very end of the second season, reach their 'zenith' forms, where they each become pure manifestations of their element. The drawback of this is they very nearly lose their minds in doing so.
Pinkie Pie's Pinkie Sense, despite being common knowledge to everyone who had prior knowledge of her in the show, had never been evident before the episode "Feeling Pinkie Keen", and has seen little screentime afterwards. It almost seems like she only had it to provide that episode's Aesop in teaching Twilight Sparkle to not adhere so rigidly to logic and to what can be explained.
Twilight Sparkle herself defies the trope constantly. Spells brought out on the spot, such as the one she uses on the parasprites in "Swarm Of The Century", and the Cutie Mark creating one she tries on Applebloom, fail or backfire more often than not. The ones that do work are justified by her having researched and practiced them previously. Also, she constantly seems to have merely forgotten about spells she's used without a hitch that could significantly aid the current issue.
The Animated Adaptation of Beetlejuice is an unusual variant in that Beetlejuice displays all kinds of weird powers, but since the show is a comedy rather than a "good vs. evil" show, it's typically done through Rule of Funny more than anything else.
It's not a "superpower," but similar to Batman, the eponymous heroes in SWAT Kats often got new gadgets as the plot demanded. The worst of them were arguably the cockpit-cutter from "The Giant Bacteria" (a missile specifically tailored for extracting a pilot from his plane, which was never seen again) and the real offender, the Wire-Clipper Missiles from "Night of the Dark Kat," which seemed perfectly tailored for capturing Hard Drive in his Energy Being form.
Megas XLR abused and lampshaded this trope, including having a button in one episode that was labeled "same button Coop used a minute ago" which, of course, produced a completely different effect.
In Kung Fu Panda Legends Of Awesomeness, Po gets new powers each time some mystical artifact passes by the Jade Palace which he eventually has to use against some bad guy who wants it (even though he's responsible for causing property damage along the way). Of course, most of these powers are never mentioned again.
Mocked in the "Good Time with Weapons" episode of South Park. When the boys were all playing as ninjas, Cartman kept on giving himself all sorts of powers, much to friends' annoyance.
Kyle: Okay, hang on guys, I'll use my special power to see into the future, and find out where we should head next.
Cartman: Hold on you guys, I actually have another power. I can see into the future too, but better than Kyle. Let me try.
Kyle: God damn it, Cartman, you can't keep making up powers!
Stan: Yeah, dude, that's like the fifth power you've come up with!
Cartman: I am Bullrog, and I have lots and lots of powers.
Cronus, the Big Bad of Class Of The Titans seems to pull new powers out of his godly butt almost every episode, which he would never use again, despite how helpful they might be. Yes, he is a Greek God, but still.