* In ''Fanfic/MementoVivere'', a VideoGame/FinalFantasyX fanfiction, Jecht finally gets his sword-fighting act together during the battle on [[spoiler: Mushroom Rock]].
* In ''Fanfic/TheWizardInTheShadows'', at the final battle of the Black Gate, all seems lost. [[spoiler:Harry]], the most powerful weapon in the arsenal of the Light, is dead, the Sword of Gryffindor that allowed him to draw on impossible amounts of power is inert, and the forces of good are sorely outnumbered. Then Ginny -- up until this point merely quite powerful -- picks up the Sword and says a LittleNo. The Sword responds. Cue three giant tornadoes of fire and a whole world of hurt being laid on Mordor's forces.
* ''Fanfic/RiseOfTheGaleforces'': [[BadassFamily Everyone.]] [[BigDamnHeroes Just...]] [[TheCavalry everyone.]] [[spoiler:Special mention goes to Violet in particular, since her finally being allowed to cut loose and demonstrate to the bad guys what she is truly capable of is pretty much the ''reason'' the fanfic itself manages to outlast all the hell it gets put through.]]
* In the ''FanFic/TamersForeverSeries'' [[spoiler:Terriermon drops his goofy attitude right before opening up a can of whoop-ass on [[BigBad Dae]][[HeroKiller mon]]]].
* In the ''FanFic/PonyPOVSeries'' [[BadFuture Dark World]] timeline, Derpy Hooves has spent a thousand years cursed to walk around [[EvilOverlord Discord]]'s castle until she does it enough times to earn her children's freedom from their own curses, all while being constantly attacked by a murder of blackbirds that are trying to eat Dinky (who's been [[AndIMustScream turned into a muffin]]). She does this without a single complaint, staying focused on her task... until Twilight and the other [[HeelFaceTurn redeemed]] Elements of Harmony convince her to help fight against Discord in order to save Dinky and Sparkler without abiding by Discord's fixed rules. Derpy then proceeds to prove her worth as the new Element of Loyalty by helping to fight the blackbirds, playing a key role in defeating them.
** She does it again in the fight against [[TheStarscream the Valeyard]]. She spends most of the fight holding back due to the Valeyard being a Discorded [[Series/DoctorWho Doctor]], but when he eventually pushes her too far and she finds out he's actually the Doctor's SuperPoweredEvilSide, she then proceeds to go into a TranquilFury and ''[[spoiler: [[NoHoldsBarredBeatdown beat him to death]]]]'', ensuring that he'll change back into the Doctor.
*** This is pretty much a major aspect of her character; when there's not a fight to be done, she's a kindhearted [[TheDitz Ditz]], but once her friends or children are in trouble, she gets serious and is more than capable of kicking flank.
** Back in the main timeline, the Crusaders do this during the Wedding Arc, in response to [[spoiler: Sweetie Belle and Spike getting brainwashed]] -- despite just being fillies, they manage to sneak past the Changelings, knock two out, [[spoiler: snap Spike out of it]], and then cover their escape by [[spoiler: ''summoning a water elemental'']]. Heck, the whole reason the adults let them help anyway is because they can tell by how determined they are to help that they'd do it even without permission anyway, plus, with the amount of stuff they get up to and survive on a regular basis, them taking things seriously makes them an actual threat.
* [[DirtyCoward Hobbes]] and [[HarmlessVillain Dr.]] [[BunglingInventor Brainstorm]] get a few of these moments during ''Fanfic/CalvinAndHobbesTheSeries'', and when it happens, [[SugarWiki/MomentOfAwesome things tend to get serious with them.]]
* In the ''Series/BuffyTheVampireSlayer'' / ''ComicBook/PowerGirl'' crossover ''FanFic/OriginStory'', Alex Harris (a Kryptonian trapped in the Franchise/MarvelUniverse) and her domestic partner Louise are attacked by the telepathic assassin Typhoid Mary. Mary knocks down Louise, then turns her attention to Alex and proceeds to use her mental powers (against which Alex has no defenses) to ''wreck'' Alex. Turns out ignoring the normal, untrained girlfriend was a mistake when Louise proceeds to clock Typhoid Mary from behind with a bottle of sparkling grape juice, then continues to beat the assassin with it until Mary is unconscious and bleeding on the floor.
* At the beginning of ''The Celestial Renegade'' in ''Sailor Moon: Legends of Lightstorm'', this is the reason Luna takes Sailor Moon to Lightstorm. Lightstorm causes this to happen with Sailor Moon over the course of a single afternoon. And that was before he came to Tokyo and began training the rest of the Scouts.
* In ''[[https://www.fimfiction.net/story/237978/how-trixie-somehow-saved-hearths-warming How Trixie (Somehow) Saved Hearth's Warming]]'', Trixie is her normal hammy, bragging self and just manages to skirt by in the situation she's gotten herself into (namely saving Hearth's Warming) by being MistakenForBadass. However, after [[BalefulPolymorph being turned into a nutcracker]] by [[Theater/TheNutcracker the Rat King]], she goes into a HeroicBSOD until [[spoiler:she finds out Vixen was desperately trying to finally stand out among the rest of Santa's reindeer and is having a HeroicBSOD as a result of their capture. Trixie quickly realizes this means the two of them are NotSoDifferent, both feeling OvershadowedByAwesome by somepony else and just wanting to stand out]]. This realization causes Trixie to snap out of it and devote everything she has to getting them through this. [[spoiler:She ultimately stages a prison break and defeats the Rat King via quick thinking and plenty of IndyPloy, saving not only the two of them but all the Rat King's slaves.]] Even Trixie is surprised.
* Havoc does this in the Fanfic/ElementalChessTrilogy. Just like in ''Manga/FullmetalAlchemist'', he's a bit goofy and loves to make jokes, and has a powerful nostalgic streak. When things go south for his beloved commanding officers, however, he steps up and takes charge, reminding everyone just why he's one of Roy Mustang's most trusted subordinates.
* Chapter 13 of ''FanFic/TheCommission'' has Qrow intercept Ruby [[CurbStompBattle and kick her ass totally]]. This gets her desperate enough to [[spoiler:bust out her Spring Maiden powers ([[YouKillItYouBoughtIt complete with a flashback of her meeting the girl before killing her for it]]).]]
* ''Fanfic/OnceMoreWithFeeling'': Shinji used to be wimpy and non-confrontational, but after returning to the past, being aware of what will happen if he fails, he is way more dangerous and fights like a berserker.