Popeye would spend a considerable amount of time each cartoon getting his butt kicked by Bluto/Brutus. Then he got his spinach on and it was over. Sometimes, this point came when he really got angry, and said, "That's all I can stands, 'cause I can't stands no more!"
Parodied as a Noodle Incident in The Simpsons episode "Monty Can't Buy Me Love," where Mr. Burns and his team try to subdue the Loch Ness monster in Scotland. When all else fails, Mr. Burns himself approaches the monster with a stern look in his face. The scene then unexpectedly cuts to Burns' helicopter in the air, with the monster tied up and swinging below. Mr. Burns explains to his admiring team:
Burns: I was a little worried when he swallowed me, but then, well, you know the rest.
Uncle seems like an eccentric weirdo who's too old to fight effectively, but when he wears the Dog Talisman, an artifact that gives its user the strength of youth, he shows off his martial arts skills by pulverizing a specially hired assassin who beat Jackie to within an inch of his life.
Jackie himself would prefer to live quietly and spends most of every episode running away. Then you threaten his family (or the world) and he starts handing you your ass. Bad day. Bad day. Bad day.
In Teen Titans, goofy, silly Beast Boy would occasionally get pissed off and remind both villains and viewers that he was capable of turning into any animal alive or that had ever lived. In one episode, he thoroughly thrashed Slade or rather a roboSlade, though the two were usually interchangeable despite the fact that this was someone who regularly whooped the team's designated badass, Robin. This is especially evident in "Titans Together", where the Titans, both organized and honorary, have been decimated, and it's up to Beast Boy to lead the handful that remain against the Brotherhood of Evil.
Brain:(to Robin) You will fall, one by one. Who among you can possibly stop me now? (cut to Beast Boy, surrounded by the destroyed parts of the robot drones sent to capture him) Beast Boy: Now try and follow me. (presses a button, blowing up his ship and everything around it)
Happens with Flash in Justice League Unlimited. Twice. The first time has him destroying his robot double by punching through its chest and then making it explode by vibrating his arm super-fast, and thentaking down a Brainiac/Lex Luthor fusion by drive-by getting a run up for each punch so quickly he travels around the world (multiple times) and then PUNCHES EVERY MOLECULE OF BRAINIAC OUT OF LUTHOR while he screams in agony. The second time comes in the next season, when his mind has been swapped with Lex Luthor, who does the vibration trick on some walls and threatens to do the same to someone's head. (Flash doesn't make a habit of this due to the property damage it tends to cause, thus proving that despite appearances, he is more conscientious than pretty much the rest of the Justice League.) Another moment of The Flash Getting Dangerous would be when he sabotaged Luthor's plans for the robbery of the century, kept any innocent bystanders from getting killed... and to top it off, thoroughly cuckolded the completely oblivious Luthor with Tala, to the point that the only one unhappy with the mutual restoration was Tala herself.
Tala: Is that you, Lex? Luthor: Of course it's me, you idiot. Tala: ...Oh.
In Gargoyles, Broadway and Lexington were typically the comic relief and the Smart Guy in most plots, respectively. But every once in awhile, they'd be pushed a little too far, and remind their enemies (and the audience) that they were full-fledged warriors that could bend steel and break through concrete with their bare hands. Especially if you try to pull a gun on Broadway.
At the very end of the real last episode/movie for Kim Possible, Ron steps it up and masters his Monkey Kung Fu in time to completely destroy the aliens and save Kim.
Metalocalypse's Charles Foster Ofdensen, Dethklok's mild-mannered manager/lawyer/accountant. Hints are given throughout the first season that Ofdensen is more than what he seems, but we don't see this full until the finale, which manages to be Crowning Moments of both awesome and heartwarming. A somewhat unique example, in that the villains make a point of taking his awesomeness into account in season 2 and adjusting accordingly.
Dib of Invader Zim is an almost perpetual Butt Monkey trying to tell a Cassandra Truth, but under the right circumstances he's able to unleash his inner awesome and defend the Earth like never before. Like the time he piloted Mercury. Indeed, Jhonen Vasquez remarks in the DVD commentary that "Dib's Wonderful Life of Doom" is pretty much just an excuse to show Dib's Crowning Moment Of Awesome as he leads Earth's defense fleet against the Irken armada and single-handedly brings down the Massive.
Sewer Urchin of The Tick is a semi-scatterbrained guy who talks like Rain Man and gets knocked around pretty often. However, this is because he's out of his element. In the sewers, he's Bad Ass to the bone, to the point where The City's sewer workers talk about him with hushed tones of awe.
Urchin: Down here I'm considered the apotheosis of cool.
Arthur(to The Tick): Did he just say "apotheosis?"
Starscream is usually a comedic figure, as exemplified by his death montage, being killed over and over again by Megatron in the span of a single speech. However, when it comes to actual combat he is a badass, taking down Ultra Magnus, head of the Autobot Elite Guard and the entire Autobot army, with one shot.
Bumblebee and Bulkhead of the same series would also qualify for this as out of the main line-up they are the youngest and most comical of the Autobots. Particularly in the series' finale when they take on Shockwave, who is mostly owning them in the first part of the fight, until Bulkhead rushes him and floors the guy so hard that his chest smashes, his weapon is knocked away and he even has to resort to pleading for his life. And he did it with one punch.
Recess featured an inter-grade dodgeball match, with one kid staying out of it. However, when the older kids nailed a kindergartner, this mild mannered boy showed up in a serape, calling himself "El Diablo." He proceeded to take down an entire GRADE in the span of 6 seconds. Said kid happened to be Gus, the mild meek kid of the gang.
The awesomeness is heightened by the fact that the injured kindergartner was Hector, who hero-worships Gus so there was a hefty dose of Big Brother Instinct thrown in there.
Jake Long has a level of danger inversely proportional to the density of his slang. If he's casually hurling trash talk and indecipherable jive, expect him to recklessly endanger himself or someone he cares about, or otherwise trip over himself. If he drops the slang altogether, it usually means something horrible has or is about to happen, and he's ready to administer an epic beatdown to stop it.
Jake's muggle dad gets his own such moment when he does some on-the-fly researchin the middle of a battle against the Dark Dragon's shadow monsters, finds the weakness of said monsters, and improvises a method of unleashing said weakness (light). The summoned creatures which had been giving the gathering of dragons all they could handle are wiped out in a matter of seconds. Jake taunts the Dark Dragon by lampshading this.
Iroh from Avatar: The Last Airbender. He gets to do this a number of times, the first being when he is kidnapped by earthbenders just before the winter solstice. He nearly manages to escape on his own, buying time for Zuko to show up. They proceed to lay the smackdown on them without even firebending (that would have been unfair). Another is at the North Pole, after Zhao destroys the Moon Spirit. Then there is the second season premiere, in which he redirects Azula's lightning and kicks her overboard just as she's about to finish off Zuko. Finally, there is the scene when Iroh demonstrates how he got the nickname "Dragon of the West." Moral of the story: do not underestimate someone who once broke through the outer wall of Ba Sing Se, no matter how harmless he may seem now.
In episode 10 of the all three of Tenzin's children. Jinora sent the Lieutenantflying, Ikki took out a bunch of chi-blockers with her air-scooter, and Meelo gassed the rest of them. Oh, and they did this to protect the series' token Bad Ass, Lin Bei Fong.
Then there's Bolin for that matter. When he's not being the Butt Monkey, he kicks a fair amount of ass with his own Earthbending.
The Avatar State, depending on your POV, can be seen as this trope. Basically, when the Avatar's life or loved ones are in danger regardless of personality, meaning that even if the Avatar is as gentle as Aang, or on the opposite end as Hot-Blooded as Korra, their bending power increases considerably and they go into an epic state of destructive fury. So if the Avatar's eyes go solid white and you're not the guilty party, take cover - and if you are, well, please stick your head between your legsand kiss your ass goodbye.
The Trope Namer himself, Darkwing Duck. He's goofy, grumpy, and has an ego the size of a small planet. But tick him off, and the guilty party will find itself on the receiving end of Quack-Fu and an intelligent, strategizing brain that understands both chemicals and criminals. Simply put: He's Disney's equivalent of Batman. Mess with him and you will lose.
The Toiletnator in Codename: Kids Next Door pulls one of these in a rage-filled, paranoid moment against the very villains who constantly belittle him. Mainly applies because he is fought seriously by most of the villains he takes down in the sequence. Unfortunately, he essentially runs out of ammo by the time the actual heroes show up.
Ding-A-Ling Wolf, Hokey Wolf's little sidekick once bashed a robot repeatedly with a massive stick longer than he was because the robot was trying to kill Hokey.
Winx Club: An S2 ep shows Griselda blocking every single attack from her students as part of Magical Defense Class, including Bloom (though to be fair, she was thinking about her phone call with Sky from earlier in the episode). Then you get to the fridge and realize that she didn't have any significant role in the final battle of the previous season, when that ability would have been useful. The Secret of the Lost Kingdom makes up for it by having her putting up a shield for Stella, Flora and Tecna during an attack from Mandragora.
Robot Chicken does this a lot, but the best example would probably be the sketch in which Mafia-installed union reps are brutally killed... by Bob the Builder and crew.
In the episode "Octi-Evil": Blossom and Buttercup, having spent the better part of the episode fighting with each other, bury the hatchet to save Bubbles.
Also, The Movie, which had the girls becoming pariahs for their role in not only trashing Townsville but helping Mojo Jojo take over with an army of apes. The girls serve a self-imposed exile on an asteroid till they hear the Professor being threatened. They return and kick monkey tuchus.
"Not So Awesome Blossom" has our redheaded heroine in a state of shellshock through the episode as she is incapable of having anything go right for her. But she steps up and hits it out of the park at Mojo Jojo's lair when Mojo has Blossom's sisters and the Professor in dire straits.
Bubbles is normally the most timid and ditzy of the 3 girls, but as the pilot and episode Bubblevicious show, push her too far and she WILL kick your ass.
When Blossom's brains and Buttercup's brawn fails to down a giant monster in Three Girls And A Monster, Bubbles uses the only tact left: disarming charm and politeness.
The Mighty Heroes seem like ineffectual stumblebums in the first half of an episode, but once they escape the Death Trap and regroup, they are unstoppable!
In Star Wars: Clone Wars, Yoda spends most of the time offscreen in the Jedi Temple. When the Separatist invasion of Coruscant begins, he heads to the front lines and starts forcepushing colossal troop carriers.
In Adventures of the Galaxy Rangers, Walter "Doc" Hartford spent most of the series as a Deadpan Snarker and "computer guy" who was nowhere near the other three when it came to a fight. But when a Evil Chancellor steps in and threatens to kill the king they're trying to ally with? Doc picks up a sword, brawls him, and hands said evil adviser his ass. All the while admitting that he learned fencing from "Miss Abercrombie's Charm School!" This came up again when he was back on Tarkon and took down the show's nastiest villain singlehandedly using stuff he picked up in "charm school." Which has led a great many fanfic authors to wonder just what kind of Charm School Miss Abercrombie was running in the first place.
Launchpad McQuack often filled out this trope. Sure, he was typically bumbling and accident-prone, but when necessary (especially toprotect the nephews), he could pull off some amazing feats of aeronautical skill, as well as general badassness. Perhaps this is what led them to team him up with Darkwing Duck.
Fenton Crackshell should be mentioned as well. While he was smarter than Launchpad, he still had a tendency to screw things up. But he was the superhero Gizmoduck, and he occassionally showed that he was heroic and could save the day even without the Gizmo suit.
Twilight Sparkle is pretty frequently the butt of jokes and the entire plot of the show so far has been her learning social skills. Just don't threaten her, her friends, or her town, lest she remind you that she is the magical prodigy student of a solar god-empress.
Doubly so Fluttershy — usually timid as anything, and unable to stand up for herself, but threatening others can make her angry enough to forget herself and suddenly have enough attitude for a grade-A Bad Ass, as well as a Death Glare powerful enough to cow fully-grown dragons and win staring contests with cockatrices. While normally a weak flyer, Fluttershy can also fly well enough to keep up withRainbow Dash when her friends are in danger.
Pinkie Pie provides a more mundane example. When she realizes she hasn't been particularly responsible caring for the twins she's babysitting, she becomes far more serious and strict. It lasts about as long as it takes the pegasus foal to realize he can fly and the unicorn foal to start using her magic.
Pinkie Pie's "Pinkie Sense" is mostly used on the show to predict falling objects for slapstick purposes. Then she uses it to save the lives of at least four ponies in a horrific construction site accident.
In an earlier episode, "Griffon The Brush Off," Pinkie doesn't take Gilda seriously no matter how much she tries to tell her to "buzz off," only telling herself that she's jealous of her (despite the latter in fact being a Bitch InGriffon'sClothing) and that Gilda just needs to improve her attitude. But when she sees Gilda blowing a gasket at Fluttershy...
When he sees his beloved Rarity being kidnapped by the Diamond Dogs, "Little Spiky Wikey" just outright starts beating them up. He loses, sadly, but puts up a good fight. Not to be outdone, Rarity manages to inconvenience the Diamond Dogs with wit and whining.
In the season 2 finale, the entire team successfully fights off dozens of monsters. Dozens. Pinkie Pie weaponizesher Party Cannon, and Rarity slaps some into submission with a grin on her face. Even Fluttershy manages to trick a few. Problem is, the Big Bad has thousands of mooks to spare. It is still the second legitimate action sequence on the show, and demonstrates that the team is quite capable even without the Elements of Harmony.
Good old Applejack, an earth pony who mainly serves as the Team Mom, often gets overlooked despite being strong enough to knock every single apple from a tree simply by kicking it. Then Season 3 comes around and she takes out a trio of timber wolves with nothing but rocks and her own cunning.
Seriously, don't screw with the Cutie Mark Crusaders. When they've finally been pushed too far by Babs Seed the A-Team music plays, they build an entire motorized parade float in less than a night, fit it with a booby-trap (made from an egg timer) that will send it careening off a cliff at precisely the right time, and lure Babs into stealing it with a Batman Gambit. They even plant a mattress to land on when she shoves them out of the way, and the only reason it fails is because they have a sudden attack of conscience and save her (The plan itself works beautifully). Seriously, how do these kids not have their cutie marks yet?
The episode "Maud Pie" introduces Pinkie's sister, Maud. Even after prolonged contact, she is quite clearly an Emotionless Girl who is only interested in rocks. But when Pinkie gets caught in an avalanche, Maud reacts while everypony else is still gaping, covers several hundred yards in seconds, then pounds a house-sized boulder into gravel with a pair of jackhammer hooves.Jaw Drops all around.
In Twilight's Kingdom Part 2, once Tirek destroys the library, all bets are off and Twilight goes full throttle on him with her power.
Isabella is already competent as is, but the kid's gloves come off if Phineas is in danger of being hurt.
All of Phineas and Ferb's friends and inventions get to do this during the climactic battle in The Movie.
For the first season of Transformers Prime, Soundwave kept away from the front lines; the only fight he got into was with the kid sidekicks of the Autobots (his drone also fought a single Helicopter once). In the finale Airachnid tries to convince the Decepticons to abandon Earth, and Megatron. Soundwave effortlessly tosses her around, forcing her to abandon her mutiny. No wonder Starscream wanted to stay on his good side. He's since dethroned Wheeljack as Most Badass Character in the show by kicking the everloving slag out of him. Soundwave Superior, indeed.
When the usually idiotic Cosmo finds his wife and godchild in danger, he gets serious to the point of becoming GODZILLA.
Poof qualifies as well, in the episode "Poulter-Geeks". Like father, like son, apparently.
Family Guy: When Quagmire plots to kill Jeff, Joe refuses to go through with it, since it would be murder. But once he hears Jeff abusing Brenda next door, Joe responds with this... Let's waste this dick.
Mr. Bogus will very often swing around into this trope, due to his status as the Butt Monkey, despite being the main character of the show. However, when push comes to shove, he will face off against any threat that comes his way. In fact, he once took on a rather hideousEvil Knockoff of himself that threatened him and pushed him around repeatedly while he was experiencing a streak of bad luck.
Clue Club: In "The Walking House Caper," a mysterious monster figure locks Larry, Pepper and D.D. in a house. Larry uses his communicator to contact Dotty at home base in hopes she'll call the police. Dotty and the show's two dogs, Woofer and Wimper, take it upon themselves to lure the monster away from the house so they can rescue the others. And Dotty does have the foresight to call the police as well.
In SheZow, you know (s)he's serious whenever SheZow says "No more mister nice girl!" or "Time to get SheZow-y with it!"
Mabel from Gravity Falls may be a girly-girl Cloud Cuckoolander, but she can be dangerous when provoked (especially if you threaten her brother Dipper). In the pilot episode alone, she defeats an army of gnomes with a leaf blower.