Adventure Time has a plethora of characters like this. The Ice King is a complete idiot, but has shown great power and surprising strategy when the time came for it. LSP, usually a dumb bratty teenager, was the only one of the gang who was unfazed by a zombie attack, proceeding to clobber the zombies with a hammer and a 2X4, even punching the zombies in the face! Even Lemongrab had a quick one in his first episode- although he cried after getting punched in the stomach, he actually fell from the window of the tall tower and SLAMMED his head into the ground hard enough to leave a large hole he had to pry himself out of... and he was totally uninjured, thanks to his super-thick skin.
Lemongrab's been further hinted to be this, with his SoundSword.
The Sultan from Aladdin: The Series of all people is like this. He seems to be a bumbling, almost childlike figure most commonly, but he was a pretty good adventurer and warlord in his younger days, and he still retains something of his old skills.
If you need definitive proof that he's more than he seems, remember this: the guy who's all of about three-foot-nothing in height and of a distinctly round shape, who chuckles more merrily than Santa Claus and is often-times found playing with little figurines, once wooed Queen Hipsodeath, the queen of the Amazons because of his sheer badassery when rescuing his daughter from them, a plight even Aladdin hadn't overcome!
It's implied that he was quite the swashbuckling adventurer in his youth, making him a combination of this and Retired Badass.
He defeated the entire army of Shade Demons in the series finale by grabbing a cermonial vase with his fanny pack and reflected moonlight off of it. Jake even lampshades it proudly.
Sterling Archer, title character of the series Archer is the world's dumbest secret agent. But he is a secret agent, and can dole out a secret agent level ass whooping when the need arises.
Aang from Avatar: The Last Airbender was an example of this trope in the first episodes whenever he tapped into the Avatar State. Character Development, a showcase of the fact that he is a master airbender, and the realization of how scary he is in the Avatar State contributes to the lessening of this carefree nature.
Bumi treads the line between this and Obfuscating Stupidity. He appears to be an insane old man. And he is. He's also built like Arnold Schwarzenegger (normally concealed by his cloak) and one of the greatest Earthbenders in history.
When the Fire Nation attacked his city with overwhelming force, he surrendered and let them close him in a metal coffin to prevent him from Bending. When an eclipse stripped the Firebenders of their power, he bent with his face to break free of the coffin, kicked the Fire Nation out of his city and defaced Ozai's giant statuein less than eight minutes. Repeat, he defeated an army in less than eight minutes and still had enough time to deface a metal statue as big as Lady Liberty if not bigger, and then threw it down a cliff. But he's also, to all appearances, genuinely nuts.
And then there's Sokka. At first he seems like a guy who wants to be a capable warrior but is constantly outstripped by the others. Then, as the series goes on, he turns into aGadgeteer Genius with a space sword and is a much more capable leader and a skilled fighter to boot, who has gone toe-to-toe and WON with such deadly foes as Mai, Wan Shi Tong, and Combustion Man.
We also have Bolin – who is characterized as naive, somewhat immature, and impressionable – from the sequel series The Legend of Korra. However, in "Night of A Thousand Stars", after finding out from Lu and Gang that some waterbenders are after the president, Bolin apprehends the criminals and catches them in the booth where President Raiko and his wife are sitting. Bolin earthbends them against the ceiling and down onto the Pro-Bending ring below, where he fights and defeats them (albeit comically) in front of spectators. Afterwards, one of the mooks reveals that they were hired by Varrick, and Lin Beifong and her police confront the mogul and his assistant, Zhu Li, just before they can make an escape.
And then he learns how to bend magma. While he's still the same goofball he always was, he now is the only known living non-avatar person to have that ability and has the title of being the second-most destructive person to live in the avatarverse.
District Attorney Harvey Dent was a tragic variation on this in the two-part "Two-Face" episode of Batman: The Animated Series. Although not a moron by any measure, he's described as "so clean he squeaks" and refers to himself as "Mr. Goody-Good." However, repressed memories of having been bullied at school as a child (in the comics that inspired this episode, Harvey was a victim of child abuse, but there was no way the animators were touching that one) have bred within the adult Harvey a deep-seated, schizophrenic anger that occasionally manifests itself as "Big Bad Harv" and eventually emerges as his "Two-Face" persona after his face is scarred. Before the scarring, however, he goes to see a psychologist in an attempt to save his sanity and - after being cajoled to do so by the doctor - introduces her to the "Big Bad Harv" persona. He lifts the chair on which he's been sitting and tosses it through the doctor's window, shattering glass everywhere and frightening the poor woman. Moments later, the good Harvey returns and is completely baffled by the havoc wrought by his evil alter ego. ("Did I do this?")
Harvey Bullock as well. Most of the time he's portrayed as such an overweight bumbling moron it's hard to imagine why he hasn't been fired. Then in episodes like A Bullet For Bullock you see why he hasn't lost his job: despite his shortcomings he's a damned good detective who breaks rules to get results (not because he's corrupt) and able to hold his own in a fight almost as well as Batman.
Ben Tennyson usually is portrayed as this, though how much of a moron he is varies considerably according to the different entries of the franchise. In the original show, he was rather immature and incompetent most of the time, but would get effective when fighting an actual threat. Later installments go more for a Brilliant, but Lazy approach. By now, he usually appears as immature with a huge ego... who also is a Living Legend who saved the universe multiple times.
Odd Della Robia in Code Lyoko is often shown to be an impatient slacker, far more interested in his various dates and lunch than studying for classes or saving the world. His crappy grades and his many, many failed plans showcase just how stupid he appears to be, and he isn't faking this: he is pretty Book Dumb. However, his slacker attitude hides his rather impressive abilities, both physical and mental. He has no combat-related "powers" on Lyoko, but he doesn't need them to do well.
Also, for a single episode of season 4, William II falls under this. As stupid as he is, he does go toe-to-toe with the real William and win. Sort of.
Don't let his lack of booksmarts fool you, there's a very good reason Numbuh 4/Wallabee Beatles of Codename: Kids Next Door is Sector V's hand-to-hand combat specialist and resident Blood Knight.
Of course, there's also Numbuh 2/Hoagie P. Gilligan Jr., who may act like a goofball most of the time, but has actually proven himself to be the team's most efficient Gadgeteer Genius.
His mother Betty has shown to be no slouch either, as shown in Operation T.O.M.M.Y.
Likewise, Numbuh 3/Kuki Sanban may act like a ditzyGenki Girl, but threaten her stuffed animals, and YOU WILL DIE A SLOW AND PAINFUL DEATH!!!!
Daffy Duck as a whole could fit this trope, his attempts to prove his superiority over Bugs and look like a suave hero nearly always fall flat, but when a true push comes to a shove, he can prove rather cunning and formidable. Even excluding earlier shorts, this is the same guy that beat the crap out of the Tazmanian Devil for stealing one dollar from him, oh and he has beaten Bugs at least once...
Roadrunner normally gets by on being The Fool, but on the occasion that he does realize what's going on he's very capable of defending himself.
Penfold on Danger Mouse has a moment in the episode "Chicken Run." While trying to evade a giant chicken pecking at him and DM, Penfold jumps into a reservoir of a growth formula and grows larger than Nelson's Column. He single-handedly (literally) captures Greenback and Stiletto, garnering DM's praise as a national hero.
Penfold: Oh, thank you sir. Mum always wanted to know what I'd be when I grew up.
Jack Fenton in Danny Phantom. Granted he's an eccentric inventor, a bumbling ghost fighter, and an embarrassing parent. But when his family is in danger, he'll show everyone just how competent he can be, then proceed to kick the opposer's ass. Vlad (his old college friend turned supervillain) learned this the hard way. Twice.
From the same show, Comet Guy also fits this trope. With as much power as Superman and fewer brain cells than Bizarro, this bumbling, moronic superhero manages to get his act together when Steelbeak threatens St. Canard with his remote controlled rubber wrecking ball. Even invoking his Weaksauce Weaknessnote being forced to dance upon hearing a bell doesn't stop him.
Brittany Taylor in Daria is ditzy, airheaded, shallow, dimwitted and basically incapable of doing anything right. It's probably not unfair to presume that inside her skull there are a handful of neurons that swim around aimlessly in the otherwise empty void, never coming across each other. And yet, if she is put in a combat situation, she suddenly becomes a gifted tactician and charismatic infantry commander.
Droopy falls under this in several of his animated shorts. Repeatedly humiliated by his adversary, he never reacts...until it looks like he's about to lose "The Dame". Then he responds, in the same monotone as always, "You know what? That makes me mad," and can suddenly fling his enemy across the room with ease.
Duck Dodgers himself has moments such as these. Even despite being incompetent 95% of the time with his successes mostly due to his Cadet, he can manage some pretty amazing feats.
Such as saving the entire Green Lantern Corps after Sinestro captures them all single-handedly.
Or single-handedly defeating the Martian Queen in order to save the Cadet, with a Star Trek II homage, to boot. While played as if he really didn't know what he was doing, some of his tricks (most of the ones involving the Evaporator, for example) were actually quite cunning.
Jonny of Ed, Edd n Eddy, in the Big Picture Show. Several others as well, but particularly Johnny.
Ed deserves to be notably mentioned, as at one episode, he went into a nasty mood to a point where he proved he could even intimidate his brattylittle sister Sarah.
Jimmy as well. Normally Jimmy is a naive little kid that spends his time playing with Sara, but when motivated he can be frighteningly cunning. Twice he comes up with scams that are incredibly successful, and once he puts all 3 Eds through a Humiliation Conga of epic proportions to get back at them for a wedgie.
The Fairly OddParents! gave Cosmo this role in an episode when Timmy ended up in a best-of-three contest with another child, with the loser forfeiting their fairy godparents. Cosmo wins the first contest by accident when the bananas he conjures distract the giant gorilla he needed to rescue Timmy from, and in the final round his teasing causes the opposing fairy to mess up and zap Timmy instead of his own godchild, causing Timmy to win the contest.
There was also that one time he watched a video called "Five Second Massive Pecs," and then proceeded to kick large amounts of ass against Crocker...who was in a mech suit.
Family Guy actually has a few. Glenn Quagmire is a sexual predator, but an absolutely ace pilot (it's "Hidden Badass" because it's so rarely we actually get to see him do his piloting job). Plus, it turns out Adam West was a KGB sleeper agent who managed to single-handedly beat the crap out of Brian, Stewie, Dan Aykroyd, AND Chevy Chase all at once when accidentally activated.
Fry from Futurama. Not only is he a Crouching Moron, Hidden Badass, but he's exactly the right kind of moron, as he lacks the Delta Brainwave, allowing him to save the universe on more than one occasion, like when he infiltrated the Infosphere to defeat the Brainspawn. Fry is a badass BECAUSE he is a moron.
My name isn't Slick. It's Zoidberg. John ***ing Zoidberg.
In one episode of G.I. Joe, three Cobra Mooks dogpile on the simpleton Bazooka, which causes him to exclaim, "Darn it, swallowed my gum!" and knock them all out with one punch each.
Orko in He-Man and the Masters of the Universe (1983). On Eternia, this Trollan native is the comic relief buffoon who's famous for being unable to make many of his spells work properly. In the eyes of even his friends, he's little more than the court jester, albeit with a brave heart. However, it turns out that's because the laws of magic on Eternia and Trolla are the reverse of each other. On Trolla, he's revealed to be one of that planet's greatest arch-mages who (with only a little help from He-Man) almost single-handedly saves his people from enslavement by a half-dragon servant of Skeletor. Even on Eternia, when his magic does work, he can be extremely competent.
Prince Adam himself is this in the 80s cartoon. He puts on a lazy, clumsy, cowardly, combat-incompetent rich kid act to help erase any suspicions that he might be He-Man. Every so often, however, he reveals a tremendous depth of knowledge on Eternia's ancient cultures and science, he's been able to use his intellect to defeat bad-guys when even He-Man's strength had failed, and with one notable exception that saw his father get kidnapped, when he is forced to play the hero as Prince Adam instead of He-Man he can be surprisingly effective.
Inspector Gadget, of all people, occasionally demonstrated this. Uusually he wasa total joke who always chased after the wrong clue while his preteen niece and her dog captured the criminal behind his back, but when Penny and Brain were in real danger from Dr. Claw's traps in one or two episodes, the normally Too Dumb to Live Inspector would skilfully use his gadgets to save them.
GIR of Invader Zim is even more inept than Zim himself. His brain is literally made of chewing gum, coins and a paper clip and he seems to have both feet firmly planted in space and his mind on waffles or tacos. However, when issued a command, his eyes occasionally turn from a bright blue to a glowing red and he acknowledges the order like a perfect Irken SIR unit. He usually forgets the order or becomes distracted by something else within seconds, but when Zim locks GIR into this "duty mode" after one-too-many failures that weren't due to him, the little robot becomes scarily competent and almost kills Zim after, correctly, determining he is the greatest threat to "the mission".
Zim himself can sometimes qualify. Sometimes. When he is not allowed to have his decisions affected by his ego, he can actually be competent and quick-witted, especially shown in "Hobo 13". However, most of the time, he really is a moron, and it's what usually prevents him from conquering Earth in the first place if Dib is not stopping him.
The Flash, in Justice League Unlimited appears to fall into this category. For the most part he's a goofy dope who doesn't take much seriously and is always the one to crack a joke at the expense of drama and badass-ism. However his role is not to be underestimated (in one universe his death motivated the League to turn into a totalitarian dictatorship). Many people found it surprising to discover he's a forensic scientist in his normal identity. His big transforming moment, however, came during his Crowning Moment Of Awesome in the episode "Divided We Fall" where he taps into the Speed Force, circumnavigates the globe several times in a matter of seconds in order to make his acceleration infinite to defeat Luthor-Brainiac by punching him with an infinite amount of force. All of a sudden he went from comic relief to downright dangerous. Plus, in the aforementioned Justice Lords episode, Flash took down the Justice Lords' Superman, which is even more impressive considering that this version of Superman has turned into a dictator and was presumably dead serious, and the Flash didn't even appear to be tired at all.
Also during that battle, not only was he the first to defeat his Justice Lord robotic double (even though there wasn't one in the alternate universe), he was also the only one who wasn't affected by his double's Hannibal Lecture speech, meaning not only that he doesn't have any doubts in his abilities as a hero, but it furthers his role as the heart of the group.
Adding onto this, everyone else needed assistance on their double, except Batman who beat his just as fast. He took his on single-handedly and was the first to finish. While everyone was busy with theirs, he just shrugged off the doubts his double was trying to create and whooped its metal ass.
Back in Justice League, when the team split up for an episode, Flash is attempting to interrogate a mook who insults his technique — "You're no Batman." Flash drops him off a building, races down and catches him at the bottom. He got the info.
When Lex Luthor got a hold of his powers once, he almost took out the entire Watchtower single-handedly, effectively proving that Flash is far more powerful than most people realise. And not only that, he's aware of this, and so doesn't use his powers nearly as much as he could.
Ron Stoppable from Kim Possible seems useless much of the time. His primary contribution to most missions appears to be serving as a distraction, and even after he's exposed to the Mystical Monkey Power, he admits that his skill at monkey kung fu "comes and goes." But when necessary (i.e. when Kim is busy, incapacitated, or not present), Ron displays extraordinary skill in both martial arts and general resourcefulness. As a matter of fact, when one reviews the majority of episodes, one finds that Ron is the person to actually foil the villain's plot du jour while Kim is keeping the help busy.
His Mystical Monkey Kung Fu is shown three times, especially in the Grand Finale where Ron suddenly becomes badass enough to get a Chunky UpdraftBattle Aura that Kim, Rufus and even Shego just stared at him while he beat those aliens' asses. It's as close as the series ever came to a Super SaiyajinShout-Out.
Dr. Drakken: What happened? Shego: I think the sidekick just stepped up...monkey-style!
Chester. Spike's yappy little sidekick from Looney Tunes. Much, MUCH stronger than he looks.
The eponymous character from Martin Mystery. Normally he's a lazy goofball who Diana has to kick into line. Mess with Diana, Java, or the Center (something that happens nearly every episode), and he WILL make you regret it.
Even in spite of his clumsiness and the fact that he unintentionally causes a lot of problems, Mr. Bogus will gradually do whatever he can to make everything right again, whether it's facing off against the much-feared Dirt Dudes or outwitting his sworn enemy Ratty.
Pinkie Pie is a Cloud Cuckoo Lander and loveable marefoal whose special talent is throwing parties. Normally her use of Cartoon Physics is limited to visual gags in the show, but when she starts using it to save ponies' lives from a crumbling construction site, relentlessly pursue a pony who can fly, and to pull a cannon out of nowhere during a fight, she becomes an unstoppable borderline Reality Warper.
It should additionally be noted that a baby dragon is still a dragon. His scales are thick enough to shrug off punishment that would have easily killed a pony, and even other dragons have commented on just how resistant to harm he is. He also has a ginormous adult form, though unfortunately it's triggered by Greed and is more of a Superpowered Evil Side. Even in his baby form, his fire breath is hot and powerful enough to melt iron in seconds and a massive stadium-sized iceberg faster than the time it would have taken to crush the spectators it was plummeting toward, and he was capable of disposing of a cursed Artifact of Doom by eating it. Moral of the story: a lot of ponies are pretty lucky their resident Butt-Monkey is such a Technical Pacifist.
In the two-parter "The Cutie Re-Mark," Twilight gets sent to various Bad Futures where various villains have won, with each one having conquered Equestria more than the last. Who managed to outdo KingSombra, QueenChrysalis, NightmareMoon, Tirek, and Discord, all of them ambitious enough to overthrow nations with magic powerful enough to do it? Flim and Flam of all adversaries, the small-time traveling con-artist brothers who tried to scam the citizens of Ponyville in two episodes. In fact, if one looks at Twilight's magical map, one can see it's flatter than in any of the other Bad Futures—that is, Flim and Flam had inflicted more damage to the country than any of the major villains.
Candace. In contrast to her second dimension counterpart, who is more a straight on badass, she is prone to panicking and tends to act rather erratically in general. But time and time again she has also shown capable of amazing feats given enough motivation (most commonly when she's trying to bust or protect her brothers).
Doof once saved his daughter and her friends from an angry swarm of bees.
Baljeet when he gets out from under Buford's thumb in "Bully Bromance Breakup", he climbs a mountain completely on his own, by which I mean he even eschews any form of equipment.
Pinky and the Brain. While he may be a massive moron that has screwed up Brain's plans countless times, Pinky, when he gets his act together can come up with plans to take over the world that actually WORK.
Rigby from Regular Show can open up a serious can of whoop-ass when he wants to.
Pops, particularly in "Really Real Wrestling".
Chuckie Finster from "Rugrats" and "All Grown Up!" might have a ton of phobias and very low self-esteem, but when things go south, he's often the one saving the day, especially if his Berserk Button (protecting his family and friends) is pushed hard enough. His Crowning Moment of Awesome comes in the second movie, when the 2 and a half year old kid takes control of a giant animatronic dinosaur and beats up an ADULT MAN controlling an equally powerful robot, and arrives in time to save his father from marrying an Alpha Bitch .
Throughout the history of Scooby-Doo, both Scooby and Shaggy have both transcended their lovable coward facades to show they can really come through when they need to. "Bedlam In The Big Top" has them using the Phantom Clown's hypnosis against him.
Maggie from The Simpsons. Despite being a baby she has taken down a gang of mobsters, saved Homer from drowning, and saved Homer and Bart from The Head of the EPA when he was about to kill Homer and Bart in the movie. She also shot Mr. Burns.
Or Homer himself who normally is an idiot and a jerkass. Especially when his children are involved and in You Only Move Twice shows that Homer can be quite competent and motivated in his job with the right kind of a boss. Even if the said boss is an Affably Evil super villain.
Marge when she was a supercop who only quit because of the corruption on the police force, plus she beat up EVERYBODY in Moe's Tavern while she was a 'roided-up bodybuilder. Do NOT underestimate her, especially when you hit the Mama Bear's Berserk Button.
The entire Simpson family, and most of the secondary characters, have this trait. It's the Rule of Funny.
Pip becomes a super dodgeball player after being repeatedly insulted. His biggest Berserk Button, though, is being called French.
And while it might be more accurately described as "Crouching Moron, Hidden Psychopath", Cartman definitely applies in "Scott Tenorman Must Die". After being repeatedly fooled and humiliated by Scott, Cartman executed a Batman Gambit on his friends, and utilized THAT to murder Scott's parents by proxy, then chopped them up into a chili to serve to Scott.
Butters gets a surprisingly high number of these moments.
Beast Boy in Teen Titans is usually portrayed as, while not an actual moron, certainly far less combat-effective than Robin, Raven, Cyborg, and Starfire, primarily due to his habit of choosing an Awesome, but ImpracticalT-rex form that falls through weak surfaces, gets stuck under low bridges, and generally achieves very little. Or not going with the T-rex form in the situations where it would be truly effective. However, when sufficiently riled up, he turns (metaphorically) into an unstoppable engine of destruction, capable of annihilating pretty much whatever takes his fancy.
And then there's Beast Within. For those unaware, basically Beast Boy gets hit with chemicals that force him to give into his inner bestial aggression. This results in his transformation into the Beast Man. Cue curbstomping of entire rest of Titans. Multiple times.
It's worth pointing out that, other than Robin, Beast Boy is the only veteran superhero on the team. Given his activities with the Doom Patrol he might even be the most experienced (and it really shows in the series finale).
Michaelangelo in the 2003 Teenage Mutant Ninja Turtles displays this several times throughout the series. While normally he is the most irresponsible, laid back, undisciplined turtle, on occasion when fighting enemies that have incapacitated or captured his brothers, he displays ninjitsu prowess, agility, and technique rivaling that of Leonardo, who trains constantly, whupping hordes of monsters (or single big bads) that had previously defeated his other three brothers. Displayed most prominently in the episode "Grudge Match." However, this seems to be somewhat random: while there are times when Mikey steps up when the team's back is to the wall, there are other times when he just gets knocked out cold with the rest of them, so I guess it depends on the needs of the story.
Lampshaded with Mikey's victory over The Entity in Notes from the Underground, where he is complimented by his brothers for "really stepping up".
It's been said that Mikey has enough natural talent that, were he to set his mind to it, he could surpass even Leonardo in combat prowess. Keep in mind, real-world nunchaku fighters, like Bruce Lee, typically only wield one at a time; Mikey wields two consistently and with a great degree of skill, despite his lazy habits.
This trait carries over into TMNT 2012. In an early episode, we see that Mikey has mastered "Fighting without Thinking," which Donnie has to learn in order to defeat the PsychicVillain of the Week.
In an online animated comic, Splinter bemoans the fact that Mikey has so much raw talent, but wastes it due his lack of drive.
He also had this trait in the original animated series. One notable example was during a training exercise where Leonardo and Donatello were on one team and Raphael kept trying to get Michaelangelo to help him fight back, but Michaelangelo just wanted to enjoy the sun and eat pizza. Leonardo and Donatello were winning very easily until Donatello's garbage catapult ruined Michaelangelo's pizza. The exercise was over a few seconds later.
Tom Dubois from The Boondocks. Normally he's a lovable coward with an extreme and irrational phobia of prison rape. However in "A Date with the Booty Warrior", when he takes some young boys on a tour of a state prison gone horribly wrong, he manages to man up at the last minute to fight off the titular Booty Warrior, the most feared rapist in the jail.
Rufus and Amberley of The Dreamstone are usually buffoonish Kid Sidekicks who rely more on the Urpney's own stupidity to save the day. When faced with a genuinely menacing plan however, they can sometimes be very tactful. Arguably played both ways the Urpneys also have odd moments as Not So Harmless Villains.
Billy from The Grim Adventures of Billy & Mandy is naturally a happy moron, but he has his moments where his anger is so intense that even Mandy fears him. His father Harold is very similar in personality, and is a former Navy SEAL.
Bubbles in The Powerpuff Girls. While technically the whole concept and style of the show has echoes of Grotesque Cute, Bubbles is the cute and ditzy "sugar" in "sugar, spice [Buttercup] and everything nice [Blossom]" and occasionally gets to go berserk.
Reddy from The Ruff & Reddy Show. He may be dense and a little chicken at times, but when it comes to Ruff and anyone smaller than its antagonist, he'll step up to the plate.
The Sewer Urchin from The Tick usually he's a smelly idiot who just stands around commenting on things and is rarely useful in combat, however he's very competent in the sewers where he lives, he knows all the creatures that live there and their weaknesses and he even has his own arch nemesis The Sewer Czar whom he thwarts on a regular basis.
Palmer and Sasha from Titan Maximum are both ego driven, booze swilling, dumbasses. But as the hicks on Eros, and Titan security found out, they didn't become pilots of the titular robot for nothing. They can both kick your ass very easily.
A portion of the cast in Thomas the Tank Engine have substandard intelligence or are a Cowardly Lion, but some have had their moments of badassery. Take Oliver, for example, who was shoved down a turntable well by a group of unruly trucks, and was subsequently taunted for it by other trucks. His reaction? Ripping the trucks' leader in half. Yeah.
One of the more consistent subjects of this are Donald and Douglas. In a story that never made it to TV adaptation, Douglas caused a delay by shunting one of Thomas's coaches into the wrong siding, and the twins were being vague with their numbers to avoid having him sent back to Scotland. In their first episode of the show, Donald crashes into a Signal box and Douglas breaks the Spiteful Brakevan; however, the former followed Donald disciplining the Brakevan for delaying Douglas' trains (one of the few times an engine crossed the trucks without them paying back), and Douglas only crushed the van because it held back another train and Douglas pushed "hard enough for three engines" as described by Edward. The two went on to save Henry from a massive snowbank and Douglas, the twin with more goofs on record, went on to smuggle Oliver off the other Railway in "Escape" to save the latter from scrap, despite shivering at the mere word before.
Captain Fanzone of Transformers Animated is often portrayed as the bumbling, technologically inept Comic Relief — although he's hardly Plucky. However...when it comes down to it, he shows remarkable resourcefulness and fairly good combat skills — in the episode "Survival of the Fittest", he manages to knock a monster twice his size to the floor with one punch.
And in "This Is Why I Hate Machines" he proves himself to be a damn fine cop, using the Autobots on Cybertron's apparent fear of organics to get information, as well as easily realizing he and Ratchet were being tailed by Jazz, and that he was on their side.
The Venture Bros.: Dean Venture snaps and beats the crap out of Dermott, a kid twice his size, after Dermot insults Triana.
21 and 24 are bumbling mooks. However, thanks to being dangerously Genre Savvy, they can do things even the most competent henchmen wouldn't dare attempt. Well, until one of them goes and does something genre-dumb.
The Monarch: "I hate that those two are my best men."
Then Hank plays it straight in season 04's "Pomp and Circuitry." He tries to join SPHINX and passes every single test, including foiling Hunter and Shore Leave's intentions to mind-wipe him.
In Wildfire, Sara's "adoptive" father John Cavanaugh is revealed to be this in one episode.
Also Tigger of Winnie-the-Pooh, most evident in the Disney incarnation, which is no less brainless than he is the original novels, while having saved the lives of almost all his friends in the Hundred Acre Wood from an avalanche to name at least one example.