Married... with Children's Al Bundy. On occasion, for whatever reason, Al - schlubby loser Everyman Al - would take it upon himself to beat the living shit out of some poor bastard and he would do so with the psychotic glee you would expect from a man with often literally no other joy. Of all the things Al Bundy failed at - and Al Bundy failed at almost everything he put his hand to - he never once lost a fist fight (or was even mentioned to have lost a fistfight) in the show's entire history.
iCarly: Gibby. If you are a friend, but betray his trust, or someone who dares kidnaps his friends, he won't think twice about beating the fudge out of you.
T-Bo, a background character who runs the Goovy Smoothy and is normally seen just putting random items on sticks to sell and being wierd, reveals himself to be one in iStill Psycho when he helps Mrs. Bensonrescue the iCarly gang from Nora and her equally insane family. Not only does he kick down the basement door so Carly can rescue Spenser, he singlehandedly takes down Nora's father.
Played with in Disney's The Suite Life: Apparently London Tipton is quite the athlete (notably in volleyball), but only when she's mad.
In Get Smart, Maxwell Smart may be a bumbling idiot most of the time, but when he realizes the situation is becoming make-or-break, then he buckles down and becomes nearly unstoppable.
R.J., Trickster Mentor of Power Rangers Jungle Fury, seems at first to be nothing more than an overly-mellow pizza restauranteur. Then he steps out of the kitchen and reveals himself by taking down a squad of Mooks without breaking stride, and suddenly the guy who seemed like bumbling comic relief a few seconds ago takes on a whole new light. He barely changes at all, busting out the crazy martial arts skills while remaining as calm and carefree as ever.
River from Firefly is a Crouching Crazy Hidden Badass. Most of the time, she tends to be a Cloudcuckoolander with some endearingly whimsical moments mixed in with seriously disturbing fits and a few Ax-Crazy moments. But mess with her brother Simon or her friend Kaylee, and she will take you down, whether by gunning down three men with her eyes closed or orchestrating an impressive Batman Gambit. And that's even before the Big Damn Movie.
Drusilla exhibits only the loosest connection to reality, and spends most of the time behaving like an idiot child (e.g. cooing over her pet bird, which died because she left it in the cage and forgot to feed it). But when she does fight, she's extremely dangerous, and capable of killing a Slayer with a slash of her fingernail.
Xander. Whether it was magic tuxedo, skill with a crane, or just a combination of borderline Too Dumb to Live and luck, Xander managed to be a useful and functional member of a team that included a slayer, a powerful witch, a werewolf, a vampire, a half-demon, and a high school librarian.
From Stargate SG-1: Colonel Jack O'Neill. Probably more a case of Obfuscating Stupidity, as you couldn't get to officer rank (much less Colonel) without being fairly smart; he's just a very "military" guy and would like to be told that yes, this alien/Ancient technology/doodad will do what he thinks it will, instead of listening to a lengthy technical explanation about why it will do what he thinks it will. He acts Book Dumb in everything remotely scientific. He never reads Carter's reports, he refuses to listen to Daniel explain some ancient piece of writing or technology, and acts as if nothing matters except The Simpsons. But when some dumb Goa'uld or other bad guy threatens his team or the Earth, he gets very serious, and reminds you just why he's leading this team. In the season 5 episode "Rite of Passage", Cassandra observes that Jack is smarter than he pretends to be.
He also happens to be an amateur astronomer (a single comment about an accretion disc results in Daniel hitting a figurative brick wall while talking). When he's not using the telescope on his roof to spy on neighbors, that is.
He's only an amateur in the sense that he isn't paid for doing astronomy. It turns out he's got a Masters Degree in astronomy and really is that smart. He's also smart enough to know that he doesn't need to know everything his subordinates know, just that they know about whatever situation is at hand.
Adrian Monk: Socially inept, obsessive-compulsive and afraid of everything. But he manages to solve the most baffling crimes and he can hold his own in a fight — he was a cop once, remember. And if you even so much as insult Trudy, he will hand you your ass. In the episode where the suspect was a marathon runner, who grabs the key piece of evidence and takes off on foot. Monk proves that he used to be a great runner in school by giving chase (especially in his new sneakers which he got from his idol).
Otoya Kurenai from Kamen Rider Kiva, who at first blush seems to be nothing more than a foolishly self-absorbedThe Casanova who doesn't have a serious bone in his body. But then he starts pulling Batman Gambit after Batman Gambit, demonstrating an incredible degree of character judgement, and kicking monster butt with an experimental Powered Armor — typically stolen from its intended user as part of said Batman Gambit. It's no wonder that near the end of the series, the Fangire Queen, normally tasked with killing "race traitors" who fall in love with humans, has herself fallen for Otoya and would go on to bear their child, the show's protagonist Wataru.
Brendan "Hot Dog" Costanza fits the trope quite well. Most of the time, he's just screwing around and too busy having random rashes. But in the cockpit, he's more than competent.
By the end of the series, most characters who weren't introduced as straight-up badasses (and are still alive) have turned out to fit this trope. Felix Gaeta's leading an armed mutiny, Laura Roslin's raging refusal to surrender when it appears that said mutiny has succeeded, Romo Lampkin's escaping from a marine who had a gun to his head by stabbing the guy's jugular with a pen, and Gaius Baltar going all Rambo on the enemy Cylons during the battle in the Series Finale.
NCIS's Anthony "Tony" DiNozzo. He's a Chivalrous Pervert and, at times, a Man Child. An example usually comes up Once an Episode. His backstory is that he decided to go into the police force after he, a civilian out for a walk, saved a young boy from a house fire, and failed to save the boy's younger sister. He became the single best officer in the department before becoming Gibbs' right hand.
Example of the "Crouching Moron" part from Season 2's episode, "Bikini Wax". DiNozzo and Kate are going to a prison to interrogate the cell mate of their prime suspect. Tony lectures Kate on the way there about how they're supposed to keep their guards up, since prisoners have nothing to lose. A few minutes into the interrogation, Tony notices an Alpha Chi Delta tattoo, and...
Example of the "Hidden Badass" part...pretty much every time he's involved in an interrogation. Not only is he skilled at making the ones interrogated say more than they should by annoying them, he can make the ones interrogating him say more than they should. Prime example from "Aliyah": Eli David, head of Mossad, accidentally says too much to Tony while he's being interrogated...and while Vance, Gibbs, and Ziva are watching.
The 1991 series Step by Step had Sasha Mitchell's Cody Lambert as the beach bum slacker nephew of Patrick Duffy's Frank Lambert. Though normally a clueless "Valley Dude," in one episode when braced by a group of bikers in a bar, Cody showed why he's played by the man who made two "Kickboxer" films. After clearing the bar of bikers, he reverted to goofy flake with the following dialog;
Carol: Cody, I didn't know you knew karate!
Cody: Cha, me neither! I'm gonna have to go back and re-read some of my old diaries!
Jason Stackhouse spends most of True Blood constantly doing absolutely stupid shit, because he's dumb as a stump, and is often led about by just about anyone. Threaten his sister Sookie, however, and he will not just step up; he'll absolutely take you down and make the audience's jaws drop. But he's still dumb as a stump for the most part.
This appears to be somehow related to Sookie being around. When she disappears for months, she returns to find him a full-fledged cop, having become much more responsible in the meantime and even running the police station, more or less, when the sheriff turns out to be a drug addict.
Brilliant but baffled Professor Yana in the three-part 2007 series finale of Doctor Who. He does a dramatic Face-Heel Turn as he remembers what a badass he is as The Master.
Rose Tyler could qualify. She certainly has her fair share of ditzy moments; she has to get saved a lot, and sometimes, if she's feeling particularly helpful, she's shouting ineffectually at the villains. She does, however, absorb the power of time and become a demi-goddess at the end of series 1, sacrifices herself for the universe at the end of series 2, and blows things up like all-get-out at the end of series 4.
While he's hardly a moron, the Doctor himself can fit this trope at times, particularly in his tenth incarnation. Say you're an alien invader. You captue this odd fellow in a suit and hi-tops; he appears to be batshit insane, jabbering on, often in rather inane ways. And just after he finishes talking, you realize he has just taken down your entire armada.
Case in point: the Family of Blood. The Doctor (using his human disguise/alias John Smith) stumbles into the Family's ship, trying to stop them from destroying the academy where he hid from them. While negotiating with them, the Doctor stumbles about pressing random buttons on the ship before turning over his fob watch (which holds his Time Lord powers). The Family tries to absorb said powers, but they are not there. The Doctor then drops the bumbling human act and reveals he's back to normal. And has just set their ship's reactor to overload. The kicker is that after the ship blows up, the Family realizes that the Doctor hid from them because of what he would do to them as punishment, not out of fear for the Family itself.
The Second Doctor was probably the most representative of this trope. While all incarnations of the Doctor are capable of deadly seriousness and incredible skill, and most (Exception for the fifth Doctor) were usually goofy, Patrick Troughton brings the apparent goofiness up to eleven. The vast majority of the time, the second Doctor pretended to be a bumbling idiot, constantly stressed out and panicking. However, when faced with the right danger, he drops the act and becomes dead serious and sometimes even diabolic. This is especially pronounced whenever he encounters Daleks or Cybermen.
Companion Rory Pond is clumsy, awkward, useless in a fight, has a serious inferiority complex concerning the fact that his wife to be seems to love her imaginary friend more than him, and generally is mostly only on board the TARDIS for her. But he will not take crap if someone is in trouble and by the end of the series has become a major mythical figure and badass fighter. Even after the timelines are reset he remembers living for 2000 years guarding his wife inside Pandora's box, and is an extremely competent fighter, able to face down an army of Cybermen if necessary. River had to get those genes from somewhere, I suppose...
Amy Pond: (to her infant daughter, Melody, as both are being held prisoner by The Silence) "I wish I could tell you that you'll be loved. That you'll be safe and cared for and protected. But this isn't the time for lies. What you are going to be, Melody, is very very brave. But not as brave as they [The Silence] all have to be. Because there's somebody coming. I don't know where he is, or what he's doing, but trust me. He's on his way. He's the last of his kind. He looks young but he's lived for hundreds and hundreds of years. And wherever they take you, Melody, however scared you are, I promise you, you will never be alone. Because this man is your father. He has a name, but the people of our world know him better... as the last Centurion."
Strax. The series goes out of its way to emphasize how useless he is, unable to adapt to any situation where the answer is not simply killing an enemy. Whenever the situation really does require killing an enemy however, he is the last person you want to go up against.
Vir from Babylon 5 is introduced as a timid and shy comic relief, used to being pushed around by his boss Londo. However as the show goes on he gradually shows more and more backbone, first by not backing from a huge monster (a hologram, but he didn't know that) when sent to deliver a message to technomages, then by telling off an emissary of Eldritch Abominations in the most awesome way, and finally by retorting to an insult and a spying attempt by borrowing his Londo's duel sword and trashing the offender's stall and forcing him to confess at swordpoint.
WPC Shaz Granger is hardly a moron, she's just sweet and isn't as badass as the rest of CID. It's kind of hard when you're a plonk in your mid-twenties, trying to live up to the examples of Gene Hunt and Alex Drake. Even Ray and Chris are pretty badass, holding their own in gunfights and having multiple Big Damn Heroes moments - Shaz doesn't even carry a gun. And then 2.08 hits, where a bent copper is holding Chris hostage, about to shoot him. A gunshot sounds, and it's Shaz, holding a smoking gun and wearing her wedding dress. To cap it all off, she quips "How you doing, baby?" to Chris.
Vince Teague, the mild mannered small town newspaper editor in Haven. When a menacing and muscular ex-con shows up in town and threatens Vince's little brother, you can see the until then goofy exterior slough off and his inner badass reveal itself. A couple seasons later it is revealed that Vince is the leader of the Guard, a badass organization of Troubled people who have protected other Troubled from persecution since Haven was founded.
The live action TV version of Cutie Honey had Honey as a ditzy high school student who could take on entire swarms of mooks.
Agent Bill Hoyt of Undercovers. He's usually the nerdy, sycophantic tech guy for the Blooms; but in the episode "Xerxes" he single-handedly takes down the villainess after learning that she had almost manipulated him into falling in love with her.
Boardwalk Empire has Eddie Kessler, Nucky's bumbling assistant who often messes up and is often treated as a Butt Monkey. When an assassin comes after Nucky, he makes the assassin miss his shot, takes the assassin's gun away and then coldly shoots the guy in the back as he is trying to run away
While few people would consider Nucky to be a moron, he is a politician and talks like one so some opponents assume that he is just talk and can be easily pushed around. When three goons come after him in his own office, he dispatches all three of them by himself and then wages a bloody and ruthless Mob War against their boss.
Richard Harrow is a disfigured Shell-Shocked Veteran who has trouble talking due to his injuries and at first glance people might assume that he is brain damaged. However, he is probably the deadliest individual on the show and becomes a One-Man Army when needed.
This is pretty much Castiel from Supernatural. Perpetual confused look, especially when it comes to humans but piss him off and he can kick your ass to hell and back.
Also Gabriel in a sense. This is an archangel who is pissing around messing with humanity.
To quote Dean: "Word to the wise: don't piss off the nerd angels."
Criminal Minds has a few of these. It's to be expected from members of the team-they are FBI agents after all-but a lot of the Unsubs and quite a few would-be victims fall into this category. One example that springs to mind is the strung-out prostitute from "Legacy" who looks to be a disposable victim when the UnSub picks her up but manages to make her way through the killer's torture factory-including walking barefoot through a roomful of broken glass-and survives to see him taken down at the end of the episode.
Todd "The Todd" Quinlan from Scrubs is an utter meathead, seemingly only capable of communicating through innuendo and That's What She Said jokes. But he is also a ridiculously talented surgeon, one of the best in the hospital.
Highlander's Methos poses as a mild-mannered, sheltered scholar with the Watchers; even with his friends who know he's the oldest living Immortal, he'd rather lounge around drinking beer than accept a challenge. And he would just as soon not talk about how he used to beDeath of the Four Horsemen.
Methos: Just because I don't like to fight, doesn't mean that I can't.
In social game shows like Survivor and the American Big Brother, a good (albeit boring) strat is to make everyone think you're not a threat to them, so they won't target you for eliminations. Then, you start playing much harder and make them regret not taking you out in hindsight.
Attempted but failed with Brett and Ashley. Both Brett and Ashley just kept their mouths shut and hid behind numbers so they wouldn't be targeted. Then all of a sudden, they start competing well in challenges and force the dominant alliance to vote each other out first. Immediately the person in charge of the game thinks Oh Crap because this person they perceived to be a nothing is actually not bad at the game. Unfortunately; Brett & Ashley both failed to win the final immunity and were therefore the final member of the jury.
Before either of those three, we had Lillian Morris. She was very emotional and would cry at the drop of a hat...and became Fairplay's personal Butt Monkey. After a bit of Fridge Brilliance where he avoided elimination at the final four and had to face two people who weren't good competitors, Lil immediately takes to the final immunity challenge and doesn't move a muscle, while Fairplay starts trying to cut a deal, only for Lil to constantly shoot him down and taunt him to break down his spirit.
Hiro of Heroes is a Wrong Genre SavvyOtaku who spends most of each season suffering from severe Plot-Induced Stupidity. But when he gets his act together and wades into the fight, he will own you. He has basically singlehandedly defeated Sylar (a couple of times), two seasonal Big Bads, and an entire government black ops agency. The only villain on the show who was ever any match for him was Arthur Petrelli, who had his same power (among many others).
Walter Sherman of The Finder seems a harmless, goofy man who's just two steps short of an institution. It's not until he's strung out from a bad Find and faced with three bad guys that we're reminded he was an Iraq veteran. He does so by killing two of them with their own guns before snapping the last one's neck.
Mystery Science Theater 3000 has Gypsy, who at one point beats down her creator Joel after he was being mean to the other robots by cheating at 'Rock, Paper, Scissors', and Professor Bobo in the episode The Giant Spider Invasion , in which he defeats a race of pod people assimilating the Mads and the So L crew.
Ru Pauls Drag Race has Shangela, who didn't even survive the first week of Season 2, come back Season 3 as an opening plot twist... and proceed to last through over half the season (notably winning a comedy-routine based challenge in the process!)
Bob Sweeney from Justified is an overweight, unpromotable constable whose cockiness has found him on the wrong side of a gun a couple of times. Push one of his beserk buttons or test his loyalty to Raylan, however, and you unleash a whole different beast. To date, he has taken down two attackers on separate occasions with a concealed knife, withstood a vicious beating and opened fire on a couple of guys with an AK-47 because they made fun of him.
Raylan Givens: People underestimate Bob at their peril.
Chang of Community is a Cloud Cuckoo Lander whose general plot role is just to be annoying. However, over the course of Season 3, he becomes completely insane, eventually taking over the school and installing himself as a Stalinesque dictator and nearly killing the entire student body.
Penhale from Doc Martin, heavier on the moron than the badass admittedly, but he came across as a very professional and scary copper when he dealt with the evil loan sharks threatening Bert. And when his estranged wife shows up apparently unaware that they've been divorced for four years, he's clearly still desperately in love with her, but the first thing he does is ask her the date, confirming that she's not well.