Happens in TTA in the D-Bug Infiltration arc of Season 2, and later in Season 3, where The Ditz announcer duo are revealed to have powerful Summon Magic.
In Unforgotten Realms, when Sir Schmoopy stops screwing around and starts fighting an enemy seriously, he's capable of some really amazing stuff.
In Episode 4, he singlehandedly destroys an army of skeletons and their necromancer by activating a self-explosion curse on himself, and swapping his soul into an indestructible crystal so that he could survive it.
In Episode 6, he formulates a brilliant strategy that allows the party to defeat a giant ogre in a battle of attrition.
And in the last episode of Season 1, he and the bumbling paladin Roamin mow down a pirate ship's worth of Kobolds in the series' biggest fight scene to date, and the whole affair ends with Schmoopy leveling up and using an explosion to kamikaze the leader.
Except it turns out that that plan failed and the ship exploded for a completely unrelated reason.
Homestar Runner and Pom Pom both fit this trope. Homestar is a terrific athlete after all. Pom Pom is a stellar martial artist.
Red vs. Blue: most of the characters get their moments of this.
Most exemplified when Caboose learns "how to be mean". Although his battle cries are... less than intimidating, the sheer efficiency with which he dispatches two full teams of fighters is a little unsettling.
And there's the Grand Finale of the Recollections trilogy, in which even after learning they were designed to be nothing more than cannon fodder, our whacky band of misfits rally together to defeat The Meta.
The Supreme Six were one of the least respected teams of superheroes in the Global Guardians PBEM Universe. All six heroes (Calculus, Photon, Omnipresent, Prototype, Stonewall, and Thunderstorm) were former teen sidekicks "all grown up", and as such were generally the butt of jokes because, in the public perception, they were still sidekicks. And then they fought the Fatal Four, one of the most feared supervillain teams in the world... and won. Handily.
Shadowhunter Peril has several characters who embody this trope, most notably Etzel and Veronica. Although Etzel has yet to fully unleash his inner potential, it has been strongly hinted that he was an extraordinarily successful assassin who would not be afraid to do what had to be done if he situation arises. Of course, being that he's almost constantly surrounded by several Physical Gods he's never had much of an opportunity to display this. Veronica, on the other hand, is basically a Genki Girl who spends her days cooking up random, inedible meals and annoying the hell out of the rest of the protagonists. Because her lethality is generally directed at the good guys for the sake of humor, it's pretty easy to forget that she has a portable tank inside her Bag of Holding, as well as nearly every single weapon that's ever existed. Some notable examples of her being Let's Get Dangerous are when she took down a Behemoth demon with said tank, engaged the Greater Demon Lilith in an intense aerial swordfight above a decaying New York City. Then there was the time she was training Wynter, an unruly, teenage Daylighter, and Veronica got tired of her snarkiness and delivered Wynter a Curb-Stomp Battle that completely humiliated the vampire. Also, her Mama Bear side once took over when Ethan got hurt, and she reacted so violently that she even managed to disturb Puriel
In the Whateley Universe, Vox is just a protagonist's girlfriend. She's pretty, and she has Siren powers so she sings beautifully. But when mercenaries in power armor attack Phase's birthday party in "Ayla and the Birthday Brawl", she shows what she's been learning in class. She can force people to do her bidding. "Shoot your leader."