A character has an ability that they cannot show off to or use in front of others because of nerves. When under pressure, the character can't perform as well, making others doubt that character's ability.
Usually, once under pressure, the character is unable to perform at all, looking like a complete idiot as they stand there unable to do what others are asking them to do. Other times, it has a more mild effect, causing them to simply make stupid mistakes that they wouldn't any other time.
The trope name is a more technical term for Stage Fright (see Real Life examples below).
If all we ever see is the character cracking under pressure, then it starts to edge into Informed Ability territory. Compare Centipede's Dilemma, in which a character loses an ability by thinking too hard about it. Related to Dead Air, an occasional occurrence on radio.
Anpanman himself from Anpanman. Despite being a powerful superhero, he is horrible at onstage acting. He stutters, speaks his lines in a very forced tone, and blushes like mad. This leads to problems when he's roped into performing (normally by Shiratama-san or the Den Den Troupe). However, as soon as Baikinman attacks the stage, he forgets all of his fear and goes back to being the one thing he truly is: a superhero.
An episode of Cinderella Monogatari features an acrobat named Mary who is nervous about performing in front of the prince - she sees everyone as the prince. Mary's first performance in the episode fails when she messes up and everyone laughs at her. Cinderella tries to help her, but is unable to make Mary's insecurity go away. Charles decides to help out and helps Mary overcome her anxiety.
Sorata from Sakurasou no Pet na Kanojo suffers this in Episode 8 during his presentation for his video game idea. Naturally, this doesn't go very well for him. Subverted come Episode 10, where he performs just fine.
One story in Archie Comics involved Reggie getting stage fright during his first attempt at stand-up comedy, leading to Jughead heckling him, which got Reggie mad enough to reply and then go into the rest of his routine.
Arthéon from Noob, according to one of the early short stories.
James: [Wallace] got the lead part in the school play, but on opening night... I never knew it was possible for someone to forget so much so quickly without a severe blow to the head.
One of the superheroes in Mystery Men has the power to become invisible — but only when nobody is looking.
In My Favorite Year, Alan Swann is a beloved swashbuckling film star. When he learns he has to perform in front of an audience (and on live television) for his latest gig, well...
Todd in Dead Poets Society is actually quite a good poet, but has a bit of a problem with reading/speaking in front of people, which leads to him taking notes at Dead Poets meetings rather than doing any of the actual reading. At first his aversion to public speaking is so strong that he refuses to join the society at all.
Ron Weasley has this problem in Harry Potter books five and six. He's a good Keeper when no one is looking, but he gets too nervous to play when people are watching him play.
Happens to Petra in Ender’s Game, crippling her usefulness to Ender. This is not because of an inherent inability to deal with nerves, but because of the enormous weight of pressure and responsibility that Ender had placed on her.
The Great And Powerful Turtle in Wild Cards suffers from this when using his telekinetic powers. This gets in the way of his desire to be a superhero until he gets access to some military surplus battleship armour and a scrap Volkswagen Beetle.
In Lonely Werewolf Girl one of the biggest problems Dominil has acting as manager for werewolf punk-rockers Beauty and Delicious is preventing them getting completely drunk due to their stage fright. Yep, werewolves with stage fright.
In Moving Pictures, clickie superstar Ginger is terrified to step out of the coach in front of a cheering crowd, having only acted in front of a camera crew rather than an audience. Victor snaps her out of it by suggesting that she pretend it's a click, and it works: she can handle the attention if she stays in character.
A childrens' book about a boy getting the lead role in the school performance of Robin Hood brings this up. "The more mistakes I made, the more nervous I got. The more nervous I got, the more mistakes I made." It gets so bad that he ends up mixing up words; when the script reads "Listen to the minstrel", he comes onstage and announces "Listen to the Smartie!"
"Randy's Dandy Lions" is a children's book written by Bill Peet that features a troop of performing lions that suffer from what the book refers to as 'cage fright.' This results in the firing of their trainer and the hiring of a new one. This does not end well.
The Brady Bunch: "You Can't Win Them All," from the fourth season, was about Cindy earning a spot on a Quiz Bowl-type show for elementary students ("Quiz the Kids," with Chris Knight's father, Edward, playing the quizmaster). Cindy's huge ego (the central focus of the episode) underlied the fact that she studied hard for the program ... only for it to come crashing down when the cameras started rolling and the live TV broadcast began. Cindy fails to so much as mutter a syllable the entire time - all she can do is stare blankly at the camera throughout the show - and Mike reassures his disconsolate, humiliated youngest daughter afterward, "It can happen to anyone."
Family Ties: The first season episode, aptly titled "Stage Fright," had a High School Bowl-type program in its plot, where the highly intelligent Alex, captain of his school's quiz bowl team, is forced to turn to his dunce of a sister Mallory when a teammate falls ill. Alex is well prepared for the show and victory for his school is assured just by his mere presence ... only for him to unexpectedly come down with stage fright. Alex is unable to answer one question or spit out a coherent statement, while Mallory (the underachiever) is relaxed, despite only answering a few of the questions right.
Game shows are a prime example of this trope. Many shows have qualifying tests, which producers use to select the contestants they believe will perform the best and have a chance to win. While many contestants selected to play the game do well, many others simply do not ... not necessarily because of the unexpected difficulty of the material or because they're facing a Ken Jennings-type champion, but because they simply are unable to relax or perform well under pressure (i.e., when he/she is expected to do well). As such, contestants sometimes unintentionally say the wrong thing when asked a question everybody knows the right answer to ... if they are able to give any response at all; the contestant will almost always - unless they were honestly stumped or truly believed their answer was right - admit their error once they realize their mistake or the host reads the right answer.
Pat Sajak, host of Wheel of Fortune, has said many times - especially after an easy Bonus Round puzzle goes unsolved - that playing the game at home or from the studio audience is much different than a contestant playing the game, and that nerves and the pressure to do well take their toll when the cameras begin rolling.
At least one champion from The $100,000 Name That Tune recounted his experiences from the $100,000 tournament in Jefferson Graham's "The Game Show Book"; in an elimination round, he knew the titles to three songs ("Yesterday," "America" (from West Side Story) and "Tie a Yellow Ribbon 'Round the Old Oak Tree") but was either unable to answer before being buzzed or press his lock-out buzzer fast enough, and he owed his lack of success to extreme nerves (particularly since he knew a large prize was up for grabs). At least one other book - pitching advice to prospective game show contestants - included its author's own disastrous experience on the Jim Perry version of Card Sharks; she wrote that, despite being relaxed beforehand, she became completely unnerved when told she was going on air to play the game, and those nerves - plus a lack of support from others in the contestant pool, whom she had thought were her friends - contributed to a pair of quick losses to a dominant champion.
Played with in Married... with Children. Marcy is anxious about having to deliver bad news at a presentation to her bank executives. She sees a psychotherapist who conditions her to associate public speaking with sex. This not only relieves her performance anxiety, but causes her to have an orgasm during the presentation. She's soon in demand throughout Chicago as a speaker delivering bad news.
In the Homestar Runner short "A Decemberween Pageant", Strong Sad gets the part of The First Decemberween in the titular pageant. He gets so nervous that he locks himself in the bathroom before his scene, and the rest of the cast are forced to improvise to extend the play. When Strong Sad finally does come on-tage, it turns out that The First Decemberween's entire part consists of walking on-stage and saying "What?" (or alternately, that Strong Sad's part was supposed to be larger, and no one in the audience noticed or cared that he messed it up).
In an episode of Birdz, Eddie's friend Gregory is shown to have a fine singing voice, but is afraid to perform onstage. At first, he has Eddie lip-sync to him, but then feels guilty over pushing Eddie into the spotlight this way and runs off. Eddie finally fakes a sore throat, forcing Gregory to step in as his understudy and overcome his stage fright.
The page image is from "Hurricane Fluttershy", when Fluttershy remembers the mocking stares and constant bullying of her past.
It's further only when observed by other ponies; she goes off to train with her animal friends and is fine when they are watching, but as soon as they don Uncanny Valley pony masks, Fluttershy falters.
It happens again in "Filli Vanilli", where Fluttershy is too afraid of performing and as a result hides behind a curtain while another pony lipsynchs to her voice. When she accidentally knocks the curtain down and outs herself as the real singer, she has a nervous breakdown.
Happens with Rainbow Dash in "Sonic Rainboom" after she crashed during every practice session and was being upstaged by Rarity.
Sweetie Belle has an instance of this in "The Show Stoppers." Like Fluttershy, she has a good voice but is shy about singing in front of others, prompting her to do props and costumes for their show instead of singing, which goes to the Hollywood Tone-Deaf Scootaloo. She's clearly gotten past it as of "For Whom the Sweetie Bell Toils," when she writes, directs, and acts in a play.
In Equestria Games, when it's time to light the torch in front of the entire stadium, Spike's unable to trigger his fire breath.
Truth in Television for many people: glossophobia, the fear of public speaking, is commonly listed as the most common phobia in humansnote as much as 75% of all humans demonstrate mild symptoms of Glossophobia, though the actual diagnosis rate of Glossophobia is much lower. When referring to people specifically performing something (like a play or a music recital), it's colloquially called Stage Fright.
The fear of public speaking is so common (some say people fear it more than even death) that there's an old joke that goes "Most people would rather be the guy in the coffin than the one doing the eulogy!"
Everywhere in amateur/high school productions of plays or musicals. It can merge into Bad Bad Acting eventually if the performer does not try to keep it under wraps. The way to spot it is a refusal to put down their arms, frequent gesturing to emphasise words and/or legs placed firmly on the stage, complete with bobbing in some performers.
Barbra Streisand has admitted to having terrible stage fright before every performance.
Steven Tyler of Aerosmith was extremely reticent about singing in the studio in the band's early years.
Peter Gabriel used to suffer stage fright as frontman of Genesis. Wearing outlandish costumes onstage helped him overcome it.