In Eyeshield 21, Hiruma likes to brag that he has a kicker on his team who can score a field goal at 60 yards, over half the field away from the goalpost, calling it the 60 Yard Magnum. Musashi, the kicker, repeatedly refutes the point, saying it's just an intimidation factor (later he says he's never made one from further than 55 yards...which is still an impressively long kick). Then it comes down to the last second in the Christmas Bowl, with Deimon trailing 42 points to 44 and possession on the 50 yard line (for those who don't know, that puts them 60 yards from the goal post because of the 10 yard long end zone). What follows is one of the most beautiful and kick ass scenes in the entire series.
Hiruma forced Sena into the identity of "Eyeshield 21", the ace running back from Notre Dame High School, so he wouldn't be scouted by other sports teams and Mamori wouldn't flip about him playing such a dangerous sport. As Sena later discovers, the mythos that Hiruma had built around the identity is largely true concerning the real Eyeshield 21 — the latest in a series of running backs to wear that number at Notre Dame. It's revealed in the final chapter that after the World Cup, Sena actually became the real Eyeshield 21 in the US.
¡Three Amigos!: Ned mentions that a passing biplane is the same kind that he flew in one of his prior movies. At the end, the other Amigos need him to fly this plane to escape from El Guapo's army. Ned confesses that it was his stunt double who actually flew the plane. They climb aboard the plane anyway, and Ned flies like crazy.
In Memphis Belle bombardier Val has not exactly gone out of his way to deny that he is almost qualified as a doctor, even if he never specifically claims so. When Danny is wounded during the mission and the others look to Val to save him, he finally fesses up, admitting he only took two weeks of medical school before enlisting. The Captain, Dennis, gives him a speech about how he's the closest thing they got, and he goes off to save Danny's life...
Faintly justified; some flight simulator games, notably Microsoft's Flight Simulator, are accurate enough to be used in training real pilots.
In fact, as MythBusters showed, landings can be somewhat automated on modern commercial planes assuming there is no damage to systems or especially bad conditions. Further, a flight controller was able to talk Jaime and Adam through their simulated plane landings... and then showed an easier way it could be accomplished.
Mystery Men: Mr. Furious supposedly has the superpower of rage-induced super-strength, but, when called on it near the end of the film, he reveals it's all an act. However, when rescuing the Love Interest from the clutches of the Big Bad, he becomes genuinely furious, genuinely gains rage-induced super-strength, and starts kicking ass.
Film - Animated
Chicken Run: Fowler talks constantly about the time he spent in the Royal Air Force. At the end, the chickens expect him to pilot their homemade aircraft to freedom, and he admits that he was in the Air Force as a mascot, not a pilot. He ends up flying anyway. Interestingly, he's not the least bit ashamed at being a mascot, and seems surprised that the others actually expected him to be able to fly a plane. I mean, he IS a chicken. Fortunately, Fowler was observant.
Ginger: Fowler, you have to fly it. You always talk about "back in your day." Well, today is your day.
Bunty: You can do it, you old sausage.
Fowler:(beat, then salutes) Wing Commander T.l.Fowler, reporting for duty.
The title character of Rango finds himself boasting big about having killed several men with a single bullet and, after accidentally accomplishing the feat, being made sheriff, at which point he has to do it for real.
A Star Trek: Deep Space Nine novel once featured Sisko bragging about being able to hit a target with a phaser blast using a mirror. Later, that exact situation crops up on the station (with Odo becoming the mirror); Sisko pulls the stunt off but afterwards admits he had exaggerated his ability.
Ciaphas Cain is constantly on the receiving end of this because of his (partly accidental) status of a Fake Ultimate Hero. The best example probably being in the first novel, when his friend assumed he'd want to sneak into an enemy camp for fun (although he is quite competent he's also an admitted Dirty Coward).
In the Septimus Heap series, a ghost asks Septimus if he can perform a certain spell and he says "almost"—not because he knows anything about the spell, but because he knows the ghost will get horribly upset if the answer is "no". Of course, later on they need the spell to heal a dragon-boat and they scramble to make it work.
Subverted in Harry Potter and the Chamber of Secrets. Gilderoy Lockhart's an amazing fighter of dark-forces- werewolves, hags, vampires, he's fought them all and managed to write about them. Except in reality he's an inept fool; pretty much everyone realises this and their suggestions about Becoming the Boast are mostly out pleasure to see him squirm. Eventually, Harry and Ron genuinely force him to help face the Basilisk, since he's the closest thing they've got. Instead, Lockhart subverts the trope, attempting to memory-wipe them and abandoning Ginny to die.
Live Action TV
An early episode of Star Trek: Deep Space Nine has a Bajoran, Li Nalas, who is famed as being a great resistance leader, who later admits he is nothing of the sort. It's something of an inversion in this case, in that his reputation was not his own doing (he was, in fact very uncomfortable with it), but actually spread by other Bajoran freedom fighters who needed a hero to inspire them. He did actually kill an infamous villain, but it was mostly a lucky accident (he stumbled upon the guy bathing in a lake, shot him in his underwear when he saw him running to his phaser, and only found who he was after someone else came across them). After that, as his legend grew, he got credit for almost every successful campaign by the resistance despite never having actually been in charge or heavily involved in carrying them out. But when the time comes, he shows that he can be every bit the legend that people already believe.
Even after proving himself, Li Nalas shows that he still considers himself unworthy. His Last Words are "Off the hook," reflecting his relief that he'll no longer have to live up to a reputation he doesn't think he deserves.
In an episode of Seinfeld, George claimed to be a Marine Biologist to impress a woman. Later, he and that woman were walking along a beach when they came upon a crowd gathered around a beached whale who is having trouble breathing. George removes a golf ball from the whale's blow hole and saves its life.
Played with in Scrubs. Elliott insists that her brief endocrinology fellowship has made her an expert on the subject, when in reality she's just very good at hiding textbook pages all over the hospital. When Dr. Cox tries to expose her by scheduling her to give a lecture about endocrinology, she panics until JD makes her realize that all her obsessive efforts to hide the fact that she was faking it has led to her becoming a genuine expert on the subject.
Brad Vickers of Wild ARMS 2 claims himself as the "Hero of Slayheim" in order to keep the heat off of his friend Billy, the real hero. He proves himself more than capable of carrying the mantle once he regains his confidence, however.
In Chapter 3: The Paper Hero of Mega Man X: Command Mission, X and Spider go to rescue some POWs inside a prison. There they meet up with a hero in green armor Steel Massimowho in reality is just a random cowardly reploid in the armor of the real one, who has given it to him, before he has been stripped down to practically nothing and hung out to dry. The fake Massimo finds him and cries over him. The real Massimo tells the fake one that he’s glad he gave him his armor, and that he can be a strong fighter if he tries. Massimo begins the game pathetically weak, with average LE but no offensive or defensive strength to speak of...at LEVEL 1. And even at level 1, his LE is close to that of X at level 10.
Captain Qwark of the Ratchet & Clank series. Throughout the series he boasts about how amazing and heroic he is, but always fails to live up to it (he's even a villain in the first game). In the third game however, he fakes his death, but when Ratchet and Clank find him, Clank tells him that there's still a chance for him 'to be the hero he always wanted to be'. Cue his entrance during the final boss battle in which he helps you fight Dr Nefarious.
He's also helped out in the more recent Crack in Time, despite maintaining his pathetic qualities.