Basic Trope: Character brags about having skill, only to reveal they lied about it when called upon to perform said skill... And then succeed anyway.
Straight: Bob claims to be a skilled pilot. Later, Alice needs him to fly a plane. But when she volunteers him to fly, he reveals he's never flown an actual plane. However, he takes the pilot's seat anyway, and manages to perform well enough to save the day.
Exaggerated: Bob constantly brags about being a fantastic flyer and a doctor who's repeatedly performed life-saving surgery. This convinces Alice to take him on an extremely dangerous mission. Eventually, their friend Eddie's life depends on Bob's talents — only Bob chooses now to reveal that he's been lying all along; he's only ever flown remote-control planes and is actually squeamish about the sight of blood. He still fixes Eddie up in record time.
Downplayed: Bob claims to have run a ten-second hundred meter dash when his actual best time is just under eleven seconds. However, he figures out that Alice's plan doesn't require him to run from a standing start, and manages to just beat the clock.
Justified: Bob was being held hostage, and claiming to have a useful skill was his only way to survive.
Bob was only lying/exaggerating some of his skills — turns out he really is a doctor, but never got his pilot's license despite really wanting to learn how.
Bob reveals he lied, then saves the day through some other means... which he's also bragged about in the past, only this time, he wasn't lying.
Bob really was a pilot — but for a completely different kind of plane than he claimed.
Averted: Bob never claims to be a pilot.
Enforced: The ending where Bob crashed the plane fared badly with the test audiences, so they rewrote it.
Lampshaded: "Well, Bob, I guess you weren't lying after all."
Invoked: Eddie prompts Bob to exaggerate what he's capable of in order to impress Alice. Bob isn't aware (although Eddie is) that Alice is sizing him up to take on a special assignment...
Exploited: Bob actually has been studying to become a pilot, but he's been so underconfident that he has never attempted to fly solo — so his trainer arranges for him to brag about it to Alice so he will be forced to step up.
Defied: Bob refuses to try.
Discussed: Alice asks Bob "This isn't going to be like in the movies where some unqualified rookie said he could fly when he couldn't but ends up saving everyone, is it?"
Deconstructed: Alice built part of her plan around Bob's proclaimed piloting skills; without that ability, her plan is crippled. Even if Bob pulls off an unlikely save, she remains furious with him — and the others are furious at both of them, holding Alice responsible for not checking his qualifications instead of just trusting his word.
Reconstructed: Bob listed skills that are at least related to some of his actual skills/knowledge so that he can tell himself he wasn't fully lying (e.g. claiming to be a hacker just because he uses the computer a lot, claiming to be a doctor because he took some biology courses). When called on he still admits he was full of it, but his actual skills allow him to fulfill this trope just enough for everyone to get out alive.
Played For Drama: If Bob can't perform the skill asked of him, they're all going to die horribly. Not the best time for Alice to find out he was deceiving her all this time...