In Yu-Gi-Oh! GX Fubuki is said to be one of the top duelists of the school and is even lauded as the best duelist they have left after Kaiser graduates. The one duel he has as his true self ends in a loss chalked up to a decidedly amateur mistake. (His duels as Nightshroud involved either Judai or Hell Kaiser Ryo.) In the manga, he more than lives up to his reputation.
It's well-established that everyone fears getting Retsu Unohana upset with them, from 11th division bullies to heavy-weights such as Kyouraku and Ukitake. Although she's the military's top healer, her dominant skill is actually sword fighting. Since she's never allowed to fight, her reputation with the sword informed only, and the reason for her scary reputation unexplored. The final arc resolves this, revealing her to be the 11th division founder and first Kenpachi. She trains Zaraki in a Duel to the DeathTraining from Hell scenario, proving that Beneath the Mask she's even more of an Ax-CrazyBlood Knight than Zaraki himself is; she actually reduces Zaraki to tears of despair at one point.
9th Espada, Aaroniero Arruruerie claims to have copied the abilities of over 36000 Hollows. Before he's killed, we see him use at most three of them.
Taken to an extreme with Chojiro Sasakibe. His funeral reveals he possessed Bankai but had always refused to use it. Justified, however, in that the manga has long introduced us to the concept of Shinigami who hide their Zanpakutou abilities for personal reasons; Aizen, Gin, Ikkaku and Yumichika are the most well-known examples. Choujirou therefore just slotted into an existing story concept. His case isn't helped by the fact that the one time we see him enter a fight, at the end of the Soul Society arc, Ichigo knocks him out with one punch.
Mashiro can hold her mask for 15 hours. But the only time she fights with her mask on it breaks after a few minutes.
Shinra from They Are My Noble Masters is supposed to be a world-famous, talented conductor. Yet all we see her do is waving her staff around in a very unprofessional manner and answering stupid questions from her musicians. The music that results from her conducting is also not really noteworthy.
In Fairy Tail, there are several moves which represent this, although it would be classed more as Informed Power then ability. The Iron Dragon's Sword is stated by Gajeel to "be able to tear through anything it touches" yet has not hit a single target, presumably due to fights ending rather quickly should it actually hit. Whilst there are other moves that fit this bill, such as Laxus' Raging Bolt, but the biggest offender is the Abyss Break. It's supposedly a devastating spell that is capable of wiping away an entire town. The first time it's attempted to be cast, it's power source is removed, the second time, the user faints before being able to finish casting it. Abyss Break is the only spell to have never been officially cast despite being mentioned practically once every other arc.
In fact, any such spell that's potentially able to end a target's life can be predicted as likely to miss or prove nowhere near as effective as claimed.
Yukito from Air supposedly managed to support himself for years by performing tricks with his magic doll, but in the TV series and manga he hardly makes a single yen with his act. He fares a bit better in The Movie, though.
Gates, the villain of Full Metal Panic!! The Second Raid, is supposedly an extremely skilled and dangerous Humongous Mecha pilot who leads a team of specially trained hunter-killers for Amalgam, all of whom are equipped with extremely powerful BlackBox mecha (for a comparison, one such mecha in the hands of a lower-standing member of the organization fought Sousuke to a standstill three times, killed several redshirts and mortally wounded a Mauve Shirt during the first season). Alas, five minutes after actually entering combat and proving his 'fearsomeness' by killing an overstrained and mentally unstable girl who was using an inferior machine, he and his entire team are bowled over by Sousuke in one go like so many mooks.
Gates is a good pilot, but Sousuke is one of the best in series; once he got his mech's Lambda driver working properly, it took away the one advantage that Gates had over him.
Moreso, the mechas (AS for Arm Slaves) Gates and his allies used, equipped with Lambda Drivers, known as Venoms by Mithrill, were designed to fight regular AS, much like counter-AS. The 'Arbalest' (Sousuke's AS) was a counter-counter-AS, and this is in fact an informed ability, since all it is depicted like afterwards is simply a more powerful Lambda Driver. On the side, Sousuke wasn't the best AS driver out there...sure, he was good, 'a specialist', but there were many who were better, e.g. Ben Clouseau, and by word of god, Kurz at least (said in the later novels), and many more
Tohdoh is described as a great general (having beat Knightmare Frames during the invasion without any of his own), only to have disaster strike whenever he is put in a leadership position, the most obvious instance being the season finale where he manages to get nearly his entire army either killed or captured. Code Geass was quite big on informing of someone's genius outside of Lelouch.
Justified in-universe as Tohdoh admits on several occasions that his reputation as "Tohdoh The Miracle Worker" is inflated far beyond his actual abilities. Also, in many cases where Tohdoh was given command (like the example above), he took over partway through a battle with incomplete information and/or when his forces were already losing. Lelouch could in fact succeed in impossible situations like these so this may also be a case of Overshadowed by Awesome.
The Knights Of Round. Most of them are killed by Suzaku with ease, and Luciano Bradley and 4 of his subordinates, together, manage to defeat a sniper in melee, but even the ones who live don't show impressive ability. Anya's greatest skill is using her Knightmare's giant cannon to blow stuff up, and with a cannon that big, I'd be shocked if she missed. Gino is the only one who shows actual skill, and he's left in the dust by Knight of Seven, Suzaku Kururugi - so much that he's not even a challenge. At least he can go toe to toe with every other pilot in the series... Except, you know, the Black Knights' own ace, Kallen. But that's not unexpected, so that's... a 2 out of 7?
Dorothea Ernst and Monica Kruszewski get the worst of it though, no doubt owing much to rushed planning for the second season that left them mostly out of the spotlight, getting only enough battle screen time to get one-shotted by Suzaku. Fan favorite Nonette Enneagram was spared the indignity by being kept entirely in the background. Speaking of Nonette, she's been said to make even Cornelia nervous.
In addition to being a Chess Master, Lelouch is said to be an actual chess master, and the show at least tries to demonstrate it by showing fragments of his games. However, the few times his games are shown, his moves and thought processes, while good for the camera, aren't going to win games against any remotely skilled player in real life.
Worse still is Schneizel, who's said to be the only one who can beat Lelouch at chess (which doesn't sound so hard, considering). It's probably for this reputation that the gathering crowd hails his move, in which he puts his own king in check, as a brilliancy, even though that move is just plain illegal.
Punie from Dai Mahou Touge is supposed to have nigh-infallible submission techniques to bring down her opponents, but everybody who knows a bit about martial arts should see that it's a bit silly. Most opponents don't even defend against her approaches—and when they fall down they don't even try to get up anymore, making it all too easy for Punie to perform a lock on them.
Stealth and black ops for ninja in general. Ninja teachers will take any chance they get to lecture their students on the subtle arts of subterfuge, sabotage, assassination and spying. In practice they are as stealthy as a barrage of fireworks, and their chief concern is fighting other (equally flashy) ninja.
ANBU is the main exception to this rule, but see below for how that is working out for them...
The Chunin examinees are said to be the best Genin in the world, but all nine of the rookies fresh out of the Konoha academy, as well as Team Guy, whose members have been ninjas for a little over a year, all make it to the finals while many more experienced ninja fail although in Kabuto's case, he did it on purpose to gather intelligence on the competitors.
Shikamaru's supposed IQ of 200. It only gets strained applications that come off as hokey at best, not to mention isn't integral to his character (though he is the Guile Hero of the group) and being extremely dodgy in the first place. Same with Sakura's supposed smarts (In Part 1, she has high intelligence and genjutsu skills, but no abilities that make use of them); intelligence might be the worst trait to have in the series.
Kakashi Hatake is said to have "over a thousand" different abilities he has copied from people over the years. Unfortunately we only see about six of them over the course of hundreds of episodes.
Kakashi is better off than Sarutobi Asuma or especially Yuuhi Kurenai. Asuma is supposedly one of the most powerful guys in Naruto, and besides defeating cannon fodder Sound nins, he next opponents were the Kisame and the immortal Hidan . Kurenai on the other hand, had faced undefeated Itachi.(Doesn't help that the only thing she really does in the plot afterwards is get knocked up by Asuma shortly before he died.)
The worst offender, however, goes to the ANBU. They're sent on all the most dangerous missions, any Bad Ass main character has spent time in the ANBU, and overall, supposed to be the SEALs of the ninja world. When they actually get into a fight, they're degraded to mook status, those formerly Bad Ass masks now cementing their status as Cannon Fodder. The fact that they're anonymous but allegedly elite makes them custom-made for The Worf Effect.
Hanzou, leader of the Village Hidden in the Rain, also suffers from this. The audience is told of his almost unstoppable power and skills but we never actually see him in combat. This is then mixed with The Worf Effect when Hanzou's assassination is used to hype up Pain's own power, emphasized even more by the fact that Jiraiya, Tsunade, and Orochimaru all lost to Hanzou during the war. Both of these tropes are eventually averted for Pain when he proceeds to kill Jiraiya and blow up Konoha proving that the rumors of his abilities are no exaggeration.
Naruto himself became a victim of this trope throughout the first half of Shippuden. Practically every mention of him was accompanied by a comment over how strong he is and how much he has improved, yet the only improvement he ever really showed was a bigger rasengan, and the constant need to get bailed out of any dangerous situation by his teammates. Thankfully, he got better as the series went on.
Naruto does demonstrate knowing how to use his techniques, particularly the Shadow Clone jutsu, more efficiently than before, and actually can fight against the Akatsuki rather than being forced to run from them or rely on another for protection. Unfortunately, his opponents are still quite difficult, and Sasuke has also gotten stronger.
The newest victim of this trope is the 2nd Hokage. Despite doing all of nothing in the one fight he has on-panel in the series, much, much later chapters have revealed several capabilities of his that have never been seen, including creating the Edo Tensei technique, which allows the user to use golems of dead people to fight their opponent, and being a master of Space/Time techniques, the same techniques used by Madara and Minato Namikaze, the Fourth Hokage, which make them Nigh Invulnerable.
Sasuke has actually flirted with this trope from time to time. While he's without a doubt very skilled, when you actually take a closer look at his Win/Lose ratio Sasuke doesn't put up the best performances. A good majority of Sasuke's victories have been a result of Worf Had The Flu; Naruto specifically held back at the end during their battle at the Valley of the End, he ambushed Orochimaru when he was on a sick bed, Itachi was nearly blind, half dead and holding back and still stomped Sasuke, who only technically won because Itachi then dropped dead on the spot due to an unrelated disease.
Hayate Gekko, the Chunin Examiner was heavily implied to be pretty tough judging by the fact that he's got the job he does. However, he gets offed while listening in on two villains, one of which, the Sand Ninja Jonin was his killer.
In Hitohira, Nono is supposed to be able to beat up all of the karate club on her own, even though later she is shown to have her hands full fighting only one person, namely her female fellow club member Risaki—which even results in a tie.
The Canis Niger group of bounty hunters, who were made out to be an incredibly strong group of mages. We see them capture Nodoka (a total noncombatant) and her group of treasure hunters (who we've never seen fight), before Negi shows up and wipes them all out single handedly. The only real thing of note that they pull off successfully is luring Setsuna and Kaede into a trap.
In an in-universe example, Nagi Springfield the Thousand Master's legend says he knows 1000 spells. In truth, he knows 6. To his credit, though, most of the otherrumors tend to be true. It turns out that those 6 spells are all he needs.
Any given monster or non-humanoid creature generally fits, most notably the Sealed Evil in a Can Ryoman Sukuna no Kami, who was overwhelmed by an even more evil being Evangeline the Dark Evangel (among her many titles). Also odd was a massive black dragon which Ninja Kaede defeated while blind-folded.
One Piece: The Sea Kings are supposedly very powerful, and serve as a means of dissuading sailors from entering the Calm Belt, but they only demonstrate how deadly they are when the Lord of the Coast eats Higuma the Bear in Luffy's past, and then eats Shanks' arm. They mostly serve to show how powerful certain characters are by falling in battle against them.
There is, however, immense variance in size and power among Sea Kings. The latest arc has appearances by Sea Kings like Stripey who are bigger than good-sized islands; Lord of the Coast is probably about the size of one of their teeth. These uber-Sea Kings also, unlike previous smaller ones, demonstrate human-level intelligence.
Additionally, their primary threat is what they can do to ships rather than characters. Alot of the high end characters are Devil Fruit users. Destroy the ship and there's nothing keeping them out of the water. TKO.
Reportedly Asuka of Neon Genesis Evangelion was a prodigy who graduated from a German University by the age of 13, and though this emphasizes the competitiveness of her character, she never displays the education level she should have. Further, accelerated admission to higher learning in Germany is as much dependent on emotional maturity of the student, but her temperament doesn't suggest someone who'd be considered for Gymnasium early, much less Universitat, or even someone who'd been immersed in mature peer group, academic or otherwise. In the manga she at least shows an ability to act more mature than she actually is.
Lampshaded in The Prince of Tennis 40.5 databook. Aragaki, a character whose sole contribution was to lose a doubles match the author didn't even bother showing, is given a bio something like: "In order to hold his own on such a formidable team, he must have awesome tennis powers ... well, he should..."
Saint Seiya spends hours of footage telling us that Bronze Saints can reach the speed of sound, Gold Saints can reach the speed of light, and so forth. But when any of them need to get anywhere in a certain limit in time (read: once per arc), they run more-or-less as fast as a normal person (even slower).
Several Examples from Rave Master. The first is Mega Unit, described as The Alcatraz... while the person talking about it is looking at it in ruins after the Big Badbroke out. The next is Miltz, who's supposed to be one of the most powerful sorcerers in the world. It takes, at most, one spell to knock him down, and he never cast a single effective spell in battle either. The final one is Beryl, supposedly the strongest of the Oracion Six generals, all other six (as there were really seven) were quite a feat to beat, if beaten at all, and one later took down a demon lord. Beryl took only one hit.
The Mobile Suits suffer from this in Gundam 00. They generally have a knack for using weapons once or twice (e.g. Exia only uses its wrist vulcans twice in 25 episodes. But blink and you'll miss it!) But supplementary materials make things worse. For example, did you know the GN-007 Arios Gundam in season 2 had a beam shield? According to the official file it does, but it has never been animated. The Model Kits are another offender as well. The Special Editions do add footage of some weapons in use, but these are only glimpses in action packed sequences.
 does this to Kio Asuno, the third-generation protagonist. Initially, he's an inexperienced pilot who relies on the power of the Gundam, his X-Rounder abilities, and his wingmates to get through battle. As the series progresses, everyone starts talking about what a great pilot he is, except he's still easily defeated as soon as he's deprived of one of his crutches.
The Royal Knights from Digimon Data Squad. Not a one of them is remotely tough, always getting taken down within an episode. Even worse, it's continually talked about how powerful they are. In fact, when they join the good guys, they don't even really help, they just kind of take a back seat.
Touka in Saki is supposedly a great Mahjong player. However, every match she's in, she manages to lose horribly. Offscreen she does fine though, in some cases because of her "Cold Touka" state, which she enters without knowing of it, and has a drastically different playstyle, enabling her to defeat the main character, Koromo and professional Mahjong player Fujita in a practice game.
The protagonists from the spin-off Saki Achiga-hen are worse at this. The writers wanted to cast them as the underdogs, but in the process forgot that they need at least some skill. We only see them blundering through games on-screen which often end with them scraping by through luck or with the deliberate aid from other players. Oftentimes, they just advance through the tournament off-screen with a message of: "By the way, they just won while you weren't looking."
Konata from Lucky Star is said to be a good martial artist, but aside from that Street Fighter parody, we have yet to see her fight for real.
Lorelei Wang from Love Is In The Bag never passes the opportunity to tell everyone that she is a genius, but even the rest of the cast is in doubt of that status.
Infinite Stratos manages to give society an Informed Attribute. Since only women can pilot the titlerobots, women are the dominant gender in the series. Except we never see anything of this (Save for maybe the IS' creator being a woman). It's so bad, this fact isn't mentioned on the Lecture As Exposition like everything else and is instead awkwardly shoved into just-barely-related dialogue once in the first episode, then never brought up again.
In the Excel♥Saga manga, Elgala and Hyatt claim to be a master swordsman and marksman respectively. Elgala is seen holding a sword on a cover, once brandishes a stick like an Iaijutsu Practitioner, and claims to have gone on a real-life adventure for treasure. Hyatt died trying to throw a rock the only time she's attempted "combat".
Yoshii from Holyland is said to know how to use a knife properly, but apart from slashing some random guy once, he gets promptly demolished by Masaki when he tries to do so.
Rando from "YuYu Hakusho" is said to have 99 Techniques, but we only see him use seven.
In Tokyo Mew Mew, Aoyama and Zakuro are both said to be excellent at speaking English, but they and the supposedly native speaker Mary Mc Guire do not speak it much better than Gratuitous English; there are relatively few grammar issues, but the delivery is very stilted.