Mundangerous

He's The Hero. Being Punched Across the Room doesn't seem to hurt him in the slightest. Throw her through a wall and she'll jump back through that same hole and nail you in the gut. He's the Determinator, standing up while still on fire from that last bomb to the face. She has keen senses and lightning reflexes - just try landing a hit on her.

But then they get a soccer ball kicked into the back of the head... and it knocks them out.

The bane of the Part-Time Hero has nothing to do with the Big Bad and his Frickin' Laser Beams with dials that go Up to Eleven. It's the mundane hazards - the stray dodgeball, the unseen Banana Peel, and the abruptly-opening door - that bypass his defenses. He was able to handle Training From Hell, but God forbid he be forced to do push-ups in gym. All stamina, reflexes, and willpower suddenly disappear when the Part-Time Hero (and especially the Idiot Hero) is off-duty.

This is usually done to emphasize the fact that the characters are still human, make human mistakes, and have human problems without the need to make them too incompetent on the job. Slapstick varieties are commonly played for comic effect and ignore the characters' defenses because the scene wouldn't be as funny. Occasionally, though, injuries and sickness acquired in mundane situations are used to justify poor performance when the hero is abruptly put back on duty, or sometimes outright used to handicap the hero for a more challenging (or sometimes, just more humorous) fight. While handicapped fight scenes can be more creative and entertaining, one wonders why the Determinator was able to resist the Mad Scientist's mind control virus through force of sheer will but can't do the same for the common cold.

This trope may have some other legitimate reasoning behind, however. Whether the character's powers require conscious effort or not, the human body is designed to only work at peak capacity when necessary. It's no stretch to assume that powers work along the lines of the adrenaline rush and fight-or-flight response, and that having them working at full capacity all the time would burn too much of the body's resources.

See Armor-Piercing Slap and Megaton Punch for specific mundangerous techniques. Contrast Mundane Utility, where the hero actively uses his powers to accomplish mundane ends. Compare Wake Up, Go to School, Save the World, the social and mental counterpart to this trope.

Anime and Manga
"Arf: ... What kind of strength does that kid have...?
 * Two examples in Dragonball Z:
 * For drama with the heart virus that kills Goku in Future Trunks' timeline, leaving them without him to help fight against the androids later
 * Humorously when Krillin sees Goku napping in super sayian form (before Cell's Arena) throws a rock at him thinking he will sense it and stop it. Goku gets a rock in the face and it hurts.
 * Rurouni Kenshin—Kenshin might be Made of Iron, but in at least one Mood Whiplash moment blood is shown gushing out of his wounds after a relieved Kaoru hugs him a bit too hard.
 * Kenshin exhibited this trope on purpose in an early episode. While eating at a restaurant, a drunken customer threw a ceramic bowl, hitting Kenshin in the back of the head. Sanosuke was watching and later lampshaded it: He knew Kenshin could have easily dodged that bowl... but if he had, it would have hit Kaoru in the face and injured her much more.
 * The anime also has a circus themed episode, including Hiten Mitsurugi Ryu Lightning Umbrella
 * Played for laughs in the supplementary manga of Magical Girl Lyrical Nanoha A's when Fate, mighty Super Soldier who we've seen get smashed through buildings, is knocked out cold in a dodgeball match by the Muggle Suzuka, the Shrinking Violet member of Those Two Guys. Promptly given a Lampshade Hanging by the spectators.

Amy: I wouldn't expect any less from one of Nanoha-chan's friends."

"Daisuke: Hey, Saotome... You said you went training in China, right?
 * Evangeline of Mahou Sensei Negima is an incredibly Badass vampire who is generally considered one of the strongest beings alive. After getting stuck with a Power Limiter she's been known to trip over her own feet, giving herself a bloody nose. She blames it her lack of powers.
 * There's also Sayo, who's a ghost and naturally cannot be injured by the vast majority of physical objects. She still manages to trip over her own feet, despite the fact that she doesn't have feet.
 * In Full Metal Panic!, Sousuke's Kryptonite seems to be whatever Kaname has on hand at the time. He'll often be laid out by Kaname's halisen and he once got knocked out by a baseball base.
 * In New Getter Robo Dr. Saotome (an old man who has no notable martial arts prowess to speak of) manages to blindside and smack Ryoma "Batshit" Nagare with a pair of geta. To put this in perspective, this is just after the aforementioned Ryoma finished off three Yakuza assassins at once, restisted an anime tranquilizer strong enough to kill most men, and effortlessly dropkicked Saotome's two bodyguards.
 * A variation in One Piece: Zoro is very agile and perceptive during battle. Outside of battle, he'll get lost running in a straight line.
 * Zoro survives over the top injuries throughout the series and is the poster-child for Overdrawn At the Blood Bank, but his great rival, whom he was never able to defeat, dies from falling down the stairs.
 * Watatsuki no Yorihime of the Touhou series. Infamously powerful God Mode Sue who easily defeated some of the best fighters of Gensokyo. Knocked out cold in the Inaba of the Moon and Inaba of the Earth spinoff when she accidentally fell into one of Tewi's Pit Trap pranks (though said series canonicity is debateable).
 * In an episode of Slayers, Lina manages to bowl Zel over with a particularly energetic pat on the back, when it would normally take something along the lines of a dragon with a sledgehammer to achieve the same effect (there's an entry for Zel under Made of Iron for a reason, after all...
 * In both Ranma ½ and Inuyasha, the main characters are somewhere on the border between Made of Iron and Nigh Invulnerable. Neither series hesitates in the slightest to have comparatively mundane "attacks" (a stray soccerball to the back of the head, a punch from a ticked off ordinary schoolgirl, etc) seriously hurt them for comedy's sake.

Daisuke: Couldn't you have dodged that?
 * cut to Ranma with a softball embedded in his cheek*

Ranma: I had something on my mind."

Comic Books

 * Spider-Man was hindered in an encounter against the Black Widow by what, in nearly anyone else, was a case of pneumonia. He still broke out of her line. One good night's sleep, and he was good to go.
 * In one of his old cartoon series, Spider-Man caught a cold while the Lizard was on a rampage. It kept screwing him up, so he figured he'd have to go to the doctor's office... as Spider-man, since a blood test from Peter Parker would come back with spider DNA.
 * ... Because doctors always sequence your genome when you have a cold.
 * The original (Marvel) Captain Marvel, who survived a hundred brutal battles, died of cancer suffered from accidental exposure to a chemical weapon caused by battling the villain Nitro.
 * In the end of one early Fantastic Four story, The Thing kicks a rock (or something, don't remember the details) as a result of his usual bad tember, and breaks a toe from his own super-strength. He confines himself to bed - it's not that a bad injury, but he is horribly embarassed that him being childish hurt himself worse than many of his enemies, and vows to stay out of sight until he is ok again.
 * One Batman story from the Seventies had Bruce Wayne desperately trying to fight crime while dealing with a high grade fever. Alfred practically had to restrain him in bed to keep him from going out, and when he did, it was clear it was heavily impeding his skills.
 * Doctor Strange, the Sorcerer Supreme, is physically no more than a mature man in good shape. He is usually prepared for most attacks, but anything that can take down an ordinary person can also disable him, particularly if he doesn't see it coming. He also requires the free use of his hands and voice in order to cast spells.

Literature

 * In the Kate Daniels novel Magic Bites by Ilona Andrews, Kate and her allies are gearing up for a potential battle. The Beast Lord explains that the werewolf Alpha is absent because he broke a bone due to "LEGOS". At first Kate is confused, trying to remember a Greek monster or spell that might have disabled a werewolf, before the Beast Lord clarifies that the wolf tripped over his kid's toys and fell down the stairs.
 * Hammered home again and again in Mercedes Lackey books, who likes to remind her readers that all mages are basically squishy mortal people who can be killed just like anyone else.
 * Harry Dresden may be one of the most powerful wizards on the planet in terms of sheer evocation, but he still carries a gun. In the short story Day Off, he's confronted by a group of very-low-grade magical talents who want to throw down, and ready their wands and other magical paraphernalia to do so. Harry calmly draws his gun, which scares the crap out of them.
 * In Changes, Two BIG spoilers.

Live Action TV

 * Lampshaded by Stargate SG-1, in Nemesis. Daniel can't participate in a mission because of an appendectomy, and says, "You know it's funny. I mean after everything we've been through these past few years. And, of all things, it's my appendix that lays me out."
 * Also an example of Written in Infirmity. Michael Shanks was sidelined from filming by an appendectomy.

Real Life

 * Gen. George S. Patton, possibly the most brilliant military strategist and tactician the Allies had in WWII, Blood Knight and all-around Four-Star Badass, died after he hit his head in a minor car accident 8 months after Germany's surrender.

Tabletop Games

 * Any character with the "Combat Luck" ability from Hero System (5th and 6th editions) is prone to this. Combat Luck gives you increased defenses ... but only if you're aware of the attack and thus can try to avoid it. Blow your Perception Roll, and that club/knife/whatever is gonna hurt.

Video Games

 * A milder version can be see with Phoenix Wright, who can be knocked unconscious by a whipping from Franziska or even a hard series of slaps from seven-year-old Pearl. However, he survives  with no injuries, and only spends a few days in the hospital because of a mild case of pneumonia. He's back at court the day after he gets out.
 * Although this is due more to eagerness, the next day he's still ill but doesn't want to lie around anymore.
 * In Apollo Justice, Phoenix is hit by a car and sent flying across a city block. The only damage is to the car.
 * In Super Mario RPG, given what Mario goes through in almost every game in the entire franchise, it's pretty ridiculous to see him knocked out in one shot from the Geno doll. Some players just attribute it to being Geno, but still.

Western Animation
"Uncle: Run down side of exploding building, nooo problem. Step on child's toy, break your bones. Bwahahahaha."
 * Lampshaded by Uncle in Jackie Chan Adventures; Jackie was able to run down the side of a building while it was being demolished, but broke his leg in the next scene upon tripping over an action figure, and Uncle chastises him for it.


 * Jade also manages to do this, when she tries to demonstrate that Jackie really is an nigh superhuman fighter at Bring Your Parent to School Day. She chucks an apple at his arm in an attempt to evoke a reflex, only for the apple to smack him in the arm. Jackie jokingly teases her for having an overactive imagination and a good throwing arm.
 * Played for laughs in an episode Kim Possible, in which most of the main characters who appear in the episode are incapacitated by the common cold at some point.
 * An almost daily occurence for the Nigh Invulnerable The Tick (animation).
 * In the episode Sports a Poppin of Dexter's Laboratory that has Dexter be completely incompetent in sports, and despite his best efforts, lets his father down who was trying to teach him to be good at sports. Then at the end of the episode Dexter's dad give up and goes inside. Immediately after this a monster let loose by Dexter's sister Dee-Dee comes out. Dexter proceeds to fight it, using skills that obviously should have made him be more capable at the sports than he was.