It is a peculiar custom in brawls that almost no one ever hurts his fist in a fight no matter how hard he hits. In fact, it is almost universal. This is occasionally subverted for comic purposes or to show off how little the person punched was hurt, but in any other case, everyone has invulnerable knuckles.
It also doesn't matter if the hittee is bareheaded or is wearing a mask or helmet.
More likely to be averted by punching a wall than in most other situations.
Also a Required Secondary Power for characters with Super Strength who use it punch through things.
This is so universal that you're going to get only exceptions and subversions, because otherwise no one notices it.
Nothing to do with an echidna with a power-up.
Exceptions and Subversions
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Anime & Manga
One exception in Rave Master is the all-out slugfest between Haru and Lucia in vol. 21 when Haru succeeds in breaking Lucia's fist. Lucia just keeps hitting him with it anyway.
In Rurouni Kenshin this is very averted with Anji and Sanosuke, who use a punch technique called "mastery of two layers" and eventually "mastery of three layers". In the Kyoto Arc, Sanosuke's hand actually shatters after over-using it against Shishio, who had the foresight to wear a metal plate because he thought Sanosuke might try it.
But it is played straight with Shishio himself, who has a technique which involves soaking his gloves in a mixture of oil and gunpowder, grabbing an opponent, and setting the gloves on fire. By rights, that explosion should have done more damage to him than Kenshin.
Avoided in a non-comedic example, by all people, Knuckles the Echidna. During the third season of Sonic X, he smashes at an enemy robot's leg (said robot a prominent member of the Metarex, the Big Bads for that season), and the spurs on his knuckles snap off, causing him obvious pain. Although they do reappear the next episode no worse for wear...
In Naruto: Shippuden, Sakura puts on protective gloves before hitting anyone with her Super Strength.
When Sakura punches Sasori in the face after he admits that it's unfortunate that Chiyo didn't lose her life saving Sakura's, he tells her that he feels no pain and she'll only hurt her hand if she keeps punching him.
Rock Lee constantly wears bandages over his hands, and it's shown that this is largely due to them being almost perpetually cut up and bruised, thanks to this overly vigoroustraining methods.
Sasuke's hand bleeds after he tries and fails to punch through Gaara's Sand Shield.
Musubi does the same. It also helps that she's not human anyway.
Hajime No Ippo, being a series about boxing, enjoys subverting this trope. Ippo is informed (after he finds out for himself, unfortunately) that his hands are constantly in danger thanks to his enormous punching power. Coach Kamogawa's backstory reveals that his own Crowning Moment Of Awesome also came at the cost of heavily injured fists.
In Bitter Virgin Daisuke breaks the bones in his hands offering a Berserker Tears-fueled beatdown on a man who was trying to rape the lead female Hinako. He promptly switches to the closest stick he can find to continue the beatdown.
In the Ghost in the Shell series, a form of electric knuckleduster is used by Batou and the other Section 9 personnel. The Stun Knuckle is capable of sending a 400,000 volt ultra-high pressure shock into the target. Since they're usually fighting cyborgs, it's a necessity, otherwise you might end up breaking your entire arm, let alone skinning your knuckles.
There's also the fact that Batou and the Major both are made from titanium and carbon nanotubes. It's not like their arms can break just like that. Except when they do, though considering what she breaks them on...
In CLANNAD ~After Story~, Tomoya punches a wall hard in anger due to his father's latest actions, and he bruises his knuckles so badly that his hand needed to be bandaged up.
Two artificial aversions happen to Ranma himself: once, when he is weakened by a vengefulHappôsai to the point a toddler couldn't feel his punches, he keeps punching things in anger only to hurt his own fist. Many volumes later, third-string character Gosunkugi acquires a Powered Armor that very nearly leveled the field between him and Ranma; the armor is durable enough to bruise Ranma's fist when hit.
Also we get to see Ryouga hurting himself quite badly learning his special move where he breaks solid rock with his hands. It's very amusing, if not a real example of this trope: he's not punching the surface and hurting himself, he's poking at a boulder being swung at him and so he gets whapped in the face/upper torso with it if he fails to blow it up.
The aversion of this trope makes the audience wince in Saint Seiya's flashbacks. It's bad enough when a small boy punches a wall of solid rock and ends up with a bloody mess of a hand, but then he also tries an open-palm chop on a rock and it also sends blood flying everywhere.
During the Galaxian Wars tournament saga, when Bronze Cloths were still considered pretty spiffy, the Dragon Cloth's shield was spoken of as a legendary, invulnerable barrier which no attack could even scrape. Similarly, Shiryu's punch was said to be the strongest among all the Bronze Saints. When Seiya realized he couldn't defeat Shiryu straight on (the latter would easily block every single one of the former's Pegasus Meteors with his shield) he had the bright idea of ramming the shield with his head and trust Shiryu to try to crush his skull by trapping it between the shield and a punch. Seiya slipped out at the last second, Shiryu punched his own shield, and both shield and fist were broken.
In Holyland the main character dislocates a knuckle in one of his first big fights, freaks out about the pain and swelling, and another character pops it back in and tells him to see a doctor. Much later, the pattern of bruises on his knuckles lead another character to tell him he's slipping in his form and hitting with his smaller (ring/pinky) knuckles. He does take to wearing protective gear after some time, though.
In Kaiji, Kaiji's hand bleeds after he punches a mirror.
More often than not in Dragon Ball and Dragon Ball Z, the fighters' knuckles are scraped and bloody by the end of the fight. Of course, so is the rest of them. Nobody ever draws attention to it, though.
Between the Cell and Buu arcs, Gohan is pretending to be a superhero called the Great Saiyaman. he stops a robbery, taking their bullets and punches and (momentarily) lets the robbers get away. One of them says to the other "I think I broke my hand on that kid's face!"
Bean Bandit in Gunsmith Cats demonstrates this on an impostor at the climax of his signature story arc: he points out that in the most common fistfighting technique (boxing) the padded gloves protect the fighters' hands. He took a shot to the jaw earlier, and felt the bones in the guy's knuckles crack. So he purposefully punch parries the loser, who ends up with compound open fractures in his hand. Bean ends up looking absolutely badass - as he's been fistfighting with driver's gloves at best for his entire life, the bones in his hands are significantly stronger.
One minor villain in Hunter × Hunter had a steel plate in his fist that let him crush stone... severely bruising and nearly breaking his hand in the process.
Averted in Bleach with Yoruichi's fight against Yammy. While she stomps him into the ground, the fact that he has literal iron skin results in her hands being seriously injured. (This is the only injury she sustains, as Yammy is unable to lay a hand on her.) During her next appearance, she has armor protecting her arms and legs to allow her martial arts to be used effectively against such opponents.
In Fairy Tail, Natsu punches Gajeel in the face during their climactic battle. Gajeel, unfortunately, is using his Iron Dragon Scale technique, and his skin turns to iron. This leaves Natsu rolling around on the floor in agony, clutching his wounded hand.
"AAAAAAAAAAH It's okay! Doesn't hurt at aAAHAHALL!"
Later, Elfman transforms into a lizardman in his fight against Bacchus. The martial artist injures his hands striking the tough hide.
Actively sidestepped in One Piece, in that Sanji fights exclusively with kicks to protect his hands, which are important to him as a cook.
Oddly, this doesn't stop him from spinning on his bare hands on top of rocky or wooden floors.
Also during the Enies Lobby arc, after tanking a punch from Luffy, Rob Lucci tells him that anyone else would have broken their hand against his body hardening technique.
The Dressrosa arc introduces Bartolomeo, whose Devil Fruit allows him to generate forcefields. His favourite method of fighting appears to be letting his opponents break their own hands trying to punch him.
In A Certain Magical Index, Touma defeats the concrete golem Ellis once and for all by punching him so hard he shatters. Touma then shakes his hand trying to get rid of the pain. In the Light Novel, he says he almost broke his hand.
In a late episode of Cardcaptor Sakura, the group is attacked by flying penguin statues. Mei Ling manages to take a good number of them down with her martial arts skills, but during a lull in the fight she collapses in pain because she was just punching statues!
In Daily Life with Monster Girl, Kimihito defends his lamia girlfriend Miia's honor from two racist assholes by taking them out with a Megaton Punch. Miia and Ms. Smith are amazed by how powerful the punch is, then Kimihito collapses. He is then shown with a broken hand.
In Fist of the North Star, Kenshiro is never actually seen suffering any negative effects for punching or kicking things that the human body simply shouldn't be able handle striking. Granted, he is practically superhuman in his endurance and resilience. This is the man that once punched out a tank so hard that it exploded, after all.
Averted by the Rogue Titan in Attack on Titan, it has a tendency to hit other Titans so hard its arms start to rip apart.
Gotham Central seems to be a comic founded upon the principal of Reality Ensues, and that extends to a healthy aversion of invulnerable knuckles in fist fights. It might take place in the same fictional universe as Superman, but every main character is a perfectly normal human, and they take damage. When Renee Montoya begins to slowly and steadily descend into depression and alcoholism she starts getting into fights, and her hands are thrashed. From bareknuckle fights in back-alleys to literal bar-brawls she is bruised and beaten, and her mauled knuckles are one of the biggest signs to friends and family that something is not right in her life.
Averted in The Walking Dead, where Rick finds out who killed Hershel's little girls, and in his Unstoppable Rage he beats the killer so hard his own hand is broken. Despite medical treatments his hand never fully healed and was cut off soon after in the next story arc.
A Silver Age Batman and The Flash crossover has a common crook who is totally invulnerable thanks to a magical totem. He's eventually captured and put in jail, but uses his invulnerable fists to punch through the wall pebble by pebble.
Flash Thompson from the Spider-Man comics punches three bullies to help his new friend Felicia Hardy (not knowing that she is the Black Cat, and most likely not in danger at all). Before the beating, he says that, as a professional boxer, he is going to hurt his knuckles because he wears no protection, but that it would hurt them much more.
This trope has been subverted several times with Spider-Man, when the man does NOT hold back and holds his ground, a normal human being who's not superpowered or superskilled is not budging him. And can break his fists.
In both the comics and some of the video games Spidey has been known to wrap his fists in webbing to act as makeshift boxing gloves. By protecting his knuckles it allows him to punch harder.
Whoever is stupid/angry enough to try punching Superman should NOT expect this trope to be played straight.
Including Batman in A Death In The Family. Superman said he would've broken his hand if he hadn't rolled with the punch.
In the 1970's, when Doctor Strange was hanging out with the Defenders, he once faced a super-scientist with a Cool Chair that let him analyze and counter the mystic energies of Strange's spells. It couldn't, however, counter the physical energy in a punch to the jaw. Strange was rubbing his hand as he monologued to his unconscious foe.
In The Punisher MAX story arc, Long Cold Dark, Punisher wakes up in a hospital after blacking out after Barracuda annouced his attention to torture his illegitimate baby daughter in front of him. He was able to piece together what happened based on his injuries, which included broken knuckles.
Astérix, of all people, averts it at least once when he needs to punch out a guard without superstrength potion. Turns out a right hook to the chin from a Gaul warrior will do the trick anyway, but it hurts.
In Sin City, Dwight McCarthy hates punching people because he always skins his knuckles. When he ends up punching Shlubb, the inevitable happens. This is why he usually kicks or just shoots his enemies.
In KnightsEnd, the last part of the Knightfall storyline, Nightwing's Unstoppable Rage against AzBats leads to the knuckles of his gloves torn and the knuckles themselves bloodied from pounding on AzBats' armor.
Iron Fist has this as his primary super power; he channels his chi into his fist (or any limb) to make it invulnerable, allowing him to deliver extremely hard punches. Even without using said power, Iron Fist has conditioned his knuckles to be able to punch through stone, but just the fact the he has the power of the Iron Fist implies that he could hurt his hand if he struck something stronger, like adamantium or vibranium. With the power, he can punch Captain America's shield without being hurt.
Films — Live Action
Jackie Chan gets comedic fist injuries in many of his films.
He also gets non-comedic fist injuries while shooting many of his films (In Real Life).
Jackie himself used this as a metaphor to describe the difference between himself and Bruce Lee: "Bruce punches a man and knocks him out; I punch a man and hurt my hand."
Played with in Batman Forever, where the Riddler asks Two-Face to show him "how to punch a guy". After Two-Face demonstrates by punching someone out with no ill effect, the Riddler tries to copy him and ends up comically hurting his hand.
In In And Out, Kevin Kline's character is visibly in pain after punching Tom Selleck's.
The first Beethoven movie (the one with the St. Bernard, not the composer) has the father of the family punching out the corrupt vet, then afterwards groaning and shaking his hand — while his family admires him for manning up in the face of the family's dog threatened with being unjustifiably put to sleep.
When Vandamm wallops Leonard in North by Northwest, he has to reset his knuckles before continuing.
In The Thing (1982), when MacReady punches out Blair in the radio room, he leaves shaking his hand that he used to punch him.
In M*A*S*H (1970), when Trapper John punches Frank Burns (who totally had it coming), he seems almost as badly hurt as Burns.
Inverted in The Departed, where Billy Costigan breaks his hand punching a goon in the face, resulting in him wearing a cast on his arm.
A possible subversion occurs in Needful Things, when Norris Ridgewick clearly hurts his hand after punching Dan Keeton. However, two fingers on that hand had already been recently injured, so it's hard to say whether this was truly a trope subversion or not.
The Bride in Kill Bill plays this trope straight for most of the movie. However, at the start of her intense training montage, she bloodies her knuckles fairly severely against that damned wooden plank.
It's hinted that her knuckles are more than just bloodied. Given the way that she wakes up out of a dead sleep dreaming of that board and clenching her hands in pain, or how she is nearly unable to use chopsticks to eat, it's likely he was drilling her in a variation of the Iron Palm technique. Considering that some version of that involve breaking the bones in the knuckles and hands so that they regrow denser and stronger, the Bride probably sustained a lot of damage to her hands during those sessions.
When Mr. Pink punches Officer Nash in Reservoir Dogs he shakes his fist and even bends over in pain.
In Ip Man, after taking down the ten Japanese pugilists, Ip's knuckles are clearly bruised and bloody.
Comedic take: Hot Shots! features Topper Harley and his rival about to have a manly fistfight. First they put in boxing mouthpieces. Then they try trashtalking. Finally the rival throws a punch, which Topper blocks, and he responds. The rest of the fight scene consists of 'Ow my jaw!' and 'Ow my hand!'.
Lampshaded in Miss Pettigrew Lives for a Day: Pianist Michael punches slimy club-owner Nick in the jaw, then promptly exclaims: "I'm a pianist, dammit! This could ruin me!"
In Pretty Woman, when Richard Gere's character punches out a guy, he puts an ice pack on his hand afterwards.
In DOA: Dead or Alive, wimpy scientist Weatherby saves Helena's life by punching out a ninja who was about to stab her in the back. He comically hops around clutching his hand and asking her how "You people" do that without it hurting.
Very neatly justified in Oldboy, where main character Dae Su had been combat training for over a decade with a human outline he drew on the wall of the hotel room he was imprisoned in. For the rest of the film, his knuckles are shown to be largely callused, and he receives no hand injuries from punching people whenever he fights.
In Rocky Balboa, Rocky only stands a chance in the fight after his opponent breaks his hand during their bout.
Before that, in Rocky, Rocky shows Adrian a dislocated finger.
In Serenity, Simon is seen treating River's bloody hands after her asskicking scene in the bar.
Played straight in that most of the blood wipes clean off - it's not hers.
In Soldier, Sgt. Todd eventually bloodies his knuckles after repeatedly punching a suspended oil drum.
In The Karate Kid, Ali's arm is in a cast after she punches Johnny when he humiliates Daniel. Gotta give her points for not just slapping him.
The second film also gave us Mr. Miyagi using Deadly Dodging to injure the hands of the evil sensei from the first film.
An amusing non-fight scene variant occurs in Iron Man 2, when Tony is having his birthday party in his Powered Armor and is drunk as hell. He turns to his DJ after making a joke and gives him a quick bro-fist-bump, and the DJ winces and shakes his hand in pain after Tony turns around.
In Memento, the scene where Natalie gets Leonard to go after Dodd has a shot of Leonard feeling his hands. In the next color scene (movie time), he had punched Natalie after being provoked into doing so, but forgets this by the color scene described here.
In Gran Torino Walt is asked how he hurt his knuckles. He lies.
During the foot chase in Philadelphia in National Treasure, Ben briefly shakes his hand and says "ow" after decking Viktor.
Averted in Hunger in which an officer accidentally punches the wall and his knuckles are shown as bloodied and raw.
In Limitless, Eddie beats up a bunch of street thugs who were trying to mug him, but skins his knuckles.
In Crazy Stupid Love, Bernie punches Jacob in the face and clutches his hand screaming in pain. A few minutes later, Jacob punches David in the face with no ill effects.
In Back to the Future Part III, Marty wears a metal plate under his poncho to protect himself from Bueford's pistol. It turns out to be doubly useful when Bueford punches him in the stomach only to recoil in pain.
In Die Hard, Argyle finally gets in on the action and punches out the getaway driver...then shakes his hand, obviously in pain.
In Kick-Ass 2, when Colonel Stars and Stripes tries to punch Mother Russia in the face, she's unaffected and he breaks his hand.
Casino Royale breaks with James Bond tradition by having Daniel Craig actually look like someone who's been through a No-Holds-Barred Beatdown after a brawl with a couple of heavies working for the African warlord employing Le Chiffre in the high-stakes poker game/money laundering operation that drives the plot.
In the Wilbur Smith novel Power of the Sword, the Villain Protagonist is a boxer. At the 1936 Olympic Games he punches his opponent in the jaw so hard that he breaks several bones in his hand and is unable to continue fighting effectively. Despite this the judges award him the victory — because his opponent is African-American, he's white, and the judges are (literal) Nazis. It's worth noting that the punch which broke his hand didn't break his opponent's jaw, or even knock him down.
In the Inheritance Cycle, Brisingr Eragon breaks his hand after punching through a soldier's suit of armor. After healing his hand, he creates thick calluses on all of his knuckles to cushion his Super Strength-augmented blows.
In the Discworld novel Night Watch, commander Vimes notes that even punching someone out with brass knuckles hurts quite a bit.
Lampshaded / discussed in one of the novels in the X-Wing Series: Wes Janson is an experienced street brawler, and knows that punching his opponent in the face is more likely to break his fingers than do damage. When delivering a lesson to an arrogant aristocrat he therefore settles for breaking the guy's nose with a palm strike before getting to work on his abdomen.
In I, Jedi, Corran Horn beats up Remart in a bloody, messy, painful way that he later realizes was totally unnecessary, since he could have just knocked the guy out in a few blows. During this he broke several fingers with a punch to the face, although he knew what would happen when he did that. While being bandaged he speculates that he was punishing himself for letting go of his control.
In The Zombie Combat Manual, the martial artist interviewed in the section about unarmed combat relates a story about how early in the Zombie Apocalypse, a friend of his who was a boxer fought a zombie unarmed. At one point during the brawl the interviewee got knocked out. When he came to his friend had beaten the zombie to death, but had also completely destroyed his hands doing so, breaking every bone from the wrist forward. What's more, due to getting all that zombie blood in his open wounds, the friend became infected and needed to be put down.
In A Drop of the Hard Stuff by Lawrence Block, Scudder figures out a person on his witness list had beaten up the now-dead victim because of the damage to his hands.
Live Action TV
A staple of MacGyver was for MacGyver to shake his fist in pain after punching someone in the face.
Burn Notice addressed this in the pilot episode. Michael points out that you need to be careful not to break your own hand in a fight, so he starts a fight in a bathroom because it has plenty of hard surfaces that he can slam his opponents into without hitting them.
At the beginning of one Firefly episode ("The Train Job"), Mal punches a man out. Later, Simon offers to stitch up his knuckles.
Mal: I know they tell you "Never hit a man with a closed fist", but it is, on occasion, hilarious.
Nathan Fillion gets his hand treated yet again after Castle punches out a perp. Given the number of Firefly references on Castle, it was kinda surprising that he didn't repeat Mal's quote.
In the Farscape episode "Taking The Stone" featured Crichton knocking the episode's perpetually-stoned villain out with a single punch to the skull; true to reality, he shakes his fist in pain afterward.
And in "Die Me Dichotomy" Crichton, suffering a Split Personality Takeover and hallucinating, smashes a constantly reforming mirror with his bare fists at least six times, totally unaware that he actually broke it on the first try, and on the other strikes he was punching the metal wall behind it; when Aeryn finally pulls him away, his knuckles are bleeding.
In an another episode, Aeryn Sun punches a padded target dummy until her knuckles bleed.
In Veronica Mars Logan is seen after at least one fight with bleeding knuckles. One person he beat up mentioned that he had bruised Logan's knuckles pretty badly with his face.
In the BBC police series Luther, the protagonist gets into a fist fight with a murderer. In the next scene, he's wearing a cast on his hand.
Thrown blows in Three's Company invariably would hurt the puncher's hands more than his (or her) intended target.
A staple in Star Trek: TOS, particularly with Captain Kirk. Averted on one occasion, however, when Spock asks him if he doesn't find it painful and Kirk replies in the affirmative.
In the Buffy the Vampire Slayer episode "Doppelgängland", Willow decks Anya with a perfect right hook... then grimaces, shakes her fist and yells:
Willow: Ow, ow, happy, but OWWWW!
In another episode, when several frat boys have kidnapped Buffy and Cordelia. The Scoobies break down the door and Xander, leading them in, decks the first guy he sees. For the rest of the fight sequence, he pops in and out of frame, swinging around in wild agony and clutching his hand.
In the spin-off Angel the titular character throws a punch at Hamilton, only to pull it back, hissing in pain. His opponent doesn't move an inch.
In Community Annie finds her flying right cross to Jeff's face extremely painful. So does Jeff.
One episode of Matlock shows the innocent man on trial punching his framer in the face. He breaks his hand from hitting the other man's jaw.
A plot point in The Sarah Connor Chronicles, where Riley was apparently killed by Cameron. John looks at her corpse, and notices a number of self-defense wounds, including bloody knuckles. He then confirms with his uncle Derek, who's been fighting machines most of his life, that if he'd been fighting unarmed against a Terminator like Cameron, he would be killed instantly, and wouldn't get a chance to receive self-defense wounds.
Averted in Breaking Bad; when Hank beats up Jesse his hand is noticeably bruised and cut, which is used as evidence.
Averted in Kamen Rider Double's first movie, where protagonist Shotaro busts his knuckles open punching the Dummy Dopant-as-Kamen Rider Skull while untransformed.
Actively averted in one episode of Stargate SG-1, where Teal'c finds himself in a fist fight with a suburban thug, and ducks his head so the blow intended for his face actually hits the top of his head, causing said thug to wreck (and possibly break) his hand.
Not only did the man hit Teal'c's head, he hit the emblem on his forehead. The emblem made of solid metal. Ouchie...
In The Twilight Zone episode "The Big Tall Wish", a boxer loses his temper and punches a wall, breaking his hand.
On Scrubs, when Dr. Kelso was being an abnormally big asshole, Dr. Cox punched him in the face (out of his shoes!). He does the pained-hand shake immediately thereafter, possibly because Dr. Kelso is established elsewhere as having a head "like a mountain goat."
On Heroes, Peter winces and shakes his fist in pain after decking Sylar in the mouth for making one too many smartass comments about Peter's friend Emma. What makes this scene is that the two of them are trapped inside Sylar's head and thus the pain should be imaginary.
But pain is imaginary.
In the Smallville episode "Combat", Clark and Lois are pitted against one another in a metahuman fight club while undercover. Lois punches him in the gut, but since Clark is Nigh Invulnerable, she damn near breaks her hand.
Lois: That's not just a sixpack under your shirt, that's a steel kegger!
Mentioned and subverted in Graceland. Lauren is undercover as a Russian mobster's girlfriend when her mission implodes. Desperate to get the guy, she goes back to him again, and later shows up at home with a massive black eye to invoke her boss Briggs's Big Brother Instinct. However, after they arrest the Russian, the see that his hands were perfectly fine. Briggs later gets into a deliberate bar brawl to punch out a guy and show her what a hand that's punched someone looks like and calls Lauren out on her Wounded Gazelle Gambit.
The O.C. usually played this straight with Ryan's punches but gave an aversion when Sandy laid one on Ryan's deadbeat dad and is shown icing his hand afterwards.
Oz had a prisoner punch a brick wall in rage after hearing his cellmate make one too many racist comments and ends up with a broken hand. Not that he lets it stop him slowly chipping and attacking him later.
In Parks and Recreation, Ben punches out a guy who calls his girlfriend a bitch. Afterwards, she shakes his hand for a photo op and he doubles over in pain because his hand hurts so much.
When recounting the one fight he got into in his life in How I Met Your Mother, the first thing Ted mentions about a fight that no one tells you is that it hurts your hand. A lot.
Averted in 1st Edition Advanced Dungeons & Dragons by the "Stoneskin" spell; attacking a character protected by this spell with bare fists inflicts 1-2 Hit Points damage — to the attacker.
Also, the "crushing blow" technique from the Oriental Adventure allows a martial artist to break wood, ceramics or masonry, but on a failed roll the character severely injures his hand.
In Rifts, Titan Juicers are a variant of the standard drug-enhanced shock troops, optimized for brute strength. Their skeletons have nano-surgical reinforcements, but they still need special armored sap gloves to protect their hands if they want to punch out tanks without injury.
In GURPS, punching armor barehanded is a good way to injure and possibly cripple a hand. The Martial Artssourcebook has additional, optional rules under the sidebar "Harsh Realism for Unarmed Fighters," which is Exactly What It Says on the Tin. Of course, there are also ways around this, ranging from gloves or brass knuckles to special training or explicit superpowers, which may or may not be viable depending on the campaign.
Even in systems without explicit rules, a botch on a bare-fisted strike to something hard is asking to injure your hands.
Real Life background info, via QI: according to historical records, fatalities in boxing have gone up considerably since the invention of the padded boxing glove. In the bare-knuckle days, the pain experienced by the boxer's hands acted as a check on life-threatening injuries to his opponent, with boxers aiming for the well-padded chest and body rather than the head and jaw. With gloves, the boxers are able to strike with more force and not hurt their hands, and are able to fight much longer. This makes it easier for a boxer to cause permanent injury or death via blows to the head.
It's generally accepted in boxing that anyone who hits with power will suffer a hand injury in nearly every bout. The human body was simply not built to sustain the sorts of impact and damage that professional boxing places on it.
Subverted in Donkey Kong Country Returns. DK can punch goal barrels and Tikis all day without pain, but when he punches the moon out of orbit, he hurts his fist.
In his cameo appearance in Soul Calibur II, Heihachi can chose to fight bare fisted as his weapon. This results in every punch draining a small amount of health. Of course, defending still takes no damage.
This is actually kind of an odd example, since Heihachi is plotwise Made of Iron to the point of being nearly invulnerable, and has punched robots to pieces without flinching.
Bloodline Champions has the Glutton bloodline, whose main source of damage is punching people. Still, their right hand does have wooden spikes attached to it by default, with even more effective looking alternate weapons to be used on that hand, and the ritual they went through and that gave them earth powers might explain why their left hand works perfectly all right for it.
In Arcanum: Of Steamworks & Magick Obscura, knuckles are invulnerable against casual enemies, but punching golems or trying to break open metal containers with your bare hands will take a chunk out of your lifebar.
Punch a droid in the original Dark Forces, though, and you can punch it to death. Even the game's iconic Mecha-Mooks can be punched to death.
This applies to Vanessa from The King of Fighters series, but only in a few situations: after smashing an opponent with some of her super combos, she immediately shakes out her sore hands before returning to her standard position. She also verbally mentions it in a few of her victory poses and winning quotes. She is one of the game's few boxers and hits very hard, so this surprisingly isn't a case of Faux Action Girl.
Chris Redfield in Resident Evil 5 shakes his hand a bit after performing a straight punch melee attack.
On the other hand, he later pushes a large boulder and seems to get frustrated by it not moving, resorting to punching it. Likely Narm to many people.
Practically every single shooter with a melee function that doesn't use the rifle butt or a knife.
Halo, on the other hand, justifies this trope with Master Chief being in Powered Armor (he punches as a melee attack while holding the plasma pistol, but otherwise usually strikes with the weapon). In later games, the Chief is capable of latching aboard Covenant Wraiths and quite literally punching through their armor.
The Street Fighter games sometimes justify this trope. Most of the characters have some form of fist protection like knuckle gloves or boxing gloves. The trope is, however, played straight by a few characters, most notably by Final Fight's Cody Travers in Super Street Fighter IV, who very casually breaks out of prison by punching a hole in the wall just to go out and relieve his boredom, wearing only hand wraps.
Though it's not an aversion, it is still noticeable in the case of Pathologic, since the game has an otherwise strict adherence to realism (many of the Acceptable Breaks from Reality are averted). It's also possibly justified, since Dankovskiy and Artemiy wear gloves. Klara doesn't, but she has magic healing powers, so maybe she uses those to protect her fists.
Justified in Shatterhand, where Steve has indestructible bionic knuckles. He can even punch bullets out of the air.
And averted at the same time. The box art shows that his repeated punching has torn away the pseudo-flesh on his knuckles.
As quoted above, Team Fortress 2's Heavy Weapons Guy. Since melee weapons are for tiny baby men, Heavy uses his gigantic fists. He can punch metal walls and sparks will fly.
He can punch metal buildings so hard that they catch fire and then explode. An unlockable set of spiked brass knuckles is actually weaker than his bare meaty mitts. However, said brass knuckles seem to unlock something in the Heavy's mind, allowing him to punch even faster, even though those punches are weaker.
Another of his unlocks takes this up a notch with literal Fists of Steel. These giant metal gloves give Heavy a damage reduction from bullets so he can wade through firepower and punch someone to death. Ironically enough, they make him more vulnerable to melee attacks when wearing them.
Subverted in the case of Ieyasu from Sengoku Basara, who fights with his fists so that he also feels pain when punching enemies.
Ethan of Ctrl+Alt+Del once punched a guy so hard that it knocked a tooth out. It also broke Ethan's hand.
While mostly played straight in How I Killed Your Master there are occasional aversions when someone is not so much fighting as pounding the crap out of a defenseless foe.
This trope is mentioned, and justified, in Schlock Mercenary: The mercenaries all have soldier boosts that reinforce the bones in their knuckles, hands and upper arms and allow them to throw punches that don't hurt their hands. In most situations they also wear Power Armour over said reinforced bone tissue, which further shields their hands.
And in the opening to the GI Joe movie of 1987, Alpine punches through the cockpit of a Cobra Firebat and forces its pilot to eject.
In the battle sequence at the beginning of the Star Wars: The Clone Wars, a Clone Trooper tries to punch out a Super Battle Droid, and injures his hand. Then gets shot by the droid. On the other hand, another (more elite?) Clone tries the same thing later, with more success. Perhaps he had better gloves?
In Storm Hawks, Junko usually uses crystal-powered knuckled gloves. When forced to go without them, he initially thinks he's useless and there's an entire The Magic Was Inside You All Along plot where he realizes he doesn't need them to punch things hard... but that he really still prefers using the gloves since punching things can hurt.
In the Justice League episode For the Man Who Has Everything, Wonder Woman attempts to beat the crap out of Mongul, only for her hands to be injured, to her visible surprise.
Averted during an episode of King of the Hill, when Bobby socks Jospeh for kissing Connie.
Bobby: If my hand didn't hurt so much, I'd do it again. Eh, what the hell?
Punches him again
In an episode of The Mighty Ducks, the teams resident big guy Grin punches a giant robot, knocking it across the room.
Grin: Pain is an illusion...
shakes his hand and winces
Grin: An illusion that really, really hurts!
Not Truth in Television, naturally. Even if you don't injure your knuckles in some way - there's a reason Doctors call it a Boxer's Fracture - repeated punches will quickly cause the skin on them to split.
Before the invention of boxing gloves, fighters mostly targeted the chest, as a solid punch to the jaw is going to break even toughest man's hands.
Fist loads, taping the hands, brass knuckles, cesti, et al protect the hand of the puncher. Any additional damage, or protection, are just bonus. And if you see any old wood cuts or photos of bare-knuckle boxers, look at how they hold their hands. With bare knuckles, you strike differently to reduce the chances of damaging your hand.
Some martial arts try to make this closer to reality with training techniques that can increase the density of certain bones and build calluses that act as built-in-brass knuckles. Natural knuckle bones are porous and light, for manual dexterity. If they are damaged repeatedly, those pores scar up, resulting in bone that is stronger than concrete. Downside? Don't ever sign your name with that hand, unless you want to sign like a two-year-old.
This is also one reason some self-defense methods teach that you should strike primarily with the palm and elbows: in a fight, someone inexperienced is quite likely to injure their hand with the first good punch thrown, impeding their ability to grab or grapple an opponent. The palm can still cause very severe concussion injuries, and is easier to teach than a proper punch.
On the other hand if you must hit someone with a closed fist (especially in the face) be sure to try and strike primarily with the knuckles of of the first and middle fingers, as they are the biggest, densest, and strongest ones you have and have the best chance of protecting yourself from serious injury. It will still hurt though.
And keep your thumb outside!
And make sure your fist is at such an angle that the two primary knuckles are lined up with the bones in your arm-that way the force will be absorbed better-and make absolutely sure that in the fist, the second set of knuckles is parallel to the ground, or there'll be shattered fingers.
For those who don't get why you should keep your thumb out, this is because hitting a punch with your thumb inside can cause you to break your thumb, as you'll end up hitting with it in a position which is simply not made for hitting.
This is why martial arts tend to train you not to punch the head. The head is the most protected part of the human body. The knuckles contain the smallest bones outside of your ears. Do the math.
Martial arts that do teach you moves that involve striking the head often involve striking the face rather than the back of the head because there is less natural protection. In addition, even though it hurts to hit someone with a closed fist, it must also be remembered that those two knuckles also hurt your opponent.