Ranma ½ this is a common trick (mostly by the Tendo sisters) to subdue Ranma in female form, mostly because Ranma - a guy who's cursed to transform into a woman - is still unused to the enhanced sensitivity and because it's funny.
In Naruto Kabuto didn't mean to attack Tsunade's breasts. He was just trying to stab her in the heart, but her brobdingnagian boobage got in the way.
Doubly played for laughs, as Kabuto can't determine if his attack on her internal organs was ineffective because of her advanced ninja training or because her breasts are just too damn big to get through.
Hinata was hit with one as well.
In Change123, Ginga "boob bite" and "boob slap" Miriki.
Later, Rozai threatens to rip Okoi's breasts if she refuse to speak.
Comic Books
In the Garth EnnisSuper Hero satire The Pro, the main character is a bitter, cynical hooker with a baby who is given powers and inducted into a JLA-like organization to fight crime. In a case at the UN, she is shot by a supervillian with a laser in the gut. Bad idea. With an outraged cry "Cunt RAG!", she gives the villianess a titty-twisting that made EVERYBODY wince.
This is after the cow attacks him by milking itself and using the jets of milk as a distraction.
Played for laughs in Jackie Chan's Armour Of God when a large woman is double-punched in the chest and her breasts bounce with a sound effect better suited to something much heavier. She is rendered incapable of fighting until she stops them bouncing.
In Roger Avary's Killing Zoe the titular Zoe slaps the (male) main character's nipple for calling her a prostitute. In fairness he had just paid her for sex.
In Sudden Impact Jennifer Spencer is on a Roaring Rampage of Revenge on a group of people who had raped her and her sister years before. Before she kills them she shoots them in the groin. One woman was involved - she shoots her in the breast.
In Mallrats, Gwen sneaks up on her old boyfriend TS, who jumps and "elbows [her] right in the freakin' tit!" She responds by giving TS an upward double axe handle to the groin. (Brodie uses the opportunity to explain his opinion of karmic justice to TS.)
In an unfilmed scene from an early draft of Ferris Bueller's Day Off, Jeanie Bueller is talking with one of her friends; her friend pisses her off, and Jeannie responds, "I'll sock your tits."
In the film Kingpin, the male and female leads get into a fist fight. The female delivers a few shots between the legs so the male delivers several Breast Attacks.
At one point in Scott Pilgrim vs. the World, Scott is attacked by a cloaked figure. When he strikes back, the figure yells: "You punched me in the boob! Prepare to die, obviously!"
Sawney Bean is shown drinking blood out of a severed breast in Hillside Cannibals.
In the original Scary Movie, Ghostface stabs Drew Decker in the chest only to pull the knife out with the breast implant impaled on it.
In Ichi The Killer, Saburotortures Myu-Myu by stretching her nipples with crocodile clips and then severs them both with a single pull of the scalpel.
Literature
In On A Pale Horse, the hero's girlfriend is tortured by being stripped to her waist, and her nipples being touched by active electrodes.
A particular magic-user in The Sword Of Truth series controls people (women and men alike) by cutting off a nipple to use in Sympathetic Magic.
In Book 5 of The Faerie Queene, during Britomart's Combat by Champion with Radigund, the narrator notes that the two women did not "spare each other's most tender parts," or something like that, despite both being women and therefore knowing how much it would hurt. I guess we have here the logic behind Cat Fight and Designated Girl Fight.
In the fourth book of A Song of Ice and Fire, The Blue Bard is being tortured by the Mad Doctor Qyburn on Cersei's orders under the Red Keep. During said torture scene, he remarks about men's nipples sometimes being as sensitive as women's nipples, then removes the Bard's nipples.
In an early chapter of Friday, the heroine is tortured by having one of her nipples cut off. This being science fiction, her doctor is able to regrow the body part, but it is good for a guilt trip later in the novel when she meets one of her captors.
In The Iliad, after Athena K.O.'s Ares with a boulder, Aphrodite goes to help him. Well, Athena is still angry: "She charged at Aphrodite, overtook her and beat her breasts with clenched fists." As one would expect from the wimpy Aphrodite, "Down she sank with Ares, resistance quite dissolved."
From the Ian Fleming novel Dr No, describing the murder of a female MI 6 agent: "The man smiled broadly. Slowly, lovingly, he lifted the gun and shot her three times in and around the left breast."
Live Action TV
During a Thanksgiving episode of Friends the gang is having a game of American football for the Gellar Cup. Monica throws the ball to the inexperienced Phoebe who catches it, but it strikes her in the chest hard and she squeals: "Oh! Broken boobs!"
In one episode of Dengeki Sentai Changeman, the Monster of the Week employs fishes with huge mouths to attack/bite the team. While each team members got bitten, poor Sayaka/Change Mermaid got bitten on the breast area, with her knocking them down while yelling "PERVERT!". Fun fact is that since she's attacked while morphed, the Changeman female suits offer no visible breasts.
The Shake It Up episode "Party It Up" had a scene where it did happen, when CeCe and Rocky body bumped copying the boys... but in turn, both girls groan from the pain.
Wayne Brady apparently thinks that purple nurples are part and parcel of a female prison riot (the "persona" he got saddled with in Lets make a Date).
Charlie Runkle's unfortunate nipple mishap in Californication constitutes a rare male example of this trope.
An episode of Yes Dear has Kim accidentally doing this to the then-wife of her husband's boss while playing tennis. Good thing her husband didn't get punished for this.
Another one involving Christine boxing Jimmy's boss. (A female security guard)
Christine: Ow...she punched me in the boob. Can you do that?
Jimmy's Boss: *annoyed* Yeah.
Christine: Oh. (punches Jimmy's boss in the boob)
Music
Pearl Jam, "Jeremy": "He gnashed his teeth and bit the recess lady's breast"
Professional Wrestling
Several female wrestlers have developed a move where they hoist their opponent up in a chicken wing hold, pulling their arms behind them, and then drop them. At least one has called this move "The Implant Buster."
Webcomics
Misho inflicts one upon Nova in thisKeychain of Creation strip, causing the female onlookers to wince in sympathy.
Some of the anthro insects in Dreamwalk Journal get stung right in their Non-Mammal Mammaries (amongst other places), but it's a subversion because (being an erotic comic) the stings inflict pleasure rather than pain.
The webcomic Exiern has a classic one: The main character Tiffany is a male barbarian hero who has been trapped in female form by an evil wizard. She wants to be a man again but since she's a barbarian she absolutelyhates the local priesthood. So her sidekick Peonie ties her to the altar—for her own good, of course—and orders the priests to attempt the ritual. After the inevitable failure Peonie admits in trepidation that she'd she'd been counting on Wouldn't Hit a Girl to stave off retribution. Cut to the next page, where Peonie can't get over her shock that Tiffany "punched me in the boob"!
XI elbows Teri in the breasts to get out of a hold in Rip And Teri.
Rare male example: In the penultimate episode of The Venture Bros. season 3, Brock Samson's nipple gets cut off.
In one of the many Robot Chicken sketches, a PMS-ing She-Ra attacks a trio of villainesses. Two get standard deaths. The third has her breasts squeezed till they popped.
American Dad: A stripper accidentally gets her breasts pierced by hypodermic needles, causing her implants to deflate. As soon as that happens, her mind clears and she says, "I remember now! I was going to be a civil engineer!"
Also happens to Francine when she and Roger attempt to spice up their mundane lives by attending a party at the French consulate.
"Roger, that was terrible. We were the only people in period dress and your fake French accent got me punched in the boob."
Another episode also features a brief gag where Stan accidenly elbows Francine, causing her to complain "You elbowed my boobie!"
Stroker and Hoop: Hoop's mom is sent to women's prison for a crime she didn't commit. She consoles herself by claiming that a prison for elderly ladies can't be that bad and takes it all in stride, likening it to being roommates in college ... until her "roommate" threatens to cut her tits off.
Real Life
There is a report on the infamous rape case of Filipina actress Maggie dela Riva. According an interview of the actress, during her ordeal, she was incapacited by her captors by means of punching her in several areas of her body, including one of her breasts, before they proceeded to rape her.
During the filming of the movie The Maggie dela Riva Story which was based on the incident, actress Dawn Zulueta who played dela Riva reported that when that grisly scene was recreated, it actually led to real injuries and she actually had to be hospitalized.
In addition to having her throat slashed, rumors abounded that Nicole Brown Simpson's breasts were deliberately slashed off in the attack as well— however the coroner's report refutes these rumors entirely.
