Keitaro of Love Hina has been theorized to have breast magnets in his hands considering how often this happens to him. What usually follows is either a punch (Naru), an innocently/happily surprised "Oh my!" (Mutsumi), a scream (Shinobu), or a blush and an excited giggle (Kitsune).
Haruhi Suzumiya gets the computer club president to grope Mikuru against his will (Mikuru does not approve either). Kyon is an accessory to the fact, since he photographs the whole thing so that Haruhi can use the photos for extortion. This was modified in the animation by having Haruhi take the pictures instead, which is terribly amusing since Kyon has absolutely no reason to be in the room when this is going on except to take pictures. As a result, he just stands there acting all disgusted at Haruhi's actions.
The first example is when Nyu starts fondling Yuka to sexual excitation.
After Kohta accidentally gropes Nyu early on, she sees it as a action of endearment, like a hug. And then she apparently starts to innocently rub herself on his hand, since it apparently felt so good. Cue Kouta's cousin walking to the room.
Nyu also gropes and nearly rapes Nozomi because she misunderstood something that Kouta said, making her believe that it was OK to do it to big-breasted people.
Done in a surprisingly abrupt manner by Itoshiki-sensei in Sayonara, Zetsubou-Sensei (the episode with the roads signifying different excuses for avoiding responsibility). Occurs with absolutely no lead-up, and his "victim" Chiri tries to pressure him to marry her in order to "do things properly". Worth noting that she demanded marriage even earlier when both she and Itoshiki went to rest in nurse's office unaware of each other and she rolled on his bed in her sleep. She's... pathological (but aren't they all).
Early in Full Metal Panic!, Sôsuke performs a Diving Save to prevent Kaname from being hit by a truck... and ends up with his hand on her breast. She actually doesn't notice until Sōsuke gives her a reassuring squeeze... Made at least ten times funnier by the fact that her first reaction is apparently pure delight — which thoroughly shows on her face — for about three seconds. After which she realizes what's going on, resulting in one of the funniest expression changes ever...
Chrome Shelled Regios: Layfon to Naruki, after falling on top of her in a sparring match. Unlike most anime females, she understands it's an accident and jokes about it.
Rito also seems to have the breast magnets, although he must have one in his head too, leading to a lot of accidental Marshmallow Hell.
Also a rare variant where Rito accidentally grabs Mikan's butt (Thanks for the Glutes?). It proved very popular among the fanbase and lend to a second chapter with more of Mikan getting her ass grabbed by him.
Erio and Caro's first meeting in Magical Girl Lyrical Nanoha StrikerS had Erio's hands end up in two embarassing places when they crashed in a heap after he caught Caro when she tripped. In a subversion of the usual reactions, the two of them simply disengaged efficiently after a short moment of embarassment and the incident was quickly forgotten. It probably helped that they were both only ten. However, this lead to a Running Gag for the Fan Artists & Doujin Writers.
When Ranma first comes to the Tendō Dōjō, Nabiki proves Ranma's (apparent) gender by first poking her/him in the breast, then rubbing it. She later emphasizes her point by grabbing her/his breast again. And, when it seemed Ranma's bust size had increased in a late manga story, she proved it by... you guessed it! Groping hir again.
An anime-only scene in the first season has Sōun try and break up a fight between Ranma and Genma, both in cursed form, by pushing them apart with a hand on each chest. He then promptly realises he's cupping Ranma's breast, and his daughters have seen him doing so. He tries to claim that it's Not What It Looks Like, and promptly catches a fist in the face from Ranma due to not letting go.
In another episode of the anime, male Ranma stands up, half-faints, and braces himself. On Akane's chest.◊ As expected, a Megaton Punch follows.
In Nagasarete Airantou, Shinobu challenges an unsuspecting Ikuto to a sword fight. Before she can land the first blow, she stumbles and Ikuto rushes to catch her. He inadvertently gropes her and ends up winning because shortly afterwards she was consumed by "a warm feeling in her chest" and is unable to battle.
In To Heart 2, Takaaki falls on Karin and has his hand on her breast. Karin uses this to blackmail Takaaki into joining the UFO Research Club.
Mahou Sensei Negima! has Negi do this to Asuna a lot. It helps that most of the time it genuinely is accidental. Lucky bastard gets away with it scot-free most of the time, due to his age. Humorously, the one time he gets socked for it is when he does it while magically aged-up.
Subverted in Minami-ke when Natsuki, one of Touma's older brothers, gets pushed into Haruka resulting in a grab. Haruka knows perfectly well that it was accidental and does not hold it against Natsuki. However, the incident occurred in front of Haruka's class, so for the sake of appearances she still has to punch him out.
In the first episode of Vandread, Dita crashes her dread through the hull of the Male fleet's flagship, and right into Hibiki, throwing her on top of him and knocking them both out. When Hibiki comes to, he's unaware that Dita is still laying on him unconscious and his face is covered by her chest. He grabs a breast and squeezes and exams it for a few seconds, curious as to what it is (Men have had no contact with women for generations in the series). Dita apparently woke up only after he was done, or just wasn't bothered by it. In Dita's case, the later is actually possible.
Happens to Amakawa Yuuto, mostly with Himari. He seems to have mixed feelings on this...
Yuuto: Himari's are big after all... (Gah, what am I saying!?)
On at least one occasion, he wakes up with her in his bed, with his hand full of boob. Unlike the standard "OHCRAPIHAVETOSTOP!" reaction, he briefly considers massaging it. Then she wakes upnote and doesn't care at all. Lucky bastard..
How Sano found out Mizuki's secret at the beginning of Hana-Kimi. Mizuki had been knocked out playing soccer with Nakatsu, and guess what Sano accidentally touched when he picked her up? When Mizuki found out at the end, she was understandably distressed. In another chapter, Sano tried to put a hand on her shoulder, but missed.
In both the anime and manga version of Ladies Versus Butlers!, Akiharu is confronted by Selnia who is convinced from his appearance that he is up to no good. When he decides to ignore her she takes a swing at him, both of them trip and he ends up with his hands on one of her breasts. It wouldn't be so bad if he wasn't so stunned that he was still touching her when she recovered her wits.
Happens a couple time by Gourry to Lina in Slayers.
Once when they are laying in a field — Gourry falls asleep and rolls over, his hand landing on Lina.
And then in the second season where Xellos grabs Gourry's hand and makes him grab Lina to test a theory. Naturally, pain is involved both times.
In chapter 298, Jellal trips and grabs Erza's breasts. They are both embarrassed but Erza accepts it was a mistake.
In chapter 331, Lucy gets magically stripped by a dragon and sent flying right into Natsu. After the initial shock, Natsu declares that he has no choice but to cover her up himself... and puts his hands on her bosom. Cue reaction punch from an already EXTREMELY embarrassed Lucy.
In Gintama, Tsukuyo offers to let Gintoki grope her breasts, he declines, ends up doing it accidentally anyway, and gets a suplex for his efforts.
In the first chapter of Tasogare Otome X Amnesia, Teiichi doesn't quite believe Yuuko is a ghost, so he reaches out to grasp her shoulder, reasoning that a ghost will be insubstantial. That's just when she turns to face him, though, so he gets a handful of quite substantial breast. Yuuko is totally unfazed by this. She is a ghost, however.
In Bakemonogatari, during one of the times that Araragi is fighting with Mayoi, he gets an accidental handful. She doesn't notice at the time, but is pretty freaked out when he mentions it later ("You stole my first touch!").
Touma wakes up from his latest misadventure to find his hand grabbing the breast of Little Sister Misaka. At first he freaks about the fact that he's such a pervert that he'd do such a thing in his sleep, only for Little Sister Misaka to inform him he only grabbed her hand, she put his hand on her chest (she was measuring his vitals). Touma then realizes that, due to anesthetic, he can't actually feel anything.
Later, when Touma mistakes Mikoto for a shape-shifted Thor, he tries to dispel the "illusion" and grabs her chest. She is so shocked that he is able to leave before she snaps out of it.
Inagawa Goro, a recurring character in My Lovely Ghost Kana, frequently expresses a desire to encounter the supernatural. In one issue, he wishes that he could "reach out and touch it," and grabs at the air—while the titular Cute Ghost Girl is floating right in front of him.
In Girl Got Game, Chiharu trips and his hand lands directly on Kyo's chest. Since Kyo is pulling a Sweet Polly Oliver, her reaction is not embarrassment, but panic about her secret being uncovered (she had just removed her chest wrappings because she thought she wouldn't need them that day). Thinking quickly, she claimed that her chest was bruised and swollen.
In Mayo Chiki! this is how Jiro finds out that the resident Chick Magnet is a Bifauxnen and later he does the same to Usami. It should be noted somehow their shirts came undone when he did it.
Also Roxanne gropes Satellizer The Untouchable Queen in their first appearance, much to the participants of West Genetics' shock/horror, and accuses her of implants.
A non-mammary example occurs in Video Girl Ai. Being played in Yota's broken VCR made Ai's breasts smaller. She tells him that the VCR also turned her into a boy and grabs his hand and puts it between her legs for him to check. After realizing it was just a joke, he laughs but leaves his hand there a just little too long.
In One Piece, Luffy's introduction to giant mermaid Princess Shirahoshi. Apparently they feel like pudding.
In Rave Master Elie is running towards the rooms where Haru is fighting just as an explosion destroys the door and sends Haru flying towards her. He lands with a hand on her chest. She is not amused.
A variation is invoked early in Highschool Dx D. After Rias promised Issei the chance to grope her for completing a contract, she springs it on him so suddenly she counts to five before he can even touch her, leaving him incredibly disappointed. She compensates him by letting him feel them with his chest as she hugs him instead. Issei's goofy smile and transparently half-hearted objection show he's more than okay with doing it that way.
After Scott and Ramona reconcile in Scott Pilgrim Gets it Together, Scott notes the moment to be one he wants to last. Ramona then points out it's been spent with his hand right where the trope expects it to be.
Wolverine needs to stop Black Cat from walking into a minefield in the Wolverine/Black Cat: Claws mini-series. She's behind him, so he puts out his hand to stop her. Of course his hand lands right on her... chest. She gets angry, but Wolverine's in a cannot-be-embarrassed mode.
From an effectiveness perspective, this makes sense. He needs her to stop as quickly as possible, and touching her breasts will make her draw back immediately. YMMV on whether that was intentional.
In the This Time Round story "Friendly Hopes", Adric is already having a really bad day. Then, things get worse: he collides with Nyssa, and manages to put his hand on her breast as they both try to get up.
In The Second Try, after Asuka and Shinji have been sharing a bed for a while, she wakes up to find his hand on her chest (he's still fast asleep). It's a sign of their developing relationship that she isn't that bothered by it (a fact that she finds strange herself), but she still removes it before he wakes up.
In the sequel, Naked Gun 33 & 1/3, Drebin is at the store squeezing grapefruits to test for ripeness while looking the other way. A woman in a low cut dress walks by, and Drebin mistakes her breast for a grapefruit and squeezes it. He gets slapped.
Superbad: Evan is having a conversation with Becca in a crowded school hallway. When someone bumps him from behind, his hand actually punches her in the boob. He's embarrassed, but she assures him it's no big deal.
Fierce Creatures: Rollo reaches backwards for a piece of paperwork without looking. He... well, he misses the paper, let's say. Willa takes it in stride.
Star Trek (2009). Pre-cadet Kirk during a barfight stumbles "onto" Uhura, much to her annoyance. Kirk puts on his trademark smirk, which Uhura promptly wipes off his face with a sock to the jaw.
Simon Birch* from A Prayer for Owen Meany has the title character doing this. He is a twelve-year-old who (because of his unusual smallness) is playing baby Jesus in a Christmas play, and the person playing Mary (who just happens to be the prettiest girl in the class, who he has a crush on) is bending over him. He reaches up out of the manger and grabs her breast.
The Jackie Chan film Mr Nice Guy had a scene where Jackie was trying to avoid detection by henchmen, and accidentally copped a feel on Ms. Fanservice while trying to get her to stay close to the wall.
In the first Rush Hour, Carter has angered a female cop and walks off. To stop her from attacking Carter, Lee (Jackie Chan) unknowningly presses his hands on her chest. This is subverted, as she was in protective bomb-squad-issued gear for a training exercize, although he's still quick to remove his hands once he looks at where he's touching.
When a character in Kangaroo Jack sees various mirages in the Australian desert, he sees a beautiful woman. Believing her to be another illusion, he gets handsy. In return, the very physically-present woman punches his lights out.
Averted in Dude Wheres My Car in the school camp scene, where a blind boy is questioned by one of the alien android women. He feels the breasts of one of them in order to identify who he's talking to, and she doesn't react at all.
Alien Woman: (confused) Is this... normal?
Blind Boy: Oh, yeah. This is how blind people say "hello".
In Wild Wild West, James taps on the breasts of a woman, thinking she is Artemus in disguise.
In Spies Like Us, Chevy Chase's character is masquerading as a surgeon claiming that his hand has no feeling in it, hence why he couldn't perform surgery. He demonstrates how floppy his hand is to Donna Dixon, by letting ''flop" right on to her chest. She carries on the conversation without missing a beat.
Summer of '42: Hermie and Oscy take two girls to the movies, and Hermie puts his arm around the girl, casually putting his hand on her boob, with a huge grin on his face. Afterwards, they have this conversation:
Oscy: What did it feel like? Hermie: It felt like a boob. Oscy: Didn't it feel a little like an arm? Hermie: Why would it feel like an arm? Oscy: Because it was an arm.
Return of the Jedi: When Leia is shot outside the Imperial bunker, Han rushes to her side and 'helps' her by grabbing her left boob. Apparently Carrie Fisher didn't realise what Harrison Ford had done until she was told about it later.
The trope-naming novel, Don't Call Me Ishmael!, contains a scene in which the eponymous Ishmael forms part of a school debating team and, collapsing due to nerves, inadvertently gropes his opposition and eventual love interest.
In the first Xanth novel, a female centaur offers to give Bink a ride. While she was jumping across a chasm, Bink loses his balance and would have fallen off completely if not for a desperate attempt at grabbing onto whatever he could hold — which, of course, turns out to be her (bare) breasts. She forgives him, saying that she felt him losing his balance and it obviously wasn't deliberate (adding that if it had been deliberate, she'd have dropped him in the chasm).
In Hogfather, the newly-created Oh God of Hangovers joins Susan on the horse Binky, and innocently puts his hands around her at an inappropriate height. As he's only existed for a few hours, she doesn't smack him for his ignorance, just corrects his position very firmly.
In Carpe Jugulum, the witch Agnes Nitt awakes after the fight with the vampyres in Escrow, to discover the villagers have taken the appropriate local precautions against her waking up as a vampire. (these are many and varied around the Discworld.) She complains about poppy-seeds being poured into her bra, and adds that whoever took her stocking had better have been a woman.
Terry Pratchett really likes this one. It also happens with the arch-chancellor and Granny Weatherwax on a broomstick in Equal Rites.
In the Brother Cadfael mystery One Corpse Too Many, a wounded protagonist gives the "boy" nursing him a playful push in the chest and gets a shock. He also gets a clout on the ear.
The Dresden Files: In Death Masks, Harry accidentally grabs hold of Anna Valmont's breast when she stumbles against him while getting out of bed. The stumble was actually deliberately staged so she could pick his pocket.
Buffy the Vampire Slayer: Buffy turns invisible. Xander reaches out to find Buffy. His hands are at chest level. He quickly ascertains that Buffy's clothes are invisible too as he hurriedly explains to Anya.
In the House episode "Let Them Eat Cake", House begins to grope Cuddy in the middle of a serious conversation.
House: Why are you dressed like that? Why are you trying so hard to get my attention? Are you screwing with me? Cuddy: Are you screwing with me? House: That depends on your answer. (Cuddy says everyone knows that this is going somewhere, and that they're supposed to kiss now) (House says that they already did that and grabs her breast instead) House: Seemed like the logical next step. Cuddy: Really? I'm an idiot for being surprised.
Fawlty Towers: Basil Fawlty once mistook a guest's breast for a light switch as he was trying to turn on the light from the other side of the door. His hand just happened to have paint on it as well, leaving a handprint on the guest's top for Sibyl to see moments later and draw the obvious conclusion.
A skit on an early '80s episode of Saturday Night Live had a character travel from the future just to squeeze one of the female player's breasts.
In The Drew Carey Show, Drew accidentally does this to Mimi. His reaction is screaming in horror and dousing his hand with alcohol.
On Home Improvement, while driving his attractive female mechanic home, Tim makes a hard stop at a stoplight and reaches out to stop her from going forward, grabbing the wrong area in the process. When she asks why, he says that since he has three kids, it's basically a reflex. Unfortunately for him, the Detroit area just got cameras installed to take pictures whenever somebody goes through a red light.
A well-known Star Trek: The Original Series blooper has DeForest Kelley and Majel Barrett rocking back and forth holding onto each other while the ship's under attack. At one point he's holding her in the wrong place.
Although not filmed, Garret Wang (Harry Kim) of Star Trek: Voyager accidentally did this with Jeri Ryan (Seven of Nine) when they were rehearsing a scene. They were walking side by side through a corridor, and he made a full turn while trying to grab her shoulder, but instead the obvious happened.
According toPatrick Stewart, during the filming of an episode of Star Trek: The Next Generation he was supposed to help Gates Mcfadden's character (a.k.a. Dr. Crusher) out by pulling her out of a rock (specifically, by holding her arms). Because of certain circumstances they were in some kind of rush to do the scene, and so when the director said "Action!", Patrick, as he put it, "put both my hands on her breasts", and freaked out ("I screamed!"). However, at the end he did admit that "I'm really glad it happened".
The K DramaAnswer Me Nineteen Ninety Seven has an example in the second episode, where Yoon-Jae ends up on the ground with his hands on Shi-Won's chest after she has chased him around with a frog.
Naturally, this happened in Married... with Children. In an episode that's a parody of old fashioned "whodunit" mysteries, in the scene where the lights go out right before the murder happens, the Femme Fatale character angrily snaps, "Who touched my breasts?". Later in the episode, where Al is questioning all the suspects, he recreates the scene, and the exact same thing happen. This time it's obvious who it was. (And she slaps him for it.)
Quantum Leap: In the episode "Good Morning, Peoria" Sam leaps into a radio DJ in a town where the old fogeys are trying to ban rock & roll. At one point the anti-rock geezers cut the power to the radio station, plunging Sam and his female assistant into darkness. A moment later the assistant screams and Sam apologizes, claiming he was "looking for a doorknob".
In Tales of the Abyss, this happened to Guy after Luke (under orders from Jade) kicked him into the girl's area of the spa (it should be noted that Guy is very gynophobic). The girls were angry at first, but then realize that he was the least likely guy to grope them and Anise asked him who was bigger: Natalia or Tear. After he said Tear, they got angry and called him a pervert again, netting him the new title "Naughty Devil".
In Tales of Xillia 2, Milla trips while walking on a rather treacherous path near some lava, at which point Ludger is given the choice of catching her fall or doing nothing. The former option results in the trope and earns him a slap (And some affinity), though she apologizes for it quickly. She ends up being fine in the latter option, but she gets mad at him for letting her crash into the ground instead.
Rai on Arina in Rai (or was it Arina's?) ending in Waku Waku 7.
Chrono Trigger: Ayla does this unabashedly to Marle during Marle's Side Quest. Ayla claims that Marle isn't "ready" to have her own kids yet (of course, Ayla was comparing Marle's chest to her own, so the bar is a little high...).
Agarest Senki 2: Jainus is currently at the item store with Fiona getting supplies. Chloe, the girl who works there, ends up trying to grab something they want from a high shelf. Naturally this trope occurs when Jainus catches her falling. He gets a slap and apologizes immediately afterward, although judging by Chloe's... reaction after the mess is cleared up, Jainus has a gentle touch.
In Mio's route, Riki ends up falling on top of her accidentally and ends up with his hand on her chest. He's so distracted that he then proceeds to spend a good minute thinking about it (Mio having passed out from the fall) and when Kurugaya appears and starts talking to him about it, he actually forgets to take his hand back. When Mio awakes a couple of minutes later, she is not particularly impressed by this, but admits it was her fault they ended up falling over in the first place.
It happens again in Haruka´s route where Riki slips on a magazine in her room while moving some boxes and accidently lands on Haruka, although she doesn´t really mind.
Largo: (staring at the closed door) Something is there. I can feel it. Erika: There's nothing... Largo: (throws his hand back to shush her) SHHH!!! (Largo realizes "something", and looks back to see his hand right on Erika's chest) Largo: Wow. Those are a lot softer than I thought they'd be. (elsewhere) Yanagisawa: My God, what was that?? Piro: It kinda sounded like a truck running into the building.
In Code Lyoko episode "Vertigo", a bug in an anti-virus program turns Aelita invisible, and she runs to Jérémie:
Aelita: I... I'm invisible! Jérémie: (groping around for her in front of him) You're what...? Aelita: Hey! Jérémie: Oops... sorry.
In the second season of The Venture Bros., one of Monarch's henchmen accidentally does to Dr. Girlfriend while sneaking her into the lair. This being The Venture Brothers, he freaks out and makes a bigger deal out of it than she does. "Quick! Feel my tits!"
In one of the first episode of Teenage Mutant Ninja Turtles (1987 series), Michealangelo apparently does this to April O' Neil when he's untying the ropes that hold her to a chair. April O'Neil's expression when he's doing this really cements the scene.