Fanservice trope, and a subtrope of Marshmallow Hell and Accidental Pervert. Someone turns to leave, is running without looking where they're going or trips over (if they are One Head Taller)...and buries their face directly into a female character's bosom, pushing it apart with the same effect as a car's airbag deploying and absorbing the shock. Sometimes they bounce off, sometimes they get stuck. Hilarity Ensues.
What differentiates this from Marshmallow Hell is:
It's never intentional, on either side. Whether or not people who were watching think otherwise is part of the joke.
Can happen to both genders — in fact, since the stationary woman has to be one and a half heads taller, women tend to be the primary demographic. Pictured here is Megatokyo's Makoto's Crash into Hello with Not!Erika: linky
Available to all ages, especially including those old enough to enjoy it.
In one currently airing reality show commercial, a preteen boy and his older sister go in for a chest bump. However, the older sister is definitely outside of his age range and looks suitably more mature. So they go up for the chest bump normally, but he winds up on the floor.
In The Ladders advert at the beginning a man is pushed over by a woman chest first [1]
Konohamaru does the same thing but he likes it, that is before he realizes who it is.
Natsuki's half-trip/half-push-into the amazing Haruka in Minami-ke. She knows it's an accident,but punches him anyway to make it clear to everyone that it's Not What It Looks Like. Still worth it.
It's happened many times since then, but in a one-off special, Bleach's Toshiro Hitsugaya's very first meeting with his future lieutenant Rangiku Matsumoto is when she rams into him, chest-first. It's Meet Cutewith two zeppelins. He's knocked off his feet; extraordinary.
Ogiue's run-in(to) with Ohno at the Comic Fest in Genshiken.
A gentler one: in Mahou Sensei Negima!!, Negi's first meeting with Shizuna-sensei. It's used as a straight Marshmallow Hell to establish that he's still a child, when he's merely embarrassed. Although in this case, it's more of a "wumph" than a "boing."
From the above scene: "Can I... have a cookie?"
Ironically, the Marshmallow Hell incident that gave that trope its name was actually started by a Funbag Airbag moment: Negi didn't watch where he was going while swimming, smacked headfirst into the bustiest girl in the class, and somehow got caught in her bikini top in the process. The other girls just took it way too far (as usual for them).
Actually she blushes, yelps, bolts a short distance down the hall, then turns and yells "Idiot! Pervert! Sicko!" back at him. She does hit him later on in the episode, but that was right after she snapped out of the blank state that witnessing a highly destructive supernatural confrontation had put her into and started having a massive freak out, so it may not have had anything to do with the earlier incident.
In the following episode, once they reach the school, Shiho questions Yuuichi about their meeting while the Funbag Airbag scene replays three times in the background.
Variation: Vandread has Hibiki falling into the...other cleavage.
Comic Party has this same thing happen to the male lead in the fourth episode. He takes several seconds, during which the female even cheerfully asks if he's alright, before he finally realizes what's occurred.
Same happens in the first chapter of Tengen Toppa Gurren Lagann: Simon is walking with his head down and ashamed of being a driller... when he bumps into Kamina's waist. The only thing that comes out, though, is Kamina saying in an encouraging tone "Walk with your head tall and proud, Simon!".
A more traditional version happens with Simon landing on top of/in between Yoko shortly after they first meet. For some reason she gives a heartwarmingly reassuring smile when they make eye contact.
Maybe she just enjoys the sensation.
Or is just too thrilled over having whupped the Gunmen's arse and survived the fall afterwards.
Its an Establishing Character Moment for Yoko. This show was meant to be the anti-Evangelion, with Yoko as the anti-Asuka. Shinji's very first interaction with Asuka is getting slapped in the face because of an accidental pervert moment that entirely wasn't Shinji's fault. So Yoko has to greet Simon with a reassuring 'Its OK, I understand it was an accident' when his first meeting is the same kind of situation.
And then she grabs his head right as he's leaning back up and shoves him right back down protectively. Admittedly, this time it was to get his head down in case of Ganmen attack.
In Martian Successor Nadesico: The Prince Of Darkness, Harry first meets Minato chest-first. The poor boy ends up with a full-body Luminescent Blush before he can disengage. The dub takes it a step further, where deep in his self-recrimination (the reason he was so distracted), he calls himself a boob just before impact.
Kaoru gets one of these in the Ai Yori Aoshi manga when he bumps into an Aoi-lookalike Santa Claus.
He got a dose of marshmallow heaven from the genuine article the first night of their reacquaintance when she grabbed him in her sleep. ("I can't breathe!")
It's half the manga's humor, with the other half being the subsequent near-lethal beating that Keitaro receives.
Giant Amazon Aphelandra does this to Monkey D. Luffy in One Piece. It's more of a straight-up tackle than Marshmallow Hell though, since the amazons are actively trying to kill him at this point.
In Rizelmine Aoi has apparently gone head-first into Kyouko chest so many times that she is able to recognise her in disguise merely by the feeling of the taller girl's boobs on her face.
Read or Die The TV: when Nenene Sumiregawa is finally reunited with the considerably busty YomikoReadman, she takes a flying dive and face-plants into her chest.
Shugo has his own moment when he runs into another player's giant boobs, which jiggle kind of...oddly. Then it cuts to a ground-up shot so we can watch from below as her nipples, uh, twinkle.
Happens again in the manga. Shugo ends up running into Ouka, who was so happy to see him again after being separated for so long that she smothered him with a hug.
Baccano: Jacuzzi Splot gets one of these when he runs into Isaac and Miria in a cramped hallway aboard the Flying Pussyfoot. For extra points, Jacuzzi accidentally catches on Miria's (strapless) dress and pulls it down as he falls, meaning he doesn't just get a face full of Miria's boobage, he gets a face full of Miria's naked boobage. Poor, lucky bastard.
It's never acknowledged by the characters, but in the first episode of Inu Yasha, when Mistress Centipede has Kagome pinned to the Go-Shinboku (and thus, Inu-Yasha), Kagome's face is pretty much right in Inu-Yasha's crotch.
In Mahoraba, Asami bumps into Tachibana this way when she rushes out of her room in her new home.
Cyber Team in Akihabara features this nearly every time Hibari encounters Blood Falcon. After a while, she lets out a complaint along the lines of "Not this again!"
In Nurse Witch Komugi, the well-endowed Magical Maid Koyori accidentally kills someone this way.
In Beach Stars, Iruka Nanase has a one of these with Ono Reika in chapter seven. Ono then remarks on her "airbags" on the very next page.
Zatch Bell!. In episode six, Kiyo has his book stolen by a disgruntled patient and attempts to chase after the patient. However, he ends up running straight into the chest of a nurse who proceeds to JAM his head in there so as to keep him from running away, seeing as he's still recovering from an attack. From then on it becomes a Marshmallow Hell.
In the first episode of Maburaho, Kazuki accidentally runs straight into Kuriko's chest. She then places him in Marshmallow Hell.
The fourth Black Lagoonomake has Rock colliding with Chang this way. It mainly served to reinforce the Gender Flip of the cast he was panicking about.
In Infinite Stratos, this happens when Lingyin was running away from Cecilia. During which, she runs face first into Chifuyu's breast.
In Soul Eater Not!, Tsugumi first meets Meme when the she gets knocked over and faceplants into Meme's ample assets.
In the Amazon Lily arc of One Piece, Luffy accidentally is thrown right into the cleavage of one of the Hot Amazons. Since she's about three or four times as tall as him, it's especially...jarring.
Happens in the first chapter of Koko Ga Uwasa No El Palacio.
Happened in the first episode of Maicchingu Machiko Sensei when two boys steal the eponymous character's panties and try to run away, only for her to head them off around a corner.
Comics
Happened once in Strangers in Paradise. ("I'll give you first base, but second will cost you your legs.")
In Penthouse ComiX, this is how Young Captain Adventure meets Hericane. He's embarrassed but she's fascinated by him. This leads to Slap-Slap-Kiss and Destructo Nookie on a superhero scale a few pages later.
Happened chastely in The Iron Giant for a quarter of a second. Special Agent Kent Mansly (He works for the govern— SLAM!) knocks on the front door of the house, rehearsing his lines and staring at Hogarth's damaged BB gun when the door is opened by Hogarth's mom, Anne. Kent rapidly raises his head to talk to her and smacks it accidentally into the underside of her ample cleavage, resulting in a BOINNNG sound.
Films — Live-Action
Who Framed Roger Rabbit: while pulling up his pants, Eddie bumps his head on Jessica's bosom.
In the live movie adaptation of How the Grinch Stole Christmas, the eponymous character ends up being ejected from a garbage chute, ricocheting off a drum, pulling a banner like a slingshot, and landing with his face in the cleavage of Martha Mae Whovie. "Hello, Martha."
On my way up the stairs, though, I walked straight into the medium herself, surrounded by her entourage. I bumped right into her, my nose centimetres away from her plump pink bust.
Live-Action TV
Star Trek: Enterprise. Captain Archer and his Ms. Fanservice science officer T'Pol are captured and tied together by rebels in "Shadows of P'Jem". Whilst struggling to free themselves they trip and Archer's face ends up right between the best Vulcan peaks outside Mt. Seleya.
Two and a Half Men: Alan and Charlie are removing a passed out, scantily-clad woman from Charlie's bed, and Alan, who is carrying her by the arms, slips and falls on her. We don't see how he falls, but we do hear his muffled reply: "Oh, my God! They're real."
That would be the lovely Diora Baird, who's made most of her career off having Gag Boobs.
Video Games
This happens to Jin and Asuka during Asuka's ending in Tekken 5 Dark Resurrection. Jin is disoriented after reverting from his Devil form to his normal form, stumbles, and lands face-first in Asuka's cleavage. She shoves him away after about ten seconds, leaving him disoriented and confused.
Humorously subverted in the Metal Gear Solid 3 short "Metal Gear Raiden: Snake Eraser". Raiden time-travels to the scene of The Boss showing Naked Snake a scar on her chest, and so happens to teleport in such that he's making a beeline for her cleavage. He stares at it for several seconds, then gets bitch-slapped by her, Snake gasps in horror, and Raiden teleports out once more, but not without the Colonel shouting "TIME PARADOX!"
In Shuffle, Rin stumbles in the classroom and crashes into Nerine. She takes advantage of the situation by giving him a hug, which is a lot more forward than her usual demure personality.
In the pilot of Korgoth of Barbaria, one of the thugs in the bar fight gets flung across the room by Korgoth, and he lands face-first in a barmaid's cleavage. He doesn't leave to rejoin the fight (considering the guy he was fighting, it's an excellent idea).
In Family Guy, Quagmire makes this trope literal. His car has blow-up sex dolls in place of airbags, so when he crashes it results in this trope.
Truth in Television
It's a safe bet to say that a lot of us have face-planted into someone's breasts when not looking, yes even the girls.
In a rather literal twist on the trope name, a woman with implants was saved from serious injury in a car crash by her augmented breasts, which served as airbags.
Same goes for all-natural Playboy playmate Petra Verkaik, which literally saved her life.
