Bump Into Confrontation

A minor bump leads to overreaction from the bumped person.

John Doe bumps into Richard Roe on the street and turns to apologise, but Richard is a Delinquent or street thug, or general grump of some sort who is having none of it. Richard may demand an apology or compensation, or just start berating or attacking John. This may lead to a Knight in Shining Armor-type intervention by a third party, a Mugging the Monster moment where the challenged party is able to hold his own unexpectedly...or the bumper having to kowtow to the aggrieved person even if the bumper wasn't actually responsible.

Anime and Manga

 * Happens twice to Minako Aino and her friend Hikaru in Codename: Sailor V allowing Saito, a head of one of a different gang of juvenile delinquents and former attendee of the same middle school as Minako to rescue them. Minako gets a crush on him but
 * This is how Makoto Kino alias Sailor Jupiter is introduced in Sailor Moon, by saving Usagi from one of these scenarios.
 * Happens way back in The Kanto Saga Pokémon episode "The Song of Jigglypuff". Jessie and James bump into two random people (a large, fat woman and a thug, respectively). Because the entire town is unable to sleep, everyone is cranky and irritable, so Team Rocket gets the crap beat out of them. (The fat woman actually spanks Jessie). To add salt to the wound, the people beating on James and Jessie were the ones who bumped into them. Ash also bumps into someone, but avoids getting into a fight. Later in the episode (after Jigglypuff's forced everyone to sleep), the now less grumpy guy apologises.
 * At the beginning of the Marineford arc of One Piece, a waiter bumps into a pirate and stains his shirt. The pirate wants to kill him for it, but his fatalist captain talks him out of it—apparently, it was his shirt's destiny to be ruined that day.

Comic Books

 * Done very seriously in X Men. The team has just saved Colossus' big brother Mikhail from another dimension. Mikhail is still traumatized by the experience (after his tampering with a dimensional anomaly killed his wife and brothers in arms) but seems like he will recover in time. Then, as he is left alone in the street, he suffers a brief Heroic BSOD and just then a teenager on a bicycle bumps into him. A couple of seconds later, Colossus returns to pick up his brother, unaware that the nearest tree now has a pattern of stumps shaped like a face screaming in terror...

Film

 * This nearly occurs in The Protector between Tony Jaa and in a cameo at an airport. They bump into each other, turn and quickly strike fighting poses until they realize that it was a mistake and politely apologize to each other.
 * The titular "girl" in The Girl With the Dragon Tattoo gets into a rather nasty fight with street hooligans in a subway after she bumps into them.
 * Do the Right Thing has a scene with this, though nothing actually comes of the threats and the guy doing the bumping doesn't seem to be intimidated.

Literature

 * In The Three Musketeers, this is how d'Artagnan meets Athos and Porthos. He crashes into both of them (separately) and handles the result badly, leading to two duel challenges. (Minutes later, after resolving to mend his ways and be more polite, d'Artagnan tries to behave friendly to Aramis and only ends up embarrassing him, earning himself a third duel challenge.)
 * In Roll of Thunder, Hear My Cry the heroine, Cassie, a black girl, bumps into Lillian Jean, a white girl who is the daughter of a very influential bigot. Cassie barely pays attention to what Lillian Jean is saying until L.J. orders her to apologize. Cassie's little brother, who is a courageous boy, tells her to let Cassie alone. Lillian Jean doesn't listen, of course, and tells her to get down in the street. Cassie protests and tells her she isn't going anywhere, and that if she's so scared of being bumped, Lillian Jean should go down there herself. Then, Lillian Jean's father shows up and pushes Cassie down in the street and scolds her for being such an impudent brat. Cassie runs away from the curious crowd and runs into her grandmother, who makes her apologize, and humiliates the poor girl. Lillian Jean later gets what she deserves.

Newspaper Comics

 * In the Rip Kirby comic strip, Lady Lilliput smacks a small child with her cane for bumping into her, demonstrating she's not the harmless eccentric she first appeared. She would have done worse, but Honey intervened for mercy.

Real Life

 * Samurai in feudal Japan were rather notorious for this. They had to keep the scabbards for their katanas tied securely to them and pointed away from those who passed them on the road. If your scabbard knocked against another samurai while you were passing him on the road, it was often enough to provoke a sword duel.

Video Games

 * Mass Effect 2 brings us the assignment "Crime in Progress",where a Volus accuses a young Quarian of picking his pocket,solely because she bumped into him,and he left his credit chit in a store. The kicker? He doesn't apologize,and the cop brought in on the matter throws in a threat to arrest her for vagrancy. A Quarian on Pilgrimage is a vagrant by definition.

Western Animation

 * In My Little Pony Friendship Is Magic episode 5 (season 1), Gilda berates Fluttershy for bumping into her.
 * The Boondocks uses this as an example of Nigga moments. Two black guys bump into each other and start a shootout. Nobody is injured and they realize how ignorant they were being and work out their differences... and then the cops come and kill them both.
 * Kabuto from Tokyo Mater is actually introduced this way.