Human Traffic Jam

A group of people are walking in a straight line. Suddenly, the person at (or near) the front of the line stops walking for whatever reason. This causes everyone behind the person to bump into one another. This gag is mostly seen in Western Animation.

Anime and Manga

 * Princess Princess: As the three princesses leave the stage for a scene change, Mikoto suddenly stops, causing Tohru and Yuujirou to bump into him and each other. The reason? Mikoto's sister and girlfriend - the last two people he'd want to see him in girls' clothes, to the point he tried to have them sent home - are waiting at the other end, which causes him to black out.
 * Ghost in the Shell: Stand Alone Complex episode "Ag2O". While the Tachikomas are rolling along a corridor the one in front stops to speak to Batou and the others run into it.

Film

 * If The Three Stooges never used this, they'd miss the boat.
 * Happens in the Fellowship of the Ring film, when the Hobbits are running away from Farmer Maggot.
 * Snow White and The Seven Dwarfs
 * That one Snow White example is referenced in Psychoville, where the dwarfs are told when they go on stage and see Snow White for the first time, they stop and tumble over. It's supposed to prompt the audience to laugh.
 * Disney's The Jungle Book. While the elephants are marching through the jungle, the one in front stops and all of the rest ram into him.
 * This scene was taken from a short called Goliath II. Also serves as an Early-Bird Cameo for the elephants in this film.
 * An American Tail: as the Mousekewitzes board the boat to America, Fievel keeps stopping to ask questions, causing the mice behind him to crash to a halt, eliciting cries of "Keep walking!"
 * A Bug's Life when a falling leaf blocks the path.

Literature

 * Mentioned in Jingo. Vimes, at the head of a procession, spaces out and begins slowing down and finally comes to a halt, and the entire procession does the same, with jostling and bumping for those slow to keep up with events.

Live-Action TV

 * Another Disney example in Mars and Beyond, while the secretary is being pursued by the Martian leader and his minions. After she changes into her superhero costume, she holds up her hand to stop the leader and his minions slam into him.

Video Games

 * New Super Mario Bros. Wii. Play with four people, and there will be plenty of instances of bumping into each other, fouling up jumps by spring-boarding off each others' heads, and other such Teeth-Clenched Teamwork.

Western Animation

 * The Magic School Bus of all places.
 * The hyenas in The Lion King did this. Bonus points for one of them getting bumped off the edge of a cliff into brambles as a result.
 * Inverted on Danny Phantom. The girls in the pageant are all walking with books on their heads. Sam is at the back of the line reading hers, so she doesn't realize the line is moving at a different speed than she's walking; she bumps into the last girl in front of her and starts a chain reaction.
 * Occurs in the second episode of My Little Pony Friendship Is Magic, with Pinkie Pie in the lead and stopping abruptly when they encounter a roiling river, causing the others to bump into each other.
 * A variation shows up in "Dragonshy," where Fluttershy slips while climbing down a rocky slope, sending her hurtling down into the other ponies in front of her, which sends them all down.

Real Life

 * Karl von Clausewitz devoted a number of pages to the effects of traffic in military columns.
 * Happens all the time with groups going together to major amusement parks, concerts, festivals.
 * Anyone who has spent much time in the armed forces is probably familiar with this effect during formation runs, particularly since the guys in back can't see anything more than a few feet ahead of them.