The plot is set in motion or heavily influenced by a random circumstance or unnamed character
Unnamed Fire Nation lackey accidentally refers to Iroh & Zuko as "prisoners", forcing Azula's hand and resulting in their escape. Iroh & Zuko continue to grow and change as people. (ATLA)
Some rat lets Scott Lang out of the Quantum Realm, causing the inciting incident of Avengers: Endgame. This makes the rat the undisputed MVP of the Avengers saga, don't @ me (MCU)
Random mugger kills the two wealthiest people in Gotham, unknowingly creating the Batman. It is absolutely better for their deaths to be caused by a random mugging that's representative of the rot and crime that pervades Gotham as a whole and leaves no one safe, absolutely @ me I'm prepared to keep explaining (DC Comics)
It is not a random mugger. His name was Joe Chill and his motives change wildly with writers
Bit of the running gag and inverse, but Saitama (the protagonist) repeatedly kicks the shit out of Garou (a future arc antagonist) in One Punch Man. As a result to losing to "random strangers" Garou's growth as a villain is directly influenced by his insecurity about how easily he is beaten. Saitama being the airhead doesn't even recognize him until later.
The first example is either plot-induced-stupidity, or not a coincidence - but a white lotus operative doing his thing.