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)