1. The Split-Chinned Kid
The split-chinned kid appears in the first episode of One Punch Man and sets the humorous tone for the rest of the anime, solely based on character design.
Saitama must protect this child from the mutant, Crabrante.
Saitama doesn't really want to because the kid isn't all that cute, but he does it anyway.
This cements both the funny and heroic aspects of Saitama and the show.
2. Squashing His Own Brother
Hukegao, the mad scientist, created the biceps steroid and used it on his brother, turning him into a giant.
Although it made his brother bigger and stronger, it didn't make him any smarter.
While aiming for Saitama, Beefcake accidentally swats the wrong shoulder, squishing his own brother instead.