I should never be allowed to write for a cartoon series, because if I did I’d include all sorts of stuff just to mess with the sort of folks who like to speculate about the backstories of children’s media.
There’d be a character with a large, conspicuous facial scar. It’s not there in childhood flashbacks, so clearly they must have suffered it at some point between then and now, but it’s just plain never addressed.
Two major supporting characters have identical designs apart from their hairstyle and clothing, and are played by the same voice actor. No situation ever arises that would require them both to be present in the same scene, so the question of whether they’re related or one character with a dual identity or what never comes up.
A couple of supporting characters have a focus episode where they go off on their own little adventure for a couple of weeks. When they return home at the end of the episode, the protagonist now has, like, a prosthetic hand or something. When they express concern, the protagonist indicates that they’ll tell them the whole story later that evening – and then the episode ends. The change to the protagonist’s character design proves to be permanent, but the audience never does find out what happened.