Step on a Crack

After a little girl breaks her mom's back by stepping on a crack, she says that stepping on a line will break her father's spine. She does so, and the next-door neighbor's spine breaks. The dad thinks that this infidelity absolves him of all responsibilities and drives off, but it turns out that the daughter is the result of an entire second family, as her brother steps on a line, and the dad's spine breaks, causing him to crash his car.

Example of:
Secret Other Family