Couples of reddit, what were some of the smallest issues/events in your lives that permanently damaged your relationship.

As I am typing this, my short-tempered dad is threatening to kick my mom out of the house for her "annoying, loud voice" whenever she talks to my grandfather, who is partially deaf. He has slapped her in the arm just for responding while she was crying. I feel like this is going to damage my life and their relationship permanently as a result of a stupid thing. Any body else experienced similar things?