I know a family where this happened. The father, a University professor, was supposed to bring the child to daycare but didn't usually do that. It was the change in routine and the fatigue of being a new parent that led to his son's death. He didn't forget he had a son, but he forgot he had his son in the car. No one judges him more harshly than he himself does. He has to live with this and forgive himself every day. His wife forgave him. Pretty sure I wouldn't be able to, but she did.
Another friend, yes, met these families at a grief site when dd died, the wife forgave the dh after their 13 yo was killed in a car accident with dh at the wheel.
On the other hand, when I was a nanny, I did emergency relief care for a family whose nanny left the baby (18-24 months, I don't recall her age at the time) in the car to go to Target and shop alone. Not kidding. The police came and rescued the child. The police pressed charges; the family didn't have to do a thing.
It is very sad. When my oldest was young I would often drive her all the way to work with me forgetting she was in the back seat on days when school was out because she was so quiet. Any change in routine can really mess a parent up, kids take so much.