Once upon a time in their marriage, my Dad did something really stupid. My Mom chewed him out for it. He apologized, they made up.  

However, from time to time, my mom mentions what he had done. "Honey," my Dad finally said one day, "why do you keep bringing that up? I thought your policy was 'forgive and forget.'"  

"It is," she said. "I just don't want you to forget that I've forgiven and forgotten."