In the story, Anna forgot to say good night to her mother.
Although we may think of forgetting as normal in the hubbub of daily life, Anna is especially troubled by it. This is because Anna's mother died giving birth to her little brother, Caleb. In Anna's mind, her forgetfulness meant that she missed the opportunity to say any meaningful last words to her mother.
In Chapter 1, Anna recounts to Caleb the circumstances of his birth. Shortly after Caleb was born, Anna's mother had handed her little brother to him.
Her mother's only words were," Isn't he beautiful, Anna?" To Anna, however, Caleb was homely and plain. She also thought that Caleb cried too loudly and that he smelt terrible. Certainly, Anna had not agreed with her mother's assessment of Caleb.
Anna remembers that she had gone to bed thinking about the ugliness of a newborn baby. She also remembers that she had forgotten to wish her mother goodnight. This is something that Anna continues to regret, as her mother died the morning after giving birth to Caleb.
Anna also confesses that it took three whole days before she could feel any affection for Caleb. In Anna's mind, Caleb's birth resulted in her mother's death.