Adultery. He most likely forgot it because in the story he is not faithful to his religion. And his forgetting implies that he must have done something bad that involved adultery. The affair with 17 yr-old Abigail
In some cases, people used to look at other's shoes to see what type of wealth (or class) they had. Usually, richer people would have fancier shoes, and poorer, or middle class, would have more normal ones.
