He forgot he hit his shot. The information faded out of his memory as quickly as it was put in. Now, it turns out he was in the early stage of Alzheimer's. A lot of people do worry that the misplaced keys or glasses could be Alzheimer's. But, according to your book that kind of behavior is quite common.
Absentmindedness is perhaps the most frustrating of all the memory sins.
It's the breakdown between attention and memory. And, the examples can be amazing. Or, we've heard stories about people who've left a child in a car all day. It's a failure of our memory to take in information and remind us. We need to be reminded.