The Chofetz Chaim sighed deeply and said, "I too am a person who makes
mistakes. You know that I sell my books. Sometimes the binder
misplaces a page, or a page might be missing or have unclear print.
Even though I am careful to check every book before it is sold, there
still could be mistakes. The people who buy my books are embarrassed
to tell me when there is a defect, but in their hearts they really
care, and that is why I was also punished."
This link - from here shows a photograph of a Sefer the Chofetz chaim sold with the word "Mugah" handwritten by the Chofetz Chaim.