Famed computer scientist Don Knuth offers a modest cash reward to the first person who reports any specific error in his books. Many never cash their checks, preferring to display them!
In 1988, I found not one, but two errors in his then new book, Concrete Mathematics. I'm a pretty good proofreader / copy editor. Alas, I was not the first to find either error. I missed by 13 days in the first case.
At the time, Knuth did not use email. I wrote a letter to him about each error, and he replied beneath each letter. So I have his hand-writing twice, but only once did he write his name. Knuth is an author, and the autograph is about his book, but it's one more time I have an author's autograph not in the book.
In the image, Knuth's notes to me are a bit difficult to make out. They read: "previously reported on Oct 5 — please keep trying" and "yup — this is now the second-most-often reported error. I'm glad people are reading it... but sorry for being so erroneous... don".
(Full disclosure: In 2001, I reported what I thought was a misspelled name in a later Knuth book, but he was right and I was wrong. And that time, the correspondence was by email. No autograph.)