OK, folks, you asked for it, so now I'll share it.
The year was 1983. My best friend at the time, Haig, was a huge Trekkie. This was just after the Star Trek movie "The Wrath of Khan" had come out, and there was a Trek convention (actually, I believe it was a Creation Convention) in Boston that summer, with Walter Koenig (who plays Chekov in the film) scheduled to be the guest of honor. So, we decided to go - his father drove us up there (we were both in High School at the time), and we made a day of it.
The convention was OK; but more interesting to me was the used book dealers that were there. As an avid reader, I have been haunting used bookstores since I was very young; my dad used to take me with him when he went looking for books. So, while Haig was in trying to get Koenig's autograph, I wandered out of the main convention hall into the entrance hall, where most of the used book dealers were. My dad had recently given me a copy of Asimov's new Foundation series novel, "Foundation's Edge", as a confirmation gift (I was raised Catholic), and I was hoping to find a copy of it for him (my parents were separated and in the process of divorce at the time).
One of the small dealers, right at the very entrance to the convention hall, had a very nice hardcover copy for sale, which I purchased for a good price (I suspect it was a remainder). The dealer and I then struck up a conversation about the book, which was the first Foundation series book written in nearly 30 years. After about 5 minutes or so, a third man joined the conversation as I was expressing my opinion that it wasn't one of Asimov's better works. This third man, a small, elderly fellow with gray hair and thick glasses, began really egging me on, wanting to know why I felt this way, and very much taking a devil's advocate point of view on the subject.
Now, you have to understand, being raised in an Italian/Irish family in Massachusetts, I talk with my hands. So, in short order, the conversation became quite animated; I became more and more adamant about my opinion of the book, with this third man making comments like, "Really? Why is that?", and "Is that so?". (The dealer, by this time, had become very quiet, for reasons that would soon be obvious.) Finally, I said, "Look, Asimov really copped out on the ending. It was contrived, and it felt like a 'quick fix' for a story that felt like it was going nowhere in the first place. It's been 30 years since the last Foundation book, and this is the best he could do? I expected better from him."
All this time, I have the copy of the same book in my hands, waving it in the air for emphasis. As those last words came out of my mouth, I looked down at the book and pointed at it because, like most hardcovers, it has the author's picture on the back of the slip cover...and as in doing so, I realize, the man I'm talking to is the man who is pictured there. It dawns on me that yes, I just told Isaac Asimov, one of the finest sci-fi writers ever, that his most recent book was awful.
My jaw must have been hanging open at this point, because Asimov got this big, stupid grin on his face. He reached up, took the book out of my hand, and said, "What's your name?" I mumbled my first name name to him, and he quickly wrote something inside on the first page, then handed it back to me. I read the inscription he'd put there, which said, simply, "To ****, I promise that the next book will address all of your concerns. Thanks for the great discussion. All the best, Isaac Asimov" I managed to mumble my thanks and shake his hand, as he gave me that stupid grin again, and walked into the main convention hall without another word.
I was still standing there five minutes later, dumbfounded, when Haig came running out of the convention hall looking for me, shouting, "Hurry! Asimov is here! He just walked in!" And before I could say anything, the book dealer, who was just as flabbergasted as I was, answered, "He knows. He just told him his book sucked!"