My personal encounter might not as dramatic as Ben's (read his story, linked above), but it was so damned unexpected you can bet it left a lasting impression on me. It was around 1990, and my (now ex-)wife Peg and my brother Phred were visiting Atlanta on one of our regular trips to Georgia. The three of us were having dinner at a sushi bar on the north side of Atlanta when a tall, rather ordinary yet distinguished-looking gentleman wandered in and sat down at the bar next to my brother, who was seated on the other side of my wife. I recognized the man immediately, and though I knew exactly who he was, my brain let out an exceptionally powerful fart, and his name simply would not come to me. I very subtly nudged Peg and said, "Look, that's an actor from The Thing. He's been in a bunch of stuff. He played President Johnson in The Right Stuff. I can't think of his name!" Peg looked at the man and shook her head. "I think you're right," she said. "But I don't know his name either."
My brother, who could sort of hear what we were talking about, shook his head and softly said, "No, it's not. You're full of it."
"But it is!" I insisted. "You know, he's in a bazillion movies — a character actor."
"No. Not him."
"Is too."
Finally, a little exasperated, Phred turned to the gentleman and said, "Excuse me, but my brother is convinced you're a familiar actor, but can't think of your name."
The man smiled and said, "Donald Moffat."
At the same volume and level of excitement as Charlie Brown responding to Lucy's diagnosis of Pantophobia, I hollered out, "THAT'S IT!"
My brother's face kind of fell, but all proved well. We sat and chatted with Donald Moffat for quite a while, finding him quite amiable and full of entertaining stories. He informed us he was in Atlanta filming a movie with "that blond kid — Ricky Schroder." (I later determined the movie must have been A Son's Promise, which I have never seen, though I suppose I should check it out someday.) We did discuss The Thing in some detail, as it was even then probably my favorite horror flick. I asked him if people often recognized him when he was out in public.
"All the time," he said, "but they always think I'm their insurance man."
At that time — Deathrealm was in its heyday, so I was frequently going to conventions all over the country — I had met plenty of big-name authors, a number of movie stars, and a few other persons of note (when I lived in Chicago, I once literally ran into Tommy LaSorda — he was coming out a revolving door while I was going in. BOOM. At least he laughed about it), but more often than not it was at a con or someplace where you might expect to encounter such folks. I think this was the first time I'd met a "star" just having dinner, and, as you might expect, for the rest of that evening, I was floating on Cloud Nine.
I do still have a memento of that evening — a chopsticks wrapper that Mr. Moffat very kindly autographed, since, at the time, it was the only piece of paper I had on hand.