Apparently this happens sometimes. It suggests a phenomenon similar, if not one and the same as, "tulpamancy", which is when a thought figure is intentionally or perhaps even unintentionally generated as a physical being by concentrating or meditating on them. Here's how comic book writer Alan Moore encountered one of his own characters ---twice!
"Alan Moore claims to have met his creation on two occasions. In 1993, he told Wizard magazine:
One day, I was in Westminster in London—this was after we had introduced the character—and I was sitting in a sandwich bar. All of a sudden, up the stairs came John Constantine. He was wearing the trenchcoat, a short cut—he looked—no, he didn't even look exactly like Sting. He looked exactly like John Constantine. He looked at me, stared me straight in the eyes, smiled, nodded almost conspiratorially, and then just walked off around the corner to the other part of the snack bar.
I sat there and thought, should I go around that corner and see if he is really there, or should I just eat my sandwich and leave? I opted for the latter; I thought it was the safest. I'm not making any claims to anything. I'm just saying that it happened. Strange little story.
His second meeting with his creation was illustrated in 2001's Snakes and Ladders, an adaptation by Eddie Campbell of one of Moore's performance art pieces:
Years later, in another place, he steps out of the dark and speaks to me. He whispers: "I'll tell you the ultimate secret of magic. Any cunt could do it."
They met a third time in fiction, when Moore was written into issue No. 120 of Hellblazer by then-author Paul Jenkins. Moore is seen sitting in silhouette at the back of a bar as John Constantine (who is on a pub crawl with the reader) raises a drink to him.
Writers who had their run on the Hellblazer series have also admitted meeting the character in real life. Original Hellblazer writer Jamie Delano also claims to have encountered Constantine, during his run on the character, outside the British Museum. Peter Milligan saw Constantine at a party around 2009 and rushed after him, only to find he'd disappeared. Brian Azzarello once saw him in a Chicago bar but avoided him, saying that "the thing about John is, the last thing you'd want to be is his friend."