Diana Wynne Jones’s book Reflections: On the Magic of Writing contains two more examples of this phenomenon.
The first ‘coincidence’ happened at a time when she was working on her book Fire and Hemlock, for which the plot was, she thought, her own invention. An acquaintance railroaded her into a visiting a place where people started discussing a local legend - which strongly resembled her plot.
The second incident happened while she was working on Archer’s Goon. One of the characters discovers a newborn baby in the snow. The same acquaintance went out for a walk and found an abandoned baby!
The two incidents in Diana Wynne Jones’s own words:
The Fire and Hemlock incident
“Sometimes, however, the book comes true while I am actually writing it, and this can be quite upsetting.
Fire and Hemlock was one of those. One of the many things that happened while I was writing it was that an eccentric bachelor friend from Sussex University, who stayed with us while he was lecturing in Bristol, insisted on my driving him to some stone circles in our neighborhood. There, he began having mystic experiences, while I kept getting hung up astride the electric fences that crisscrossed the site. My outcries, he said, were disturbing the vibes, so he sent me to the local pub to wait for him.
As soon as I got there, the landlady and the other customers began talking about these same stone circles and related the local story about their origins. This story is called “The Wicked Wedding”: the bride, who is an evil woman, chooses a young man to marry, but at the wedding, the devil comes, kills the young bridegroom, and marries the lady himself.
This is the story behind Fire and Hemlock and, believe it or not, I had never heard it before - I thought I'd made it up. Well, after various other strange experiences, my eccentric friend went back to Sussex and I finished the book.”