I told it there, but I figured I'd give it its own entry, too:
Picture it: summer 1994. I was an associate editor for Byron Preiss, working under senior editor John Betancourt. Byron had just obtained the license to do novels and anthologies based on Marvel superheroes. We opened with a Diane Duane Spider-Man novel (The Venom Factor) and a short-story anthology called The Ultimate Spider-Man (this was many years before Marvel did their own comic with that title).
The book was being put together horribly late because obtaining the license was a slow process and Byron wanted the books out for the fall season come Helsinki or high water.
Because everything was running late, we had the cover done long before the stories were done. It was a nice cover by Mike Zeck & Phil Zimelman, with a central image of Spider-Man surrounded on four corners by four of his enemies: Dr. Octopus, the Lizard, the Vulture, and Venom:
We had a Dr. Octopus story. We had a Lizard story. We had a Vulture story -- in fact, we had two.
We did not have a Venom story. Remember, this is 1994 -- Venom was at the height of his popularity as a Spider-villain, and we couldn't not include a Venom story, especially since we already had him on the cover.
Mind you, we tried -- we'd sent six different pitches by six different authors for Venom stories. Marvel rejected them all.
We're past the eleventh hour at this point, and in frustration, John and I ask Marvel to tell us what they want to see in a Venom story. They give us a sentence.
It's too late to try to find a writer, so I go home, write the story, and hand it to John. He tears it to pieces, rewrites from the ground up and gives it back to me. I tear it to pieces, rewrite it from the ground up, and eventually we have "An Evening in the Bronx with Venom."
There are parts of that story I know I wrote (like the fight scenes; John's forte is not action). There are parts of that story I know John wrote. There are parts of that story where I don't have the first fucking clue who wrote it.
And that's how I sold my first story. *grin*