Here's a great Batman tale from Marv Wolfman, Len Wein & Neal Adams, that comes with a nice background story.
Marv and Len put together a 1940's inspired script on spec, that involved The Caped Crusader and Robin involved in a dark mystery, and getting trapped in a Bill Finger-ish giant ping pong ball machine.
Editor Julie Schwartz rejected it though, as he was already thinking at that time of moving away from the goofy TV show inspired plots, and back to the Dark Knight themes which Wolfman / Wein's script was trying for, but not quite achieving.
So they darkened it up a little more, and showed the story to Dick Giordano, who showed it to Neal Adams, who loved it so much he spent the next few months working on it in secret, inbetween paying DC jobs.
After completing it, Nefarious Neal boldly ( and bravely ) presented the piece to Schwartz with the words: 'Here's your next issue, Julie.'
Schwartz hit the roof: 'How dare you do this? I didn't assign it! Wait a minute! I know this story! It's by Marv and Len........... Alright, it's a great story. It's the next issue.'