Stephen King is the most successful, and one of the most prolific, horror writers of all time, and yet he'll readily admit when something he's written simply shouldn't be put into print.
You can't blame the guy for batting less than a thousand, it's honestly quite remarkable that Stephen King still has so many amazing stories in him considering he wrote his first novel The Aftermath when he was just 16 years old.
The Aftermath tops the list of stories never to be published because King considers his first novel, which is about an apocalyptic survivor trying to sabotage a military force made up of aliens in disguise, a juvenile attempt at novel writing.
The manuscript rests under lock and key in the Fogler Archive at the University of Maine alongside a short story he helped his son Owen write called I Hate Mondays, which is, strangely enough, not about Garfield the cat:
I Hate Mondays, a short story about two misfits who are kidnapped by a goon named Doctor Mindbender and forced to give up the combination to a safe.
See the 7 Stories Stephen King Refuses To Publish at mental_floss