Before Valve roped the gang at Turtle Rock to come on over and make Left 4 Dead, the Half-Life developer was working on something else. Something with...fairies.
Yup. Not aliens, or cops, or super-soldiers, or valiant knights, or space marines. A game about fairies. Valve's Erik Johnson tells PC Gamer that the game, a "weird prototype" of a thing "based on movement and mouse gestures", never got close to being a serious project (and was likely one of many prototypes kicking around the offices), but was still useful for what it did and didn't do right.
"That was a useful failure to us," adds Valve boss Gabe Newell, "because it was so clearly dumb that it made us say, ‘OK, what are we actually good at that we can do instead?'"
They could do Left 4 Dead. So did Left 4 Dead. And we're all better off because of it.
Valve were making a fairy RPG before Left 4 Dead [PC Gamer]