Steve calls it "timey-wimey creativity." I call it something else. The label doesn't really matter; the important thing is that it's the process of iterative discovery with a creative work. You write one draft, and even in the middle of that draft you discover goalposts drifting downfield, so you hurry after them. Sometimes you try to chase them down and drag them back into position, but most of the time you're better off just moving the game to where they are.
The one thing to not do is stay frozen, as it were — to assume the original idea was and has to be the best idea, to try and do justice to some Untainted Primal Vision that might not even have been all that good to begin with.
Most of the way I see this play out is when someone comes up with a great idea for a story — the Big Hook, I guess you could call it. Sometimes it's not even the idea that drives the story; sometimes it's just some Cool Thing in the story, whether or not it belongs there or even suits the story as a whole. (We talked about this problem before.) In short, it's not the idea that is the source of the problem, but the attachment to the idea, the unwillingness to see the story as the artifact of a process, with the emphasis being on the process and not the artifact.
I suspect many people do not initially recognize how their work is a moving target, and has to be one, because they also don't recognize that they themselves are moving targets. Guess what, dude -- YOU YES YOU are the artifact of a process, too! And every artifact you yourself leave behind is part of that general movement! Shocked yet?
But. I also think often of a friend of mine who over the course of something like ten years tried to begin — not even write, just begin — the same novel again and again. He never got it off the ground, let alone finished it, because every time he started to work on it he felt like his sensibilities had changed enough that everything he'd produced no longer fit the bill and had to be scrapped. Where someone else wouldn't even have noticed such a thing, he couldn't help but notice it to the almost total exclusion of everything else.