As you may know, I am a pantser. I make most of the story up on the fly, especially the details. Anyone who writes this way though, should know that they will have to go back and fix a lot of things in their early writing, because you changed something or invented something halfway through the story. If you don’t… it may become obvious that you are just pulling everything out of thin air, and it’s very important to keep up the illusion that you know what you’re doing!
For example, in the book I am currently listening to, the author introduces some aliens, and describes them in some detail. But a couple scenes later, these aliens now have a new feature–a ‘collar’ that changes color to show their emotions. This was not mentioned at all during the original description of the aliens, so at first I thought it was something they were wearing, and I didn’t know why they’d put it on between scenes. What it must be, though, is something the author made up later, and forgot to go back and put in the original description.
That may seem like a small thing, but it made me notice other instances, and now more and more the story is seeming very ad hoc to me.
Just a reminder to make sure you rewrite! It’s important.
Jonas David 
Verisimilitude – there, I know one big word. Keep at it –