1) Even before you open the document on your computer, you realize that you're going to have to research something you thought up until that second that you already knew.
2) After spending most of the day on research instead of writing, you find out that in order to make the novel remotely plausible you a) actually need to move the hero to a different location and b) actually need to set the entire novel in a different year.
3) While going back over the novel to correct the place and time, you discover that you've unintentionally stricken the hero with cognitive dissonance of epic and ridiculous proportions. This, of course, is not the problem you want the character to have. And naturally, fixing it requires going back through three chapters, then eight more to make sure you haven't missed anything.
4) And then you can finally get to work, in the last half-hour of the day before you have to stop writing to start dinner and fetch the kid, only to realize that now you can't remember exactly what you planed to add next anymore. Not that you have time.