Okay, so I've been rewriting and writing Chapter 20, and I pulled it up on Google Docs on Wednesday with an eye toward finishing it up. Anyway, I pull it up and scroll down to the last line line and think to myself, "What comes next? What comes next?" And then I looked at that line again and realized that it was finished.
Except that it isn't. It's still just a draft. It's a decent draft, but it needs polishing. It is also a tiny thing, barely more than a thousand words, and I suspect that the polishing will mostly involve spinning out some of the notions already extant in it at greater lengths. (This is something that is often true of my initial drafts. Most writers spend their time cutting. I end up expanding.)
The problem that I find myself faced with now is one of deciding whether to start this revision right away or perhaps soldiering on through the rest of the new ending first and going back to revise the whole section once that's done. I'm not sure. Both approaches have their plusses, and I have to assess which method will serve the book better. My guess would be that pressing on is the most likely winner, but I never know until I find myself at work on something.
Part of the fun is in the surprises.