Happens to me ALL THE TIME. I mean literally, ALL THE TIME, as in, I have never, ever been able to successfully complete a single, standalone book. Not ever.

There are lots of "NaNo Rebels" who write collections of short stories and all kinds of other things. I say, 50,000 words is 50,000 words. That's a heck of a lot of writing.

Personally, I count my accumulated words in November regardless of which book they end up in, so long as they're all part of the same project. But my November writing usually ends up all in the same book anyway, and I just don't get to the next book in the series until later. 50k isn't enough for a single book for me anyway.

Cheers,
Michelle