When you're editing a WIP and you end up getting ideas for one (or five) other novels you could potentially write. (This has happened to me many times.)
Bad news: You can't write it now, you've got to finish the thing you're working on. (Yes, I'm working on it as I'm writing this post to take a break from reading. Multi-tasking.)
Good news: You'll have something to write when you finish. (By that measure, I have probably six different projects waiting for me. Good grief.)
Uh...inspiration, FTW? Yep, yep.