Book-syncing marks all text flows as "non-primary."
Once again I found that, all of a sudden, I was getting "overset text" errors in a document that's set up with one continuous text flow and automatic page-adding.
This turned out to be because all the flows in my 300-page book were de-flagged as "primary," apparently by a book-syncing operation. No idea what triggered it in this instance, since I've been frequently syncing all the documents in this book as I work.
I selected the text frame on a page to see if, once again, another "shadow" frame had been added behind it. Nope. There is still only one frame per page; it just isn't "primary" anymore. I checked the master pages from which the pages were derived, and they still have one flow, which is marked as primary. The frames are all still interconnected as one flow, too.
This is a really, really tiresome defect. I haven't figured out the pattern to it, because it doesn't always happen and it's not obvious when it does.
