Here's my original set of collections, which was resulting in sync failures:
Attachment 199884
Here's my new set of collections:
I suspect that it was the combination of the Fiction (219) and Fanfiction (192) colls that was doing it, though there were other reports that they didn't get problems until the books were in the thousands. Alternately, it might've been some glitch with the collection that was resolved by nuking them entirely and re-generating them.