Somehow, categories (and in one case, DEFAULTSORT) move from the bottom of the page and get inside a reference.
I'll very roughly translate for you the comment of Drongou, who tried to replicate but didn't manage to:
<<I can't reproduce this bug (on Mac OS X) but what they have in common is the suppression of a template at the top of the page. For the reference, I have tried to manipulate one several times but that didn't lead to anything. Since it's always the first reference which is affected, I think this [...] is probably due to the template being deleted but not in an usual way. Regards.>>