While working on something else I had the same document open and minimized. When I returned to it about 30 minutes later there was a message in the autorecovery pane that said something like "Document repaired, error in the description of a bookmark". Sorry, I can't remember the exact wording but it had to do with a bookmark problem.

It hasn't reoccurred since I saved the repaired version, but I will wait a few days before I sound the all clear.