I agree that it sounds like a bug. I've had files in OpenOffice.org that had to be "recovered" for reasons usually unknown to me, but never had this happen (and I always open both a spreadsheet and a word processing document at start-up time automatically).
Possibly something like this might be avoided in future by saying "yes" to "discard changes." That's my policy whenever I wonder: "What changes are they talking about?!"
An automated backup routine can be useful, too, though it's no guarantee of anything much, with files that change often. Still, better than nothing.