Messed up upgrades and Tags
I upgraded to Wordpress 2.3.1 yesterday and do to some earlier failed upgrade all my post categories got messed up. Luckily I only have about 300 posts so re tagging everything wasn’t that painful. But word to the wise, if your running Wordpress from SVN use a branch, it was following trunk in the paste that got my database messed up in the first place.
I would recommend following WP-Hackers somewhat closely, since it would have informed you of some drastic change. I personally, didn’t have this happen to me.
PS: I really do think 300 posts is a lot.