After a recent upgrade of WordPress on Debian, my blog started spewing these errors everywhere, despite having run the upgrade script. The problem is that there are 2 upgrade scripts. Why they don’t both get executed is beyond me.
To fix it, run http://example.com/wordpress/wp-admin/upgrade.php (Not, http://example.com/wordpress/upgrade.php, which you've already run).
Then, be a Good Samaritan, and help fix the 19,500 blogs broken by this error.
