I recently upgraded my WordPress database from 2.0.11 to 2.3.1.I was writing a post, clicked Save and I got a
WordPress database error: Table <tablename>.wp_post2cat doesn’t exist
I dug a bit on the Net and found it was a plugin, in my case Goosle Sitemap plugin, doing a query like
SELECT * FROM wp_categories
when this table is not anymore in WP 2.3.1.
Updating the plugin fixed the issue.