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

nasty error.

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.