After upgrading this site to the 2.3 release of WordPress I began receiving a database error every time I tried to save a new post or make a new comment.

The error looked something like this: “WordPress database error: [Table 'wp_post2cat' doesn't exist]” and then usually a complaint afterwards about header output already being sent.

After some research I figured out the problem was with one of the various plugins I was using on this site. The actual culprit turned out to be the 2.7.1 release of the Google Sitemap Generator. A quick upgrade to version 3.0.2.1 and the error went away.

If you’re also having a similar problem with your WordPress installation, I suggest you start troubleshooting the problem by deactivating all of your plugins and then re-activating them one at a time until you find out exactly which one is causing the error.