If you happen to get a Wordpress Error 28 when you are selecting posts, managing, or even trying to view the front of your site, there is Mysql error going on. (Duh).

Anyway, in most cases, check out the space available on your Mysql slice or partition, and you’ll probably find the culprit. (IE it’s normally out of disk space for tmp tables)
I know you’ll probably have plenty of RAM for temp tables, but Wordpress won’t do that apparently.