How to fix broken Categories in Wordpress MU

Recently I found weird behavuior with categories in WPMU, they were there… But not showing on the site, or in some areas of the admin area.

I discovered, through chance that wordpress MU was storing the categories within a users session then taking that session data and displaying it in the browser.

The reason, that the categories were not showing, was because we had almost no disc space on the partition that held sessions

(that r possibly akismet breaks them, but we turned that back on and they are still working).