Fatal error: Duplicate entry '0' for key 1 query: INSERT INTO sessions (uid, sid, hostname, timestamp) values(0, 'sessionIDisdeleted', 'IPaddressdeleted', 1109177779) in /home/asturlib/public_html/includes/database.mysql.inc on line 125
This was related to somebody trying to login and a >5000 rows session table ( never cleared by cron?). Nothing but this error came up site became inusable.
I emptied the session table and that fixed it, but I wonder what was behind it. That's why I marked as bug.