Joomla Session Error
It should not happen often, but an erroneous message could occur from the frontend and/or backend. When a web page is browsed, the message could be a duplicate session_id. In this case, repairing the database may be required.
Meanwhile, when you login to the backend, there may be an error message with the following text:
Warning: mysql_query() [function.mysql-query]: Unable to save result set in /libraries/joomla/database/database/mysql.php on line 223
With either case, you should open up your mysql prompt or phpmyadmin and get to the database and session table. Then, repair the table. With any php / mysql application, you can select one more tables and analyze, optiomize and repair table(s) to keep a database in good check.