Okay, this is really distressing. A site I've done a little side work on is showing this error message when you go to the home page:
jtablesession::Store Failed
DB function failed with error number 1062
Duplicate entry '1-' for key 'whosonline' SQL=INSERT INTO `jos_session` ( `session_id`,`time`,`username`,`gid`,`guest`,`clie nt_id` ) VALUES ( 'utahnkimud7fm700kveeg22vr5','1280713226','','0',' 1','0' )
Fatal error: Allowed memory size of 67108864 bytes exhausted (tried to allocate 72 bytes) in /home2/cliftoni/public_html/libraries/joomla/error/exception.php on line 117

Any idea what is going on or how to fix it??

It is trying to insert a duplicate value into an indexed column,

and the username is empty in the query.