I found this in the log of mailpress, a newsletter plugin for wordpress.
I bolded the W3 line, is there anything can and needs to be done? Everything seems to work, I just wanted to report this...
2011-07-25 05:37:59 000000 -- PHP [E_NOTICE] 8 : Undefined offset: 0 in /var/www/knightsenglish.com/wp-content/plugins/mailpress/mp-includes/class/MP_Mail.class.php at line 746
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - Image found : wp-content/plugins/mailpress/mp-content/themes/twentyten/images/mailpress.png
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - Disconnecting from knightsenglish_com (MP_Mail)
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - Disconnected (MP_Mail)
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - Connecting to knightsenglish_com (MP_Mail)
2011-07-25 05:37:59 000000 -- PHP [E_STRICT] 2048 : Non-static method W3_Config::instance() should not be called statically, assuming $this from incompatible context in /var/www/knightsenglish.com/wp-content/plugins/w3-total-cache/lib/W3/Db.php at line 77
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - Connected (MP_Mail)
2011-07-25 05:37:59 000000 -- MAILPRESS [NOTICE] - :::: MAIL SAVED ::::
2011-07-25 05:37:59 000000 -- **** End logging **** MP_Mail_mailpress *** level : 8191 **** status : TRUE
http://wordpress.org/extend/plugins/w3-total-cache/
###edit###
removed the bold code since it doesn't work within a code section... anyway, check line 6 please.