As far as I can tell this apache error is generated for every link in my wordpress blog:
[Wed Aug 12 16:43:53 2009] [error] [client 188.8.131.52] PHP Notice: A feed could not be found at http://rococochocolates.com/blog/feed/ in /var/www/vhosts/rococochocolates.com/httpdocs/blog/wp-includes/class-simplepie.php on line 1680, referer: http://rococochocolates.com/blog/
I don't have the simplepie plugin as such, but I think this started after installing the FD Feedburner plugin.
I reported this to simplepie initially and their response was as follows:
It sounds like the plugin author should be using the force_feed()
configuration option. http://ref.simplepie.org/force_feed