Just an Improvement I want to share.

A feed with an encoding problem filled our logs without telling the feed url. Thats why I changed the corresponding line in /lib/simplepie/library/SimplePie.php, line 1386 to get more information about the mean feed:
$this->error = sprintf('This XML document is invalid, likely due to invalid characters. XML error: %s at line %d, column %d, encoding %s, URL: %s', $parser->get_error_string(), $parser->get_current_line(), $parser->get_current_column(), $encoding, $this->feed_url);