Some sites such as Tumblr will return a content type of text/xml for their HTML content if we send them a request in which text/xml has a higher priority than text/html, which will make the autodiscovery of the feed to fail with our version of SimplePie.
Another example: autodiscovery of the RSS feed of http://blog.theoldreader.com/