It seems when the URL of a post has a hash at the end, the default icon query string isn't used.
http://i.imgur.com/pBs2T.png
http://www.google.com/s2/favicons?domain=techcrunch.com
EDIT: I should point out that the accuracy is poor compared to most browsers.
It seems when the URL of a post has a hash at the end, the default icon query string isn't used.
http://i.imgur.com/pBs2T.png