This site (meta.codereview.stackexchange.com) seems to be missing a favicon. This affects not only browsers but also the inbox of the StackExchange dropdown:
I messed up. I have made a new favicon, should be in the next build
Actually the favicon does exist, but it's in the wrong format. Apparently it was saved in Photoshop's format rather than as an icon, so the browsers can't display it.