Recently I was in the private beta for Game Theory, however it didn't take off and the site was closed down. When the site was closed, it was automatically removed my from my favorites in the drop down menu on the desktop version. The android app however did not remove it from the favorites and now is reporting it as an unknown site. It seems like the site should have been removed from the desktop and android versions at the same time.
When I went in to edit the pinned sites on my android app they did not appear and were gone when I canceled. So the app will correct itself but you must go in to attempt to edit first.