This error is I think unique to Vanilla. Experienced it here, but works ok on main ES 2.1 test site...
If you view a photo album and look at a particular photo it pops up in a large popup which includes repost, like and share options... if you're not logged in and try one of those, the login popup appears behind the photo popup - so it's totally invisible and the user won't know it's there.
On the main ES RC2 demo site, the login popup manages to appear above the photo popup.