Saw this security blunder a while ago. Went onto some site and it showed me this username/password dialog (probably an apache's htpasswd or nginx one). Went away but returned quickly because I noticed I could see all content. Then I thought 'why the fuck not try?' so I dragged the auth popup thingy to the side of the screen and et voila... I could interact with the page as if nothing was wrong while the authentication popup was hovering above the page on the right!

I sat there giggling dramatically for a while.