I have a Web page in which I embed a Google docs viewer in an iFrame
I second Zhami's comment on the question. This happens when you (the user) has a google account whose login has expired. The login page shows them Email: field already filled in with your email but asks for your password. If you then click on "change user", the email field goes back to empty and then you can go back to the iframe page and you will see the document.
I think maybe we can say this is a google bug?