In Chrome (though I may have seen similar behavior in other browsers), type in http://www.google.com. You will always be redirected to https://www.google.com. Usually when I do this, I get the good old green lock in the upper left indicating everything is secure. But SOMETIMES I get the the grey lock with the little yellow warning that indicates when you click on it, "However, this page contains other resources which are not secure." It seems to go in runs where I get the good green icon for a while and then I'll get stuck with the grey lock for a while. This behavior is not specific to one website. For instance, bankofamerica.com does the same thing. Does anyone have any idea why this happens?
P.S. It's not simply the standard references to non-https links in the source because using the developer tools all requests are shown to be https except for the initial, redirected request.