I have been using Stack Overflow for a few months now. Yesterday, after I logged out from the Stack Overflow site, I again clicked on the Log In link, then the "Login with Stack Exchange" image, and I was able to Log in with out entering the my user credentials.
It needs to be told that I had not clicked on the "Remember my Password" option. And this has happened while using Firefox. Finally I cleared the browser cache and problem was gone.