I accidentally clicked on signup while selecting stackexchange option on login page, results in opening of stackexchange login form as well as signup form..
When I entered my credentials to login, it threw me page not found error.
I've attached a screen below for clearing the scenario..
1) Click on log in
2) On the log in page, click on stackexchange block, the very first one, while it's loading, instantly click on signup as well
3) Two forms will be visible on the same page, now try to log in with your valid credentials and you will see the page above..
Youtube Link for Video Reference - http://youtu.be/w-7I5oi-ntU