In dev mode, a 404 generated by the assetic controller removes the is_logged_in cookie as a result, in subsequent page loads you are still logged in but you get the top menu as if you are anonymous and if you try to log in you have a form token exception.
Steps to reproduce:
1. In Chrome, open the devtools (make sure the "Enable JS source maps" option is enabled)
2. Go to the homepage
3. Log in
after being redirected you can see in the network tab that a 404 appears for /js/jquery-1.10.2.min.map and in the header, you can see:
By the way, all headers seem to appear twice...