I just ran a composer install from within my project. This error is now produced when trying to do anything within the app:
ServiceNotFoundException in bootstrap.php.cache line 2119:
You have requested a non-existent service "fos_user.doctrine_registry".
I have "friendsofsymfony/user-bundle": "~2.0@dev", and "symfony/symfony": "2.6.*", in my composer.json. Let me know if there's any more context I can provide.