After installing a brand new sf 2.6.1 ( with sf installer), I wanted to try out the new error page function, with the local/app_dev.php/_error/500 and the local/app_dev.php/_error/500.json urls, but I got a general 500 error:

Catchable Fatal Error: Argument 2 passed to Symfony\Bundle\TwigBundle\Controller\ExceptionController::showAction() must be an instance of Symfony\Component\HttpKernel\Exception\FlattenException, instance of Symfony\Component\Debug\Exception\FlattenException given