Here's the setup:

1. Use Flex
2. Install SensioFrameworkExtraBundle
3. Run bin/console debug:container --types

The issue is that SensioFrameworkExtraBundle registers a service that relies on symfony/security. Since security is not present, this service is removed: https://github.com/sensiolabs/SensioFrameworkExtraBundle/blob/master/DependencyInjection/Compiler/OptimizerPass.php#L27

However, I believe that the debug:container command uses the container before some of these removal passes. So, this service is still present when we check to see if the class exists: https://github.com/symfony/symfony/blob/master/src/Symfony/Bundle/FrameworkBundle/Command/ContainerDebugCommand.php#L248. The autoloader causes the class to be loaded, and error because the parent class is missing.

So... long way of saying, we just need a safer way of checking to see if the service id is an existent class.