At the bottom of: http://symfony.com/doc/current/templating/debug.html

We say:

By default this means that the variables will be dumped in the dev environment but not the prod environment.

This seems to suggest that dump() will just do nothing in prod. But actually, as far as I can see, this function is not available at all, causing templates to fail in prod. That's because the DumpExtension from DebugBundle that provides dump() is not used, because DebugBundle is not enabled in the prod environment.

We should clarify that leaving dump() in your code is a big problem :).