For example; when you have a "A\MyBundle" namespace and a "B\MyBundle". The B version is only loaded in certain situation and is a child bundle of "A\MyBundle". In this bundle there is a controller and a view etc.
The view's filename is "index.html.twig" and you want to override some blocks in the B version.
You can not create the same file and extend the A one. Like:

{# A\MyByndle:Dir:index.html.twig #}

{% block something %}
   ... default markup goes here ...
{% endblock %}

{# B\MyBundle:Dir:index.html.twig #}

{% extends 'A\MyBundle:Dir:index.html.twig' %}

{% block something %}
    ... something different ...
{% endblock %}

This will throw a "reached nested level" error. Read something about missing the scope in the template loader. There are also some fixes out there, but this should be fixed and covered by default I guess?!

For now I made override the controller and action too, and use another filename for the "index" view but this should be possible in terms of overriding bundles.