ie your file 404.twig
should be
{%use "/my_blocks.twig" %}
{% block view_404 %}
{% set view = "404" %}
{{block('menu')}}
{% set tail = '404' %}
{{block('breadcrumbs')}}
<div class="no_data"><b title="data not found">{{ translate ( '404_message' ) }}</b></div>
{{block('breadcrumbs')}}
{{block('search')}}
{{block('online')}}
{% endblock %}
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because it calls 4 blocks (menu , breadcrumbs , search , online) that are defined in my_blocks.twig
{% block view_404 %}
{% set view = "404" %}
{{block('menu')}}
{% set tail = '404' %}
{{block('breadcrumbs')}}
<div class="no_data"><b title="data not found">{{ translate ( '404_message' ) }}</b></div>
{{block('breadcrumbs')}}
{{block('search')}}
{{block('online')}}
{% endblock %}
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