Well the title says it all.

{{ form_start(form) }}
  {{ form_errors(form) }}
  {% for subForm in form.components %}
    {{ form_row(subForm) }}
  {% else %}
    no components
  {% endif %}
{{ form_end(form) }}

If form.components is empty, it is not considered as rendered at the time form_end is called, causing its "container" markup to be rendered. This is especially annoying when you have expanded choices with an empty array of choices.

This is partly related to: #17579