Drupal form alteration cache issue

Drupal integration, in hook_form_alter(), injects some non Drupal Form API form as raw markup. To make the validation (errors and preserving the data) work on those, we rely on the fact that it's not cached by the form system of Drupal. For out of the box Drupal installations, it's true, but with lots of contributed modules installed, this behavior might change easily.
What we need to do is to specify $formValues['no_cache'] = TRUE; explicitely.