form_get_errors() is NULL when calling it in an AJAX form callback
I found a way to make this work.

I haven't fully investigated this for issues yet but it works with and without javascript enabled.

My use case was only running  my validation if other validation passed:

/**
 * Custom validation function for some form.
 */
function MODULE_NAME_custom_form_validate(&amp;$form, &amp;$form_state) {
  if (isset($form_state['storage']['errors']) &amp;&amp; !empty($form_state['storage']['errors'])) {
    return;
  }
  // Do our extra validation here since other validation passed.
}