public function any_error_messages() {
        $errors = array();
        foreach ($this->get_all_slots() as $slot) {
            foreach ($this->for_slot($slot)->get_sub_question_ids() as $subqid) {
                if ($this->for_subq($subqid)->differentweights) {
                    $name = $this->for_subq($subqid)->question->name;
                    $errors[] = get_string('erroritemappearsmorethanoncewithdifferentweight', 'question', $name);
                }
            }
        }
        return $errors;
    }