public function find_problems() {
        global $DB;
        static $answer = null;

        if (is_null($answer)) {
            $categories = $DB->get_records('course_categories', array(), 'id');

            // Look for missing parents.
            $missingparent = tool_health_category_find_missing_parents($categories);

            // Look for loops.
            $loops = tool_health_category_find_loops($categories);

            $answer = array($missingparent, $loops);
        }

        return $answer;
    }