Drupal Do Something on 404 and 403

This is an example of implementing hook_page_build() to run an arbitrary function on 404 and 403 pages. I used hook_page_build() here, because the HTTP header status wasn't reliably available in previous hook calls.

/**
 * Makes sure _MYMODULE_function() runs on 404 and 403 pages
 * for anonymous users. 
 */
function MYMODULE_page_build(&$page) {
  if (user_is_anonymous()) {
    $status = drupal_get_http_header('Status');
    if ('404 Not Found' === $status || '403 Forbidden' === $status) {
      _MYMODULE_function();
    }
  }
}