Returns an array of $form_state keys that shouldn't be cached.
function form_state_keys_no_cache() {
return array(
// Public properties defined by form constructors and form handlers.
'always_process',
'must_validate',
'rebuild',
'rebuild_info',
'redirect',
'no_redirect',
'temporary',
// Internal properties defined by form processing.
'buttons',
'triggering_element',
'clicked_button',
'complete form',
'groups',
'input',
'method',
'submit_handlers',
'submitted',
'executed',
'validate_handlers',
'values',
);
}