Approx. line 259 in the catalog/includes/modules/payment/authorizenet.php file should read:
tep_redirect(tep_href_link(FILENAME_CHECKOUT_PAYMENT, 'error_message=' . urlencode(MODULE_PAYMENT_AUTHORIZENET_TEXT_ERROR_MESSAGE), 'SSL', true, false));
change it to read:
tep_redirect(tep_href_link(FILENAME_CHECKOUT_PAYMENT, 'error_message=' . urlencode($HTTP_POST_VARS['x_response_reason_code'] . '-' . $HTTP_POST_VARS['x_response_reason_text'] . ' ' . MODULE_PAYMENT_AUTHORIZENET_TEXT_ERROR_MESSAGE), 'SSL', true, false));
and the error code and description will be displayed at the top of the checkout_payment page upon the return from authorize.net.