The bug was discovered by scanning the source code with a custom utility designed to detect instances where variables were incorrectly included in strings, as was the case in 0037250.

The minor issue is with logging data—in the email_api.php file, line 1422:

        if( $t_email_data === false ) {
            $t_email_sent = true;
            log_event( LOG_EMAIL_VERBOSE, 'Message $t_id has already been sent' ); <---
        } else {
            log_event( LOG_EMAIL_VERBOSE, 'Sending message ' . $t_id );
            $t_email_sent = email_send( $t_email_data );
        }