Have you ever heard of concurrent operations?

While reading some code in a legacy codebase: this should fetch the last user

$user = Userlib::createnew_user('0', $_POST["str_Username"], $_POST["str_Password"], 
  # neverending params list);

$user_id = Userlib::list_user(" 1=1 ORDER BY id_user DESC LIMIT 1 "); # OMG