 Ball 1 was removed at the first stage. Ball 2 was removed at the second stage. Ball n was removed at the nth stage, and so on ad infinitum. Since each ball has a label n, and since each label n was removed at the nth stage of the supertask, there can be only be zero balls left in the jar at the end after every stage has been completed. One can even identify the moment at which each of them was removed.