When one, or even N balls turn out to be labeled Z, the model (due to the initial zero probability on Z) continues insisting that the balls came from one of the initially-specified hypotheses.
If Pr(ball labelled Z | urn) = 0 for all urns under consideration then Pr(ball labelled Z) = 0 too, so the model tries to evaluate 0 / 0 and crashes.
If Pr(ball labelled Z | urn) = 0 for all urns under consideration then Pr(ball labelled Z) = 0 too, so the model tries to evaluate 0 / 0 and crashes.