This gives me an interesting thought, in the story all lower levels of the simulation are identical copies from the top level, in every way. The first time that they create the black ball the top level universe will diverge from the rest (they will look behind and there will be no ball) at this point there will be no more perfect copies of the original universe.

Also I wonder about the ethical connotations of turning off the machine as was suggested at the end. At what point is life "life". If the simulated beings are truly perfect, would turning it off be the largest genocide ever committed?