Unknown wrote: The variation cannot be stopped, since no two physical things will ever be exactly alike.

Assuming we can neglect position in space and movement vectors, different individual elemental particles (e.g. electrons) are already exactly alike. Individual atoms are frequently exactly alike, and the same can be said for molecules. We don't have a way to precisely copy macroscopic objects yet, but that might change with further technological development. Do you think that Molecular Nanotechnology is fundamentally impossible? If so, why?

And even if you can't build machines with absolute precision, that doesn't mean you can't build them in a way that will prevent them from being actively destructive. If you're worried that machines you build may become destructive because of construction errors, add a test stage to the construction process, and/or self-test components that will cause the machine to shut down harmlessly when an error is detected. Noticing random corruption isn't very hard, from a computer science perspective. There are plenty of cheap and effective hash functions available. By adding a bit of redundant data, you can drive the probability of a random corruption being unnotived down to infinitesimal levels.