Jackster,

No insult intended.
But humans are not perfect; far from that. Neither are machines, for that matter.

I was talking with Dub Jett, who told me that a very expensive, Israeli made Iscar tool (http://www.iscar.com/), had broken in his hands, while manufacturing a part for a Jett engine. And he is a professional; not an amateur.

A human is more likely to slip, than is a CNC controlled lathe, or mill.

It is also true that amateurs are more likely to slip, than experienced, trained machine shop workers.

And after all; a CNC machine is programmed by a human and some CNC controlled mills even record his movements, while manufacturing a part, and are simply repeating them again and again, to manufacture subsequent parts.