What do you think about when you are driving a car? Are you concentrated on driving the whole time and pay attention to other road users? I don't think so... It's similar with a production line operator. It is impossible to be focused on your job the whole time and not make any mistakes. Those incidents are by many called human mistakes, that are a result of employee's carelessness and many managers/engineers think, that defects caused by that, are employee's fault... They are completely mistaken!