Machines are not infallible. They sometimes behave in unexpected ways or fail,
Reminds me of a conversation I had with a Boeing instructor pilot with regards to the design philosophy of the Boeing 787. He said the 787 was designed on the basis it will be flown by incompetent pilots. Thus the sophisticated computer systems will do their best not to allow the pilot to crash. With huge orders for the 787 coming out of the Asian region you can see his point.