Actually, computer systems can also "not know what they are doing". They are usually designed around the assumption that they have a complete picture of what is going on, and complete control over the process. When either assumption (or both) fails, the computer will depart its zone of expertise rather abruptly. A well-designed car or train control software can often recognize the problem and shut down until help arrives; an airplane might not have that luxury. ("Is there a software engineer on board?")