From Fail Safe
If you pile all these electronic systems one on top of the other, sooner or later you're going to get a faulty transistor or a damaged rectifier, and then the whole thing just shuts down. Even computers suffer fatigue. They become erratic, they break down, just like overworked people.
Excuse me, but you're overlooking one important factor: Humans control those machines. Humans can see an error and correct it.
I'm sorry, sir, but you are misinformed. The fact of the matter is that these machines are so complex, and the mistakes they make are so subtle, that in a real war situation, you might not know whether the computer was in error, or telling you the truth.