

Obviously, but QA is based on statistic.
You need to destroy or damage the item tested If you want to check how a steel cable hold, You test it to the limit but after that you cannot use it anymore. So you get another one with the same specification. But if this one is defective for some reason, you would never know it until it fail.
Obviously.
Ok, so you have two cables, one principal and the other one as redundancy. The main one snap and in doing so damage the backup cable (or any other part that must use the backup cable) more than what falls within safety limits, so even the backup cable (or mechanism) fail. Then ? (I personally see something like that btw)
Look, I am not saying that there could not have be some problem with lack of maintenance, it looks this way, but that even fail safe and redundancy have limits to what they can do. And since some tests are destructive for the tested item you can only trust the fact that every item would be built the same way at the same quality level, which do not remove the possibility that one item end up being defective.