ABSTRACT
The faults that we do not know about, the unanticipated faults, are the ones that tend to cause failures. Not knowing about a fault means that we will not employ any techniques to cope with the effects of the fault, and, of course, those effects could be extensive. Imagine (or perhaps remember) a situation in which you were unaware that a disk could fail, but the disk did fail. Most likely, you lost all your data. Had you known about the possibility of the fault, things would be very different. This
point seems rather obvious, but ignoring the point has resulted in a lot of unexpected and sometimes disastrous system failures.
