A lot of the failures might not be obvious failures that are catastrophic, but rather something like failiure to meet spec. For examples, analog circuits that have lower SNR, less gain, lower linearity, etc. Also, lower lifetime, which is something you will only notice after a long time (A cap that is rated to 85C won't suddenly become a short at 86C, or even at 90 or 100C, but it won't live that long...)