> When talking about very low probability failure modes, it may be 
> important
> to consider the possible failures that an additional component (or a 
> few)
> introduces. It may be mounted wrong, for example, in hand-mounted 
> very
> tight SMD boards that can't have meaningful a silk screen.

True.
In another lifetime I worked for the (then only) national Telco.
People proposed installing remotely operated fault isolation devices 
at the boundary to customers' premises so you could test to see if the 
fault was 'in' or 'out'.
BUT an analysis showed that the fault rate liable to be introduced by 
the isolators exceeded the gains made by locating the faults.

> I don't think there is a mode that cannot fail :)

Indeed.
Even "stay in bed all day" while usually safish, cannot be relied on 
:-)


        Russell




 

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