Scott --

While designing the Javelin Jet we wrestled with single-point failures all the time.  The electrical guys were constantly telling us "but the systems are electrically separated, the functions are on different cards, each board has its own power supply, there are metal dividers between the boards -- why do you care that it's all in the same box?"

We responded "everything goes in one cable to that box and what if that gets cut, a fire caused by something else could take out the entire box" and so forth.  Your example of water dripping on the box or the spilled coffee in the movie "Fate is the Hunter" are examples of external factors causing multiple failures in distributed systems that happen to all reside at the same physical spot.

In my little Lancair, however, I don't think physical separation of my electrical systems will protect me from a fire on one side of the cockpit.  It may help me, though, if the canopy starts leaking or if a passenger barfs on the circuit breaker panel...

The bottom lines are:

1)  Do what you can, but realize there is a point of dimishing returns, but ...
2)  Murphy is out there ...

- Rob Wolf