A Philosophy of Software Design

[Walter Bright]

Yesterday, I watched an episode of "Aviation Disasters". In it, the airplane had 
two pitot tubes for measuring airspeed. One fed the airspeed indicator for the 
pilot, the other fed the copilot's indicator.

The pilot's one started behaving erratically, showing a very different airspeed 
than the copilot's. There was no indication of which one was wrong (or if both 
were wrong). The pilot decided to trust his indicator.

The result was a crash and everybody died.

If the pilot's indicator said "NaN" then he'd have known which one was broken.