"Anybody can build a system that works when it works, but it's how it works when it doesn't work that counts.", or some variant thereof, is a classic engineering aphorism.

I have heard this called Nebel's law. Might that be correct? -- TomAnderson 

''Probably no, google shows up no relevant hits except for this one. Spelling of Nebel?''

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I'll note that a system that never works qualifies vacuously as a system that "works when it works".  How shall these 'count'?

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Note that we're not necessarily talking about failure states (although this applies to them as well).

It's a case of "What happens when this _isn't_ such a good match?", or "What happens when a HappyCollision _doesn't_ occur?".

-- WilliamUnderwood

Perhaps a discussion of GracefulDegradation would be appropriate?  But that applies mostly to load-bearing and partial failures, I think.  I suppose you'd need to look into the different services and systems built upon them to see what counts as good or bad.  How should a security system work when people forget or lose passwords, or lose their smartcards?  

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See also GracefulDegradation, FaultTolerance, CategoryException, LimpVersusDie