[Swizec]

If it takes you 3 days to write a test that covers a once-in-a-million condition and your service gets 2 requests per day, it will take you about 500,000 days to hit that condition once.

You've increased test coverage, but was it effective? Eh probably could do something more useful with your time.

(yes this is a contrived example, adjust numbers for your situation)

[colechristensen]

Depends on what the consequence of that failure might be. It could be anything from completely unnoticed to ending somebody’s life, rarity is only one dimension of risk.