The reason why it is potentially a good idea to test that math works is human beings and computers don’t conceptualize math the same way and the line between known solid ground and buggy assumptions can be wide.

Meaning: Math != what your code is doing. There is an example of anti-intuitive testing and why a simplistic test that passes 99.9% of the time can lead to an exercise in hair pulling; Re: why Test::More?

And with regards to the original thesis: 20 extra seconds, nay, minutes!, of a test run is a *gift* if it prevents a single shipped bug. I have never once spent less than that, probably not less than an hour, diagnosing and fixing a bug in someone else’s distribution. Final note: adding tests itself is a form of thinking about the code concretely and not as a conception. That alone is frequently extremely helpful and extra tests can always be removed or factored into bigger tests as subtests.