Comment by noduerme

yeah but because the code is segmented into very discrete functions... and it's quite easy to see their call chains...

Well, I get it. You're right in principle. The truth is, though, I have had to rewrite software from scratch in new languages every 10 years, and these sorts of bugs only appear a few times per decade. Once a piece of central business logic works, it usually works forever; a change to that central logic will of course require changes and tests across the whole system, from user inputs to annual reports, but that can't be helped. I suspect you'd then have to rewrite the unit tests, too.

It's probably true that when I die, most of my software will slowly wind down and eventually be abandoned. But also that's probably true even if I were to write a lot of tests.