>You do coverage testing, which would have found the missing date -r path.

The original coreutils test suite didn't cover the -r path. The same bug would have not been statically discovered in most programming languages, except perhaps the highly functional ones like Haskell.

>You do proper code review, which would have found the missing date -r path.

And in an ideal world there would be no bugs at all. This is pointless -- we all know that we need to do a proper code review, but humans make errors.