The two jobs

A test suite has two jobs. The first is to show that the code works right now — you write it, you run it, it goes green, and you believe the thing you just built does what you meant.

The second job is the one that pays for the first. Six months from now somebody changes a line in that file, and the suite is the only thing standing between that change and production. It has to notice.

Almost every suite does the first job. A surprising number do nothing at all about the second, and — this is the awkward part — they look identical from the outside. Both are green. Both have a coverage badge. Only one of them is load-bearing.