When I write a test against existing code like this, it’s nice to see that all is well—but I’m naturally skeptical. Particularly, once I have a lot of tests running, I’m always nervous that a test isn’t really exercising the code the way I think it is, and thus won’t catch a bug when I need it to. So I like to see every test fail at least once when I write it. My favorite way of doing that is to temporarily inject a fault into the code,
