It is a way of saying that more often than not, testing will actually not cover all the bugs that you originally care about. Even when writing tests we are making assumptions about cases that "should not happen" and that actually do. And testing is not taking care of these.
It's just something to keep in mind when coding: testing as we generally do it is not a guarantee of code correctness. Other methods provide that, but testing does not. It gives a level of comfort that of all the cases that you can think of, every one of them works as expected...
It's just something to keep in mind when coding: testing as we generally do it is not a guarantee of code correctness. Other methods provide that, but testing does not. It gives a level of comfort that of all the cases that you can think of, every one of them works as expected...