Comment by tapete2

> wasn't the very concept of "libraries" created to not have to think about what exactly the code does?

Let's say you need a FFT implementation. You can write that from scratch, or you can use a library. In both cases you should use tests to verify that the code calculates the FFT correctly, and in the library case you should read the code to make sure that it works correctly and does not omit and edge cases (e.g.).