> In most testing frameworks, you’re expected to write a lot of low-level tests and few high-level tests. The reasoning is that end-to-end tests are slow and expensive and only cover a tiny amount of the program’s state space. It’s better to focus on testing all the parts in isolation versus testing that they all work together.

> In practice, global correctness does not follow from local correctness. It’s possible for every part to be individually rock-solid but the system to be broken as a whole. One way this happens is with feature interaction (FI) bugs, where different features combine to break global properties.