tl;dr; Checking your invariants matters. It’s easy to write code that violates an invariant if you’ve forgotten to verify it. When in doubt, make the constraints and assumptions you are working within explicit rather than implicit. This will help future-you avoid all sorts of pain. The Classic Missed Invariant Have you ever seen a test fail in CI or a function fail in production only to have a teammate (or yourself) declare “… but it works on my local!