A little buried since the overarching focus is on consistency but I found these two paragraphs from the blog post really relevant points as well: "You need to

A little buried since the overarching focus is on consistency but I found these two paragraphs from the blog post really relevant points as well: "You need to develop a good sense of how the service is used in practice (i.e. by users). Which endpoints are hit the most often? Which endpoints are the most crucial (i.e. are used by paying customers and cannot gracefully degrade)? What latency guarantees must the service obey, and what code gets run in the hot paths? One common large-codebase mistake is to make a “tiny tweak” that is unexpectedly in the hot path for a crucial flow, and thus causes a big problem. You can’t rely on your ability to test the code in development like you can in a small project. Any large project accumulates state over time (for instance, how many kinds of user do you think GMail supports?) At a certain point, you can’t test every combination of states, even with automation. Instead, you have to test the crucial paths, code defensively, and rely on slow rollouts and monitoring to catch problems."