alt Hacker NewsA better analogy is if one bulb in the right rear brake light group is burnt out. Technically the car is broken. But realistically you will be able to do all the things you want to do unless the thing you want to do is measure that all the bulbs in your brake lights are working.
That's an awful analogy because "realistically you will be able to do all the things you want to do". If a random GitHub service goes down there's a significant chance it breaks your workflow. It's not always but it's far from zero.
One bulb in the cluster going out is like a single server at GitHub going down, not a whole service.