There's a particular class of bug that affects all datacenters in a globally distributed service. Almost invariably it comes down to knock-on effects due to multiple things happening "the wrong way" in tandem. Those individual failures typically happen because somebody said "well yeah we should have a better way to do this, but the other components have redundancy, so shrug". If a single domino isn't tested well and has all potential failure modes well documented and tested for, it can still take down the entire chain of dominos.