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Modern internet company backends are very complex, even on a good day they're at the outer limits of their designers' and operators' understanding, & every day they're growing and changing (because of all the money and effort that's being spent on them!). It's often a short leap to a state that nobody thought of as a possibility or fully grasped the consequences of. It's not clear that it would be practical with any amount of money to test or rule out every such state in advance. Some exciting techniques are being developed in that area (Antithesis, formal verification, etc) but that stuff isn't standard of care for a working SWE yet. Unit tests and design reviews only get you so far.