There's also the issue that code designed for one purpose often gets used for different purposes. You literally can't think through all of the consequences for

There's also the issue that code designed for one purpose often gets used for different purposes. You literally can't think through all of the consequences for a design because the total number of combinations is enormous. Even when you do think through all of the possible combinations you can't know the runtime state of all of those combinations. Formal verification of software exists but can only really be trusted if running on hardware with some multiple redundancy and formal verification of its own.