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Probably not the best examples.

Sqlite famously has more test related code than database code.

Multithreading correctly is difficult enough that multiple specialized modeling languages exist and those are cumbersome enough to use that most people don't. In practice you avoid bugs there by strictly adhering to certain practices regarding how you structure your code.

You mention fsync but that famously does not always behave as you expect it to. Look up fsyncgate just for starters. It doesn't matter how well you think you understand your code if you have faulty assumptions about the underlying system.

Generally you come across to me as overconfident in your ability to get things right by carefully reasoning about them. Of course it's important to do that but I guarantee things are still going to break. You will never have a perfect understanding of any moderately large system. If you believe you do then you are simply naive. Plan accordingly (by which I mean, write tests).