Coverage isn't the same as enumeration of all possible modes. 100% test coverage might mean that all of your "wiring" fits together, but with dynamic languages

Coverage isn't the same as enumeration of all possible modes. 100% test coverage might mean that all of your "wiring" fits together, but with dynamic languages it may not as runtime information may affect your types and break things. 100% enumeration of runtime behaviors is another property, far stronger than any non-dependent static types today, but also exponentially more difficult than 100% test coverage. Good static types are a cheap way to get probably halfway between those two on a log scale.