> error, which is an unobserved, incorrect internal state For example, amount of available space on the system drive is not an internal state. However, once th

> error, which is an unobserved, incorrect internal state For example, amount of available space on the system drive is not an internal state. However, once the number reaches zero, failures of all software are very likely to happen. The software will fail regardless of static type systems or unit tests coverage. In my experience, external things like that (not enough disk space, not enough memory, unsupported instructions, broken TCP connections) cause large percentage of failures in the software I’m developing.