A good few years ago I was testing a piece of commercial, non safety critical, software. It failed at a specific point in a way I considered "interesting". I described the failure to the programmer. He looked quizzical and said "I wondered if it would do that".
Then there was a recurring fault. It happened in client systems all over the country. Nobody experienced it frequently or consistently. In fact, across the country, It happened about twice a year. Most people never had the problem. Try as we may we never got to the bottom of it (believe me, we really tried).
Software is written by humans, tested by humans (test scripts for automated systems are written by humans) and used by humans. Humans are error prone and, in the end, it means software will be error prone.