Process is not immune to entropy.

It is very difficult to assure that even experts don't make mistakes: especially when they are running through the same process again and again. They will, at some point, always make a mistake. Even the worlds most competent doctor is at risk of leaving a scalpel in a patient.

It is possible to lower the risk of such an event occurring which is why we push for best known practices (like enforcing {} syntax on all conditionals (I do), regression testing, etc.) and bodies of knowledge.