> What is the difference between a "rule" and a "mistake"? Your question is answered directly in Pullum's article, at length. He even gives a separate explicit

> What is the difference between a "rule" and a "mistake"? Your question is answered directly in Pullum's article, at length. He even gives a separate explicit example of the who-whom distinction and explains how it works and how to think about it. There is a clear reason for why this is not arbitrary.