Two singularly sensational sentences from Fowler:

Sturdy indefensibles. Many idioms are seen, if they are tested by grammar or logic, to express badly, even sometimes to express the reverse of, what they are nevertheless well understood to mean. Good people point out the sin, and bad people, who are more numerous, take little notice and go on committing it; then the good people, if they are foolish, get excited and talk of ignorance and solecisms, and are laughed at as purists; or, if they are wise, say no more about it and wait.