Hartman’s Law of Prescriptivist Retaliation

From Words & Stuff (which looks dead):

Hartman’s Law of Prescriptivist Retaliation states that any article or statement about correct grammar, punctuation or spelling is bound to contain at least one error.

Nitpickers R Us

Hartman’s law is a version of Muphry’s law, which is not to be confused with Murphy’s law.

This says:

If you write anything criticising editing or proofreading, there will be a fault of some kind in what you have written.