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