I found this online years ago. The original site is lost forever.

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

It’s 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.