Muphry's Law, as formulated John Bangsund of the Victorian Society of Editors (Victorian as in "from the Australian state of Victoria" not as in "pertaining to 1837-1901, the period concurrent with the rule of Queen Victoria") pertains to the inevitable petard-hoisting that occurs whenever you mention proofreading or editing.
1. if you write anything criticising editing or proofreading, there will be a fault in what you have written;
2. if an author thanks you in a book for your editing or proofreading, there will be mistakes in the book;
3. the stronger the sentiment in (a) and (b), the greater the fault; and
any book devoted to editing or style will be internally inconsistent.