John Bangsund of the Society of Editors (Victoria) in Australia identified Muphry's law as "the editorial application of the better-known Murphy's law" and set it down in 1992 in the Society of Editors Newsletter.
The law, as set out by Bangsund, states that:
(a) if you write anything criticizing editing or proofreading, there will be a fault of some kind in what you have written;
(b) if an author thanks you in a book for your editing or proofreading, there will be mistakes in the book;
(c) the stronger the sentiment expressed in (a) and (b), the greater the fault;
(d) any book devoted to editing or style will be internally inconsistent.
It goes on to say:
Muphry's Law also dictates that, if a mistake is as plain as the nose on your face, everyone can see it but you. Your readers will always notice errors in a title, in headings, in the first paragraph of anything, and in the top lines of a new page. These are the very places where authors, editors and proofreaders are most likely to make mistakes.
Source - Wikipedia