Muphry's Law is the editorial application of the better-known Murphy's Law. Muphry's Law dictates that if you write anything criticizing another person's editing or proofreading, you will inevitably make a mistake of your own. Note that by definition, pointing out an example of Muphry's Law makes you in turn subject to it.
John: Man, Steve really needs to take some grammer lessons.
Bob: "Grammer"? Looks like you've just encountered the effect's of Muphry's Law!