Murphy's Law, the sound principle that if anything can go wrong, it will, is widely understood.
For grammarians and all who write about English usage, there is an allied principle, the deliberately misspelled Muphry's Law, which states that "if you write anything criticizing editing or proofreading, there will be a fault of some kind in what you have written."
As formulated by the Australian editor John Bangsund, it has three components:
"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."
The lexicographer Erin McKean has coined a variant, known also as McKean's Law, that "any correction of the speech or writing of others will contain at least one grammatical, spelling or typographical error."
Muphry's Law is a particular application of the Augustinian principle that all human beings are born with an innate propensity to error. All who write, edit, and proofread would do well to keep in mind the example of the Wicked Bible, the 1631 reprint of the Authorized Version by Royal printers Robert Barker and Martin Lucas in which Exodus 20:14 omitted the word not, reading, "Thou shalt commit adultery."
Muphry's Law also explains the glee with which readers identify errors in texts written by copy editors. It has been my privilege to afford you many occasions of innocent merriment.
Example: Huffington Post, October 9, 2014, "Daily Mail Headline Features World's Most Ironic Typo":
"In a prime example of Muphry's law, which every journalist has suffered at some heart-sinking moment in their career, the Daily Mail committed a glaring typo in a story on literacy standards."
The secondary headline, under "I'm illiterate," included "a generation of hopelessly ill-equiped teachers."