Muphry's law
Definition - A law that states: "If you write something that criticizes editing or proofreading, there will inevitably be an error in what you have written."

Example -
In 2009, British Prime Minister Gordon Brown hand-wrote a letter of condolence to a mother whose son had died in Afghanistan. In it he misspelled the deceased's surname. The Sun (a tabloid) published a vitriolic criticism his carelessness; however, in their article, they also misspelled the deceased's surname.

Etymology -
The law was first described by Australian editor John Bangsund in 1992. He derived the name from Murphy's law, an adage that states that "Anything that can go wrong, will go wrong."

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