"When correcting a person’s grammar on the Internet ,invariably you make a grammatical error of your own"How?

Correcting someone's grammar does not make your grammar incorrect unless you actually have made a mistake.

Murphy's Law is simply a theory that states that, in any given situation, what can go wrong, will go wrong.

EDIT: "Muphry's Law" is the same concept, simply applied to spelling and grammar. It's not that correcting someone automatically makes one incorrect, but rather, the "law" simply points out the irony (and regularity ) of people making grammatical mistakes as they correct someone else's. The name itself is just a clever play on Murphy's, and it is, more or less, just a joke that harpoons how people behave.

Muphry's Law