Skitt's Law is an adage in Internet culture that originated on Usenet. Its precise wording is a matter of debate, but its general intent is that someone who corrects another's grammar or spelling mistake is bound to make such a mistake in the very post that makes the correction. In one phrasing, "Spelling or grammar flames always contain spelling or grammar errors."
Some view the law as a curse. Many cases could be explained through the psychoanalytical concept of parapraxis, or Freudian slip: writers who are over-anxious to assert themselves by correcting other people's mistakes express their own repressed insecurity by committing similar mistakes themselves.