The following is the Wikipedia entry, verbatim.
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.
The term Skitt’s Law appears to have been coined by G. Bryan Lord, posting to Usenet as Perchprism, in a post in October 1998 to the newsgroup alt.usage.english, in reference to a poster using the nickname Skitt. The original wording was, “The mistake you’re correcting in another’s post will appear in yours.”
Similar laws are known by a variety of alternate names; in several cases, the law was coined independently by people with no knowledge of previous coinages. Alternate names include:
- Bell’s First Law of Usenet (Andrew Bell in alt.sex, May 15, 1990): “Bell’s First Law of USENET: Flames of spelling and/or grammar will have spelling and/or grammatical errors.”
- Hartman’s Law of Prescriptivist Retaliation (Jed Hartman, April 1998): “Any article or statement about correct grammar, punctuation, or spelling is bound to contain at least one eror.”
- Tober’s Lor (in the Usenet group uk.local.birmingham, 1998, after T. Bruce Tober who postulated it)
- McKean’s Law (lexicographer Erin McKean, 1999)
- Gaudere’s Law on the Straight Dope message board (2000)
- Naruki’s Law in the User Friendly forums
- Greenrd’s Law on Kuro5hin (2002) 
One Usenet participant, Eric Kehr, jokingly referred to it as Merphy’s Law (sic)