This site calls it Murphy’s Law of Finger-Pointing, and a commenter here refers to it as “that Law of Internet Dynamics” (implictly one of many). Those names suck.
OK, this is better: this Language Log entry by Mark Liberman calls it “the Iron Law of Nitpicking”, citing Zeno at Halfway There, who summarizes it as “You are never more likely to make a grammatical error than when correcting someone else’s grammar.”
“The Iron Law of Nitpicking.” That’s a name for it that I can live with.
Reading further in Liberman’s post (I initially thought the survey box marked the end of the piece), I see that he traces the concept back further:
Jed Hartman ("Words & Stuff”, April 20, 1999): “Any article or statement about correct grammar, punctuation, or spelling is bound to contain at least one eror”.
Perchprism/Skitt: (alt.usage.english, April 26, 1999): “Any post correcting an error in another post will contain at least one error itself” or “The likelihood of an error in a post is directly proportional to the embarrassment it will cause the poster”.
Erin McKean: (Verbatim Magazine, Summer 1999) “Any correction of the speech or writing of others will contain at least one grammatical, spelling, or typographical error.”
Even Ambrose Bierce made a similar observation: [W]riters all, both great and small, are habitual sinners against the light; and their accuser is cheerfully aware that his own work will supply (as in making this book it has supplied) many “awful examples”...
Given the apparent difficulty of establishing priority, it’s probably best not to name it for a specific person. “The Iron Law of Nitpicking” it shall be.
So let it be written, so let it be done. ;)