More fiction turns to fact

I reported here about erroneous information in Wikipedia being used in reference works, thus becoming what Wikipedia calls a ‘reliable source’, and so turning a previously unreliable source into a reliable one. There is another excellent example being discussed on William Connolley’s talk page. William corrects the absurd claim that Sharaf al-Din al-Tusi was the first to discover the derivative of cubic polynomials, but is immediately challenged by another Wikipedia editor, who says

You chose the wrong example. "In the 12th century, Sharaf al-Din al-Tusi found
algebraic and numerical solutions to cubic equations and was the first to
discover the derivative of cubic polynomials." is in the Encyclopedia
of Ancient Egypt at Google Books, and it took me just 0 seconds to found it
out. Please DO NOT DELETE contents because of your POV. Please, use inline
templates instead. Cheers. –pjoef 20:10, 18 March 2011 (UTC)

William points out that the source for this disputed statement - the Encyclopedia of Ancient Egypt which at first sight looks as reliable as you could get – had itself used Wikipedia as a source. He goes on to say that “the source is therefore clearly worthless as a citation to support statements made in Wikipedia”. Yes, although I question his ‘clearly’: it may have been obvious in this case, but as more secondary sources use Wikipedia as a primary source, the problem will continue, and will be increasingly difficult to spot.