" Only eight serious errors, such as misinterpretations of important concepts, were detected in the pairs of articles reviewed, four from each encyclopaedia. But reviewers also found many factual errors, omissions or misleading statements: 162 and 123 in Wikipedia and Britannica, respectively. "
http://www.nature.com/news/2005/051212/full/438900a.html
http://www.nature.com/news/2005/051212/multimedia/438900a_m1.html
http://www.nature.com/news/2005/051212/box/438900a_BX1.html
I hope they publish more detail about this study.