"
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.