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. —Internet encyclopaedias go head to head (News @ Nature.com)
Wikipedia stood up fairly well against Encyclopedia Britannica, in a review by Nature science writers.
Update: Wikipedia’s articles were, on average, longer than EB’s. So it’s possible to spin these findings such that the news is Wikipedia has fewer errors per byte than Encyclopedia Britannica.