Erratum 1: Section PAGE REQUESTS
Server overload caused underreporting of page requests
The problem started in December 2009 and was only detected in July 2010
At that time data for April 2010 - July 2010 could still be corrected from available logs.
From Report Card July 2010 onwards underreported months December 2009 - March 2010 have been marked in the chart (greyed zone).
Underreporting was an estimated 10%-25%.
Erratum 2: Section (VERY) ACTIVE EDITORS
Editors on Commons were counted twice
From February 2010 - June 2010 a script error caused double counts for editors on Commons.
Consequently counts for all Wikimedia projects combined were approx. 5500-6000 too high for active editors, 800-940 for very active editors.
Starting July 2010 editor counts were fixed for all months shown in the charts.
Note: from August 2010 onwards editors on Commons are no longer included in overall editor total at all, on purpose, on the assumption that most of these editors also edit on one or more other projects.
Detection of double counts between any projects and languages is planned for early 2011.