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Yesterday morning, a WebmasterWorld member reported on a Google bug, where Google would append '?sa=X' on some links. Appending that extra snippet lead to a page that was not existent on the server it referred the page to, hence taking the end user to a 404 page not found page.
You can see for yourself what Google adds to the URL by doing a search on gmail for example, and then mouse over the quick link named "New features!." That link looks like http://www.google.com/url?q=http://mail.google.com/mail/help/about_whatsnew.html&sa=X&oi=smap&resnum=1&ct=result&cd=2
It is a tracking feature for Google. Google has quickly fixed the issue by encoding the URLs.
Forum discussion at WebmasterWorld.
