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This is an official statement form Google:
Marissa Mayer has now explained the error on the Official Google Blog. She puts it down to human error after a ‘/’ was mistakenly put in an update sent from the non-profit StopBadware.org to Google . The result was every URL in Google’s database being classified as “malware” until the stray ‘/’ was removed.
Hope Google wouldn't excite its users with such warnings!
IMAGE SOURCE: TechCrunch