Last year Google began warning users when a search result included a site known to infect users with viruses, worms or trojans. Early this morning something went badly wrong with that function, and Google began flagging every site in its results with the warning "This site may harm your computer."
The problem lasted about 40 minutes, and was caused by human error, as Google's Marissa Mayer explained on the Google blog:
We maintain a list of such sites through both manual and automated methods. We work with a non-profit called StopBadware.org to come up with criteria for maintaining this list, and to provide simple processes for webmasters to remove their site from the list. We periodically update that list and released one such update to the site this morning. Unfortunately (and here's the human error), the URL of '/' was mistakenly checked in as a value to the file and '/' expands to all URLs.
Oops. This triggered another huge problem: the Google warnings on every result clicked through to additional information on the StopBadware site. "This led to a denial of service of our website, as millions of Google users attempted to visit our site for more information," explained Maxim Weinstein at StopBadware. Some early media reports apparently falsely blamed the problems on StopBadware.