Saturday morning Google users might have noticed something odd, every single search results was flagged with the message "This site may harm your computer". According to Google it was minor case of human error and not the web going to the spam lords.
Marissa Mayer, VP, Search Products & User Experience explains the error in a Google blog post. Apparently the URL of '/' was mistakenly checked in as a value and saved to the malicious site database. Assigning the value '/' expands to all URLs.
Google works with StopBadware.org to maintain their malicious site database. Periodically that list is updated and released one such update was released Saturday morning with the afflicting error. Luckily the error was spotted quickly and all systems were normal within an hour. Here's Stopbadware's explanation as well as some details of how the database is maintained.