On Saturday Google listed all sites in its database as potential distributors of malware and spyware. For 40 minutes every listing brought up a warning and a click on the search result brought you to yet another warning page. The message was that “This site may harm your computer”.

The Google Official Blog says that Google flags a listing with the message “This site may harm your computer” if the site is known to install malicious software in the background or otherwise surreptitiously.

These warnings are based on a list of bad sites delivered by StopBadware.org, the Google blog says. Stopbadware, however, insists that Google generates its own lists.

The Official Google Blog notes that:

“We periodically update that list and released one such update to the site this [Saturday] 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. Fortunately, our on-call site reliability team found the problem quickly and reverted the file. Since we push these updates in a staggered and rolling fashion, the errors began appearing between 6:27 a.m. [PST] and 6:40 a.m. and began disappearing between 7:10 and 7:25 a.m., so the duration of the problem for any particular user was approximately 40 minutes.”

The Guardian reckons Google lost some 2 till 3 million dollars in advertising revenue because of this error.

We guess there is a least one Google engineer who had his or her weekend ruined.

For those who have your own site let this be a reminder: Do check regularly to see if hackers are using your site for distributing malware. If Google finds out, the company may ban your site — and this time for more than 40 minutes.

Stopbadware reports that Google continues to blacklist bad sites.

See also: My site’s been hacked, advice from Google Webmaster Central