Google shocked its users with a warning on Saturday. Those who used the search engine for one hour on Saturday evening were shocked to see a message – ‘This site may harm your computer’ – along with the search result.

In an explanation on its blog, the company blamed the mistake on human error. It apologised for any inconvenience caused to users and site owners whose pages were incorrectly labelled.

“This was clearly an error, and we are very sorry for the inconvenience caused to our users,” it said.

The company said it routinely flags any search results with that message if the site is known to install malicious software in the background or otherwise surreptitiously.

Google maintains a list of suspicious sites based on criteria developed with StopBadware.org, a nonprofit project headed by legal scholars at Harvard and Oxford universities who research consumer complaints.

The company periodically updates this list. And the error happened when it erroneously applied one of its periodic list updates in such a way that the warning would apply to all URLs.

“Fortunately, our on-call site reliability team found the problem quickly and reverted the file,” it said.