The company blamed the mistake on human error and apologized for any inconvenience caused to users and site owners whose pages were incorrectly labeled.
The glitch occurred between 9:30 a.m. EST and 10:25 a.m. EST, Google Inc. said in an explanation on its company blog. Anyone who did a Google search during that time likely saw the message "This site may harm your computer" accompanying every search result, the company said.
Google 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, a practice aimed at protecting its users. Google said it 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.
Saturday's error happened when Google erroneously applied one of its periodic list updates in such a way that the warning would apply to all URLs, the company said in a statement.
The glitch was caught by on-call staff and the file was quickly fixed, Google said. Since the updates are applied in a staggered and rolling fashion, the errors began appearing at 9:27 a.m. EST and disappeared no later than 10:25 a.m. EST, with the duration for any particular user approximately 40 minutes, it said.
"We will carefully investigate this incident and put more robust file checks in to prevent it from happening again," said Marissa Mayer, vice president of search products and user experience, in the statement.