Human error was responsible for Saturday's glitch that warned users that every site turned up in search results could be harmful.
Anyone who performed a search during the approximately one hour that the glitch was active saw the message "This site may harm your computer" on every search result.
The search company usually uses that message to flag sites that are known to install malicious software.
However, on Saturday someone applied the list of suspect sites in a way that flagged all URLs.
Google has apologised to owners whose sites were incorrectly labelled as harmful.