Google briefly labelled Google.com as “partly dangerous.” The automated warning appears to be a quirk caused by hosted content.
The warning came on Google’s own Transparency Report page, which includes details of its Safe Browsing technology. That’s the database that gives warnings in Google’s search tools and the Chrome browser when pointing to a potentially risky site.
The warning only lasted a day at most and specifically covered “Google.com” while “www.google.com” continued to be labelled as safe. That suggests the risk in question came via third-party content on a Google service, most likely Google Groups.
This created the unfortunate situation by which Google was warning its own site was dangerous while labeling rivals such as Bing as safe.
Some reports have speculated the warning didn’t come because of hosted content but rather because Google’s search tool links to risky sites. That seems highly unlikely: given that it indexes tens of trillions of pages, it’s inconceivable there’d ever be a moment when it wasn’t linking to something that posed a security risk.