Earlier this morning, every single one of Google's search results came with a warning that the site "may harm your computer," including their own properties like Gmail and Google Maps. They've since explained what happened:
Google works with the non-profit StopBadware.org to create a list of potentially dangerous sites, which is updated periodically. One such update happened this morning, but somebody typed in "/" by mistake. That simple keystroke expands to every single URL, which is why Google was universally borked. Google engineers fixed the problem in about half an hour, and it's back to normal now.
Please don't play with our heads again, Google. We trust you. If you tell us the entire internet might infect our computers, we'll probably believe you. [Google]