Here are five glaring alarm signs:
1. The Bloomberg terminal screen perhaps offers the world's first broadest reaction to the news, and it was flashing red all over with headlines early Friday morning. The screen noted heavy losses across Asian stock markets, while also noting that foreign currencies were strengthening against the British pound. Yields on U.S. Treasuries were falling, a sign of a flight to safety among investors.
2. The Standard & Poor's 500 index, which has held up in the face of worries about Brexit in recent weeks, appeared primed for deep declines Friday. At half past midnight, S&P futures, which are traded throughout the night, were down 5 percent.
3. This alert from CNBC showed how almost no market was untouched by the news. The Dow Jones Industrial Average futures were down 600 points, while gold had climbed $100 an ounce. Gold is often an asset investors buy in times of panic.
4. As noted, investors often put money into gold when other assets, like stocks and bonds, become more risky. Google search data indicated a massive spike in searches for how to buy gold Thursday night.
5. The British pound fell more than 10 percent in the wake of Brexit, a shocking decline for a rich country's currency in a single day.