Astronomers were warning an asteroid (a comet?) was due to swerve closer to Earth than any large object since the comet that wiped out the dinosaurs — in just a few days. They were all set to issue an "emergency email" and start going on television telling people not to panic too much. At the last minute, they realized it was just the Russian space probe Rosetta, on a scheduled swing past Earth to use our gravity to gain speed. Oops!