Earth got a warning shot on January 25, 2016. On that day, Air Force engineers were scheduled to kill off a GPS satellite named SVN-23—the oldest in the navigation constellation. SVN-23 should have just gone to rest in peace. But when engineers took it offline, its disappearance triggered, according to the National Institute of Standards and Technology, a software bug that left the timing of some of the remaining GPS satellites—15 of them—off by 13.7 microseconds.