From the article:
The most famous example (the one Musk was asked about) occurred when Navy pilots reported a craft resembling a Tic Tac that was moving unlike anything seen in the U.S. arsenal: They said it “wasn’t behaving by the normal laws of physics.”
The craft’s movements were, however, typical of both military and civilian UFO reports: Descending from 80,000 feet to 20,000 feet in an instant; stopping in midair and reversing direction without inertial effects; exceeding the speed of sound without generating a sonic boom; and submerging into the ocean