The afternoon of May 6, 2010, was among the strangest in economic history. Starting at 2.42pm EDT, the Dow Jones stock index fell 600 points in just six minutes. Its nadir represented the deepest single-day decline in that market’s 114-year history. By 3.07pm, the index had rebounded. The “flash crash”, as it came to be known, was big, unexpected and scary — and a new study says flash events actually happen routinely, at speeds so fast they don’t register on regular market records, with potentially troubling consequences for market stability.