PARIS — It took less than one hundred milliseconds for Aaron Brown’s run in the Olympic 100 metres to come to a devastating end.

In round one of the event at Stade de France on Saturday, Brown lined up in heat eight. The starting pistol went off — then sounded a second time. Brown, who had been first out of the blocks, became subject to one of the most merciless rules in track and field: he was instantly disqualified because of one false start.