The Olympic swimming competition ran into choppy waters on Saturday with South Korea protesting the shock disqualification of their 400 metres freestyle champion Park Tae-hwan in his opening heat.
Park, a national hero to his compatriots after he overcame childhood asthma to become the first South Korean swimmer to win Olympic gold in Beijing four years ago, was ruled out for a false start.
"It's a false start. South Korea have lodged a protest, FINA are now deliberating," said official Pedro Adrega. The final is scheduled for 1:45 ET under the roof of the wave-shaped Aquatics Centre.
Park's disqualification was the first big shock of the competition and he appeared to have made a schoolboy error on the starting block when he moved too early.
The mistake provided a flashback to his Olympic debut in 2004, when Park was also disqualified from his preliminary heat in the same distance. The youngest South Korean athlete at those Athens Games, he was so upset he hid in a bathroom for hours.
Instead of going through to an eagerly-anticipated Asian battle with China's Sun Yang, now the clear favourite for gold, Park was left to rue what might have been and focus on his other events.
"I don't know why I was disqualified, I will have to speak to my coach," said Park, the first male swimmer to win four individual medals at an Asian Games in a single stroke, after leaving the pool.
Sun said he was also disappointed for his rival: "I was really looking forward to racing him in the final and now that won't happen," he said.
Had Park hesitated only a fraction of a second, he would have been in the clear in a distance where such a delay can easily be made up.
"That's the Olympics," said Canadian Ryan Cochrane, who qualified at Park's expense in eighth place.
Timing is done by Omega, who have introduced new electronic sensor pads on the blocks for London to measure swimmers' reaction times, but only the referee can declare the disqualification.
The blocks have five positions and every swimmer will know which one to select before a race.
Park had been expected to take on the mantle of retired Australian Ian Thorpe as the dominant swimmer in the men's 400 after his Beijing win but has struggled to assert his authority on the distance.
After the last Olympics, he failed to qualify for the final at the 2009 world championships in Rome but won the title in Shanghai last July despite scraping into the last eight with only the seventh best time.