Britain's Liam Phillips dramatically crashed out of the men's cycling BMX final, as Latvia's defending champion Maris Strombergs claimed gold.
Phillips made a great start and was looking comfortable on the second straight when his foot came unattached from his clip-in pedal.
The 23-year-old from Somerset lost two places immediately and was then unable to stay upright on the second turn.
Australia's Sam Willoughby won silver with Carlos Mario Oquendo Zabala third.
After the earlier disappointment of Shanaze Reade's sixth-placed finish in the women's final, Phillips had high hopes of finishing in a medal position after an impressive series of displays in the semi-final races.
But Phillips, who suffered a fractured collarbone in May, offered no excuses afterwards.
"I probably got the best start of my life," said the Taunton rider.
"But I overshot the second jump and probably the first.
"On the step up, Willoughby changed the way he was going and I thought I was going to crash then, but I managed to unclip and stay up.
"You can't compete with those guys if you make a mistake like that.
"I'm bitterly disappointed although I'm pleased [to have made the final] as 10 weeks ago I was having surgery on my shoulder.
"I've defied all expectations to be here, let alone be in contention."
Phillips's progress through the qualifying rounds had been steady and unspectacular.
Despite an unpromising starting gate, he charged down the eight-metre ramp and was quick through the first two jumps.
A clear third through the first corner, his race fell apart on the first jump on the second straight when his left foot came clear of his pedal.
Seconds later, he was on the tarmac and out of the race.
Up ahead, the medals were supposed to be contested by the consistent Dutchman Raymon van der Biezen, Australian world champion Willoughby and American star Connor Fields.
Van der Biezen, however, picked the wrong time to have his first poor start, and had to settle for fourth, while Fields limped home in seventh.
This gave 25-year-old Strombergs, known as "The Machine", all the opportunity he needed to secure Latvia's second medal and first gold at London 2012.
Willoughby, so good in the qualifying rounds, had to settle for silver, and Zabala snatched a surprise bronze medal to make it a magical five minutes for Colombia after team-mate Mariana Pajon's victory in the women's race.