Boos replaced cheers in the velodrome as it was announced that Britain's women cycling hopefuls had been disqualified from the team sprint finals.
Victoria Pendleton, who is set to retire after the Games, and Jess Varnish prompted deafening applause and cheers from the home crowd when they appeared to secure their place in the final and further Britain's chance of more medals.
But boos rang out as it was announced they had been disqualified for a takeover infringement.
A tearful Pendleton, crowned the queen of the track after her Beijing sprint success, had no qualms over the decision, but 21-year-old Varnish was so devastated she opted out of media duties.
Pendleton said: "It was an illegal change. I came through in the change zone about a metre too early; we're talking about one hundredth of a second of a mistake there. Jess moved up a fraction too early and I just saw the door and went for it, because that's my cue to try to squeeze underneath her as quickly as possible.
"It's one of those things that happens. It's quicker than a blink of an eye. You have to stick by the rules. The rules are there to make it a fair sport. Unfortunately we fell on the wrong side of that today. It wasn't intentional, obviously."
Britons were left devastated by the news.
Alison Gibson, 49, from Camberley, Surrey, who was with husband Chris, 50, and son Tom, said: "We're devastated, absolutely gutted. We didn't really know the rules, so it was such a surprise when they said it. She must be distraught, poor thing." Tom, 22, said: "For that to happen at your home Olympics, and when they set such a good time, it's horrible."
Andy Wiles, 39, and nine-year-old son James were also in the crowd when the bad news was delivered. "It's a shame," said Mr Wiles, from Esher, Surrey. "We were looking forward to seeing the women and the men do well. It was a bit confusing. Without being too critical, it seems like quite an elementary requirement.
"I don't know how commonplace it is, maybe it was the excitement of the moment or something."