Missy Franklin won the gold medal for Team USA and viewers were shown pictures of her post-victory in a promo reel, spoiling the result.
NBC's Today show ad features Franklin holding her gold medal with her parents and the tag line: "When you're 17 years old and win your first gold medal, there's nobody you'd rather share it with."
"Clearly that promo should not have aired at that time," said an NBC Sports spokesperson.
"We have a process in place and this will not happen again. We apologise to viewers who were watching and didn't know the result of the race."
NBC has faced criticism throughout the Olympics for its deliberate tape-delay approach to broadcasting London 2012, which allows the network to air the Games in primetime.
It also faced criticism for not showing a 7/7 tribute during the Opening Ceremony. The broadcaster instead decided to switch to a Ryan Seacrest interview with US swimmer Michael Phelps.
Watch Digital Spy's video with celebrities about the London 2012 Games.
[London 2012 - The Latest Pictures]
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