The United States star, who had already claimed gold medals in the team and individual all-around competitions, had hoped to add to her tally at the North Greenwich Arena but was unable to recover from her mistake on an inside full pirouette and had to settle for eighth.
"I could have fought a little harder and pulled it around," she said. "I tried to finish as strongly as I could, but some days you either have it or you don't."
Douglas has been one of the sensations of these Olympic Games and was considered to be among the contenders on the bars, although not necessarily the favorite.
Instead it was Russia's Aliya Mustafina who clinched the gold with a flawless routine to score 16.133, with China's He Kexin in second and Great Britain's Elizabeth Tweddle clinching the bronze for the host nation. Douglas could manage only a 14.900.
After Tweddle, the American received the biggest cheer of the afternoon, proof that she has won an international army of fans as well as the hearts of the public back home.
The final gymnast to compete, she looked confident and sprung onto the bars from the board with style. But as her complex series of rotations got under way, a temporary lapse robbed her of momentum and from that point on, a medal would be elusive.
This victory was sweet for Mustafina, whose mistakes contributed to Russia falling well behind the Americans in the team event and who narrowly secured a bronze in the individual event – ahead of the USA's Aly Raisman on count-back.
China's He set the tone for the final with the very first routine, and executed it superbly to provide a tough target for all who followed her.
Victoria Komova, who missed out to Douglas in the individual, could not cope with the pressure, clipping her toes on the lower bar, making an awkward landing, and trudging off in disappointment.
By the time Douglas's turn came many in the audience were torn, wanting to see more brilliance from the young American but also willing Tweddle to hold on to her bronze position.
In the end Douglas could not repeat her magic from earlier in the week, but she remains the darling of the gymnastics competition nonetheless.
She will return to the arena Tuesday for the beam final, in which she believes she can add more hardware.
"Since the gold medals, my confidence level has gone up," Douglas said. "I'm going to come back in the beam final and try to finish on a good note."More Olympics coverage on Yahoo! Sports:
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