Britain's Jessica Ennis had to settle for silver as Olympic champion Nataliya Dobrynska of Ukraine won the pentathlon at the World Indoor Championships.
Ennis needed to beat Dobrynska by a sizeable margin in the closing 800m to retain her world indoor title.
The Briton thought she had done enough when official scoring suggested she had won when she crossed the line first.
“I'm absolutely gutted, it was a terrible long jump”Jessica Ennis
But her celebrations were quickly cut short when it was confirmed that Dobrynska had broken the world record.
The Ukrainian managed to finish within three seconds of Ennis - who ran a personal best - and set a new pentathlon world best of 5013 points, ahead of Ennis on 4965 points and Lithuanian Austra Skujyte on 4802.
"It's the worst feeling in the world," Ennis said of initially seeing her name in first place after the 800m. "I looked at the scoreboard and thought I'd won."
Ennis had begun the day in Istanbul with a brilliant run in the hurdles, dominating the field to win in 7.91s, but flirted with disaster in the high jump until clearing 1.87m at her third and final attempt.
The Sheffield athlete responded with a shot put of 14.79m - the furthest she has ever thrown, indoors or out - to lead by 10 points overall heading into the long jump.
But after an opening leap of 6.19m, Ennis followed up with 6.18m before being red-flagged for overstepping the board on her final effort.
"I'm absolutely gutted, it was a terrible long jump," she said, knowing that she had left herself a huge task in the 800m.
Russia's world heptathlon champion Tatyana Chernova had an even more disappointing day, finishing in fifth place behind Poland's Karolina Tyminska in fourth.
Elsewhere, reigning 60m champion Dwain Chambers progressed comfortably in the heats, as did fellow Briton Mo Farah in the 3,000m.
Chambers won his opening round in 6.64 seconds, matching the time of American Justin Gatlin in the previous race, and said: "It's nice to get the first heat out of the way."
Farah had to put in a sprint around the final bend of his heat with only the top four making the final, but in the end he came through in second place.
"I had to dig in hard," said the Londoner. "My aim was to slowly wind it up and try to get rid of as many people as possible. It's all about the final and trying to save as much energy as I can."
Britain had enjoyed an impressive start to day one, with Yamile Aldamamaking Saturday's triple jump final in the first round of qualifying with 14.62m - the third longest indoor jump of the year.
European indoor 3,000m champion Helen Clitheroe qualified for the 3,000m final as fastest loser in 9.02:27. She said: "It's always a massive relief to get through the heats, I'm pleased. I wanted to make sure I was in the first four or, if it was a fast time, get through as a fastest qualifier."
Nadine Okyere joins Shana Cox in Friday's 400m semi-final, while Joe Thomas and Andrew Osagie qualified for the 800m semi-final, both running 1.49:73.
In the men's 400m, Richard Buck (47.05s) and Nigel Levine (47.56s) will join world outdoor 400m champion Kirani James of Grenada in the semi-finals at 1810 GMT.
Australian 60m hurdles star Sally Pearson looked imperious as she won her heat in 7.41 seconds but Kristi Castlin, ranked number one in the world last year, went out in controversial circumstances.
The American was one of several athletes during the day to incorrectly think there had been a false start, with Jamaican sprinter Lerone Clarke another to make an early exit before complaining about the starter's gun.
