Perri Shakes-Drayton got it all wrong when it mattered most as she ran her worst race of the year in the 400 metres final at the World Championships.
The Londoner, a strong medal contender, lost her rhythm over the barriers entirely - her stride pattern all over the place from around the 200m mark - before appearing to throw in the towel in the last 50m as she came home in seventh place.
Her time of 56.25 seconds was more than two seconds down on her semi-final time and more than two-and-a-half seconds slower than her personal best set last month.
Scotland's Eilidh Child was the highest place Briton as she finished fifth in 54.86secs.
Czech Zuzanna Hejnova took gold as expected in a world leading 52.83s.
Shakes-Drayton, whose cause was not helped by hitting a hurdle, revealed she had felt a knee problem early in the race.
And after a brief trackside interview she was taken for an ultrasound on her left knee.
Holding a ice pack to the joint, she told BBC Two: "In the race it just felt like my knee was wobbling and doing something it shouldn't have been doing. It has to be seen to now.
"I'm gutted, obviously. I was doing so well and I felt so good."
She said she felt her knee go on the back straight before she hit the barrier.
She added: "I thought, 'Oh god, what's going on'. I wasn't sure. I was like, 'Please hold on' because I was still pushing. I was losing control. I don't know what's wrong with it."
It is desperately bad luck for the 24-year-old, who was enjoying the form of her life.
Five times this year she has run quicker than the 54.09 which earned American Dalilah Muhammad the silver.
She was appearing in her first global outdoor final almost 20 years to the day since Gunnell won the world title in Stuttgart and aiming to exorcise her Olympic demons after agonisingly missed out on a place in the final in her home city last summer.
The 24-year-old admitted after winning her semi-final yesterday that those doubts had played on her mind ahead of the race, but that making the final had banished them.
And she had looked on course for the ideal end to a fantastic year at the Luzhniki Stadium, having won double gold at the European Indoor Championships in March.
But when it came to the crunch - the "real deal" as she branded it ahead of the championships - it could not have gone worse.
Child, though, whose own fine form has been overshadowed by her fellow Briton this season, admitted she too was disappointed with her race.
She said: "It wasn't a great race. I lost my stride on the back straight and I was just trying to fight back the whole race.
"I did feel a bit tired tonight, but I can't really complain - fifth in a world final. I wanted to come here and get to the final, I just wanted to do a little bit better in the race.
"Seeing the times and knowing I'm quicker than that, I could have been in for a medal, I'm a little bit disappointed with that."