American Phyllis Francis took a shock Gold in the women's 400m final in a new personal best of 49.92, on a rain-soaked evening at the World Athletics Championships in London.
Francis stole in to take the World title, after Olympic champion Shaunae Miller-Uibo lost her footing in the final metres of the home straight, with the finish line in sight.
The Bahamian dropped to fourth, as Salwa Eid Naser surged in the final 50m to take silver ahead of defending champion Allyson Felix, who was forced to settle for bronze.
Naser fastest into final
Coming into the evening's final, World Junior champion Salwa Eid Naser of Bahrain recorded the unlikely feat of being the fastest athlete clocking a new national record time of 50.08, beating defending World champion Allyson Felix in Monday's semi-final.
Olympic champion from Rio last summer Miller-Uibo won her respective semi, and progressed with the third quickest time.
The Bahamian was joined by the Jamaican trio of Shericka Jackson, veteran Novlene Williams-Mills and Stephenie-Ann McPherson.
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Francis wins in remarkable finale
On a wet evening in the capital, both Felix and Miller-Uibo went out to customary strong starts. Already however, last summer's victor was opening up a sizable lead on the back straight.
Bahranian Naser was some way behind the pace-setters and as the leaders rounded the final bend, Miller-Uibo looked to be cruising to another major title.
With the line in sight, quite remarkably, Miller-Uibo lost her stride pattern and stumbled - almost appearing to suffer a muscular problem. With Felix tying up in the closing metres, Francis overhauled the rest of the field, with Naser rocketing toward the line also.
Whilst Miller-Uibo dropped back, Francis held on to to take a remarkable Gold on the line, to the astonishment of the watching crowd and the American herself.
In the men's 400m hurdles final earlier, Norways's Karsten Warholm won Gold after leading from the front in tough conditions at the London Stadium.
The 21 year-old Scandinavian held off his competitors on the home straight from Cuban-born Yasmani Copello, who finished strongly and Olympic champion Kerron Clement to take a hard-fought World title.