A horse called Doom was beaten at odds of 1-25 to become the joint shortest-priced loser in British racing history.
The hot favourite lost out to 9-1 chance Karmology in a two-horse race at Ripon.
"I really only entered her for the race because it was on our doorstep and there weren't many entries," said winning trainer Karl Burke.
It was the biggest upset since Royal Forest was beaten at the same odds at Ascot 75 years ago.
Doom, trained by William Haggas, was strongly fancied to win under Tom Marquand, who had ridden his 1,000th career winner on Monday.
The favourite finished second to subsequent Oaks winner Soul Sister last season and led with a furlong to go at Ripon.
But Karmology, ridden by Pierre-Louis Jamin, gained ground and won by a length.