Lorcan Williams escaped unscathed after favourite Flash De Touzaine crashed through the rail in the bumper at Taunton.
The 13/8f was leading the field down the back straight in the Gold Cup Lunch Standard National Hunt Flat Race, when he suddenly veered to the left giving his rider no chance of staying in the saddle.
Williams walked away from the incident and the five-year-old gelding, making his reappearance after 266 days off, was reported to be fine by the on-course vet.
The dramatic end to the card summed up the day for champion trainer Paul Nicholls, whose three market leaders were all beaten.
"I'm fine," said Williams.
"He just ducked out.
"He's a young horse. It was only his second run (for the Nicholls team).
"When a horse is going left like that there's nothing you can do."
Victory went to the well-backed mare Windswept Girl (2/1), who responded well for Rex Dingle's urgings after coming under pressure five furlongs from home.
She stayed on well to score by 13 lengths from fellow newcomer Le Crunch for trainer Anthony Honeyball, who has a 50 per cent strike rate in the last fortnight.
Dingle was celebrating a double after his heroics 35 minutes earlier on Tactical Manoeuvre (15/8f), securing the hurdler's hat-trick in a close finish.
"It was very pleasing," said Honeyball.
"The great thing is she went on the ground so you'd think next winter there's bound to be a mares' maiden hurdle she can pick up."
