Arkle Chase favourite Kauto Star suffered a shock defeat at Exeter despite nearly snatching victory after being remounted.
The 2-11 favourite was clear when he unshipped Ruby Walsh in the Weatherbys Bank Novices' Chase at the second last.
It left 20-1 chance Mistral De la Cour alone in the lead.
But Walsh remounted Kauto Star, who surged down the finishing straight to almost claim an unlikely triumph, before going down by a short head.
The horse was left unchanged as 5-2 favourite for the Arkle at Cheltenham in March by bookmakers William Hill.
Walsh was trailing by 20 lengths, and without his riding irons, when he got back on board.
The jockey believes Kauto Star can recover from his mishap to triumph at the Festival.
"He is a very good horse and I wouldn't swap him for anything at Cheltenham," he said.
"He just pitched on his head two fences out but had jumped super until then."