Un De Sceaux fell at the third-last when looking to have the Killinan Beginners Chase at Thurles at his mercy.
The 1-8 shot had his 15 rivals well strung out and was going great guns in front on his fencing debut when he appeared to crumple on landing and part company with Ruby Walsh.
The previously unbeaten Willie Mullins-trained six-year-old was outclassing the opposition, but his demise left Jessica Harrington's Mr Fiftyone (8-1) to pick up the pieces in the hands of Robbie Power, five and a half lengths clear of 66-1 outsider Smokey Joe Joe.
Harrington said: "They've got to jump. He jumped great, Robbie said they went Arkle pace and he finished tired but was entitled to be.
"They went some rattle and I'm delighted he has won as he deserved it.
"He'll probably go to Leopardstown over Christmas and is in the Grade One two-mile chase - with that under his belt he should improve a lot."
Mullins reported Un De Sceaux to be none the worse for his fall apart from a small cut.
He said: "He came out of it well and is grand apart from one little cut which any horse can get.
"I was very happy with him prior to the fall and it was just one of those things and it was a novicey mistake. I just hope he won't make a habit of it."
The Mullins team got a degree of compensation when Max Dynamite (5-4 favourite) made a winning jumps debut in the Cahir Maiden Hurdle.
Eighth in last year's French Derby, this €200,000 purchase for Rich Ricci looked potentially smart.
Leading between the final two flights, he jumped the last well for Walsh and went on to score easily by six and a half lengths from Phil's Magic.
"Max Dynamite jumped and settled well," said Mullins.
"We thought he might be hard to ride but he worked out an easy ride and did it nicely for his first time over hurdles.
"Ruby thought he should go up in trip, I wasn't too pushed but he has plenty scope."
Walsh, Mullins and Ricci completed a quick double when Clondaw Court made all to take the Rock Of Cashel Hurdle.
Supporters of the 1-2 shot never had an anxious moment. Allowed to control matters up front, Walsh did not ask to his mount a serious question as he cruised home by four lengths from Mireya.
Mullins said: "I was going to go novice chasing with him as he jumps well but he can be difficult to train so I said we'd start him off in a few hurdle races.
"His problems have only been niggly ones but it's easier to keep horses sound over hurdles than fences.
"If he stays right he'll go novice chasing otherwise he'll be kept for those staying hurdles. He has lots of ability."