Former Cheltenham Gold Cup winner Kauto Star unseated jockey Sam Thomas at the last fence as outsider Snoopy Loopy claimed the Betfair Chase at Haydock.
The 2007 Gold Cup winner was the odds-on favourite but stumbled on landing and sent Thomas sprawling.
Long-time pace-setter Tamarinbleu led going over the last but faded on the run-in to the line and came second.
The 33-1 shot Snoopy Loopy took full advantage to win for trainer Peter Bowen and jockey Seamus Durack.
Thomas, who was substituting for the injured Ruby Walsh, got Kauto Star settled well early on behind Tamarinbleu, who quickened up on the second circuit under Tom O'Brien.
Although Thomas and Kauto Star responded, along with Exotic Dancer, the mount of Tony McCoy, Kauto Star never looked fluent and a bad mistake at the third last looked to have sealed his fate.
But he battled back before making another error at the last as Snoopy Loopy made a late charge to overhaul Tamarinbleu and give Bowen the biggest win of his career.
"The horse had a lot of problems and all he needed was time," said Bowen. "I can't believe it - it is the biggest race we've ever won.
"He came there to win his race and it is just amazing. He'll have an entry in the Gold Cup and we'll see where we go from there."
606: DEBATEKauto Star's trainer Paul Nicholls refused to be too downhearted after the defeat which saw his Cheltenham Gold Cup odds pushed out to 4-1 from 11-4.
"The King George is five weeks away and it should be a different story there," he said.
"He was on the inside and not really happy, whereas at Down Royal last time out he had a lot more room and was tanking along.
"The mistake at the third-last didn't really help him but that's racing."
Earlier, Binocular claimed the easiest of wins in the Betfair Hurdle to set out on the road to the Cheltenham Festival.
The Nicky Henderson-trained four-year-old has not been out of the first two in six starts, and only lost out to another JP McManus-owned runner, Captain Cee Bee, at last year's Festival in the Supreme Novices' Hurdle.
But on his seasonal debut he had no trouble as Tony McCoy guided him to success and he is now the 7-2 favourite for the Champion Hurdle, a race McManus won three times with the legendary Istabraq.
At Ascot, another Champion Hurdle fancy, Crack Away Jack, was edged out by Chomba Womba in the Coral Ascot Hurdle.
A bad mistake at the second last by Crack Away Jack did not help his chances at all and Chomba Womba always seemed to be holding Emma Lavelle's stable star.
Chomba Womba eventually prevailed by a length with Elusive Dream a further two lengths away in third.
Betfair Chase Haydock 3m
1 Snoopy Loopy (S E Durack) 33-1
2 Tamarinbleu (T J O'Brien) 25-1
3 Exotic Dancer (A P McCoy) 7-2
6 ran. Dist: ½, 2¼
Betfair Hurdle Haydock 2m 110yds
1 Binocular (A P McCoy) 1-9 Fav
2 Los Nadis (Ryan Mania) 14-1
5 ran. Dist: 3¼, 1
Coral Ascot Hurdle Ascot 2m 3f 110yds
1 Chomba Womba (B J Geraghty) 2-1 Jt Fav
2 Crack Away Jack (Jack Doyle) 2-1 Jt Fav
3 Elusive Dream (Christian Williams) 7-2
8 ran. Dist: 1, 2
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