Sir Arkay, the mount of Switzerland's Jurg Friedli, may have been upset by the hollow sound of his own rattling hooves as he went across the top of this construction which was built from rubber-clad sleepers on a steel frame.
The horse tried to spin round on the small platform, which incorporated a sharp left-hand turn to the steep descent down the face of the bank. Sir Arkay's hind feet slipped on the facing, made from three-inch rubber bricks, and he crumpled on landing.
Shocked spectators saw the eight-year-old Belgian-bred gelding (who has won a total of pounds 20,000) hobble on three legs to the nearby exit, where a horse ambulance was waiting. John Brazier, the vet, quickly diagnosed a broken tibia in Sir Arkay's near hind leg and Friedli, a 47-year-old Swiss clockmaker and amateur show jumper, went through the misery of having his mount put down.
The incident sent shock-waves throughout Wembley Arena. Some spectators left immediately; others waited in numbed silence. Most of the riders had pulled out of the contest before it was announced, to cheering that was tumultuous with relief, that the ground jury had withdrawn the bank from the Everest Derby.
Riders who had pulled out therefore competed, but a shadow hung over the rest of the competition. Michael Whitaker, who won on Henderson My Messieur (defeating his elder brother, John, on Gammon) said that the fatality had 'soured' the competition.
But riders did not condemn the Derby Bank as a dangerous obstacle. 'I was quite happy with it beforehand,' Michael Whitaker said. 'I think everyone was intending to compete, but there was no way I would have jumped the bank after the horse was injured.'
John Whitaker had expressed reservations at the start of the show: he felt that horses might frighten themselves while clattering over the bank. 'But I wasn't unduly worried, though I admit I was waiting to see how the first few went,' he said. 'Perhaps the Swiss horse needed studs, they could have stopped him slipping.'
Most riders had intended to use studs in their horses' shoes, to give them extra purchase on the bank. But the obstacle, an innovation before the show, had already become a distressing white elephant.
Two horses did cross it safely: Mark McCourt's The Mint with some anxiety and Jessica Chesney's Diamond Exchange without turning a hair.
Fatalities in show jumping are rare. Derek Ricketts' Beau Supreme, who broke a leg when jumping a conventional course here in the mid-1970s, was the last horse to be put down at Wembley.
'This hits us as organisers and owners even harder than it hits the public - and they've been hit hard enough,' Michael Bates, chairman of the British Show Jumping Association, said. The obstacle is now destined for the scrapheap.
Michael Whitaker's win in the Everest Derby was his first of the meeting, which ended last night. 'I've won here every year since I was 16 and I thought I was going to miss out for the first time,' he said.
His brother John had gained his fifth victory on Henderson Gammon in Saturday night's Leading Show Jumper of the Year and a sixth success was in reach until the younger Whitaker snatched it away.
HORSE OF THE YEAR SHOW (Wembley Arena): Everest Derby: 1 Henderson My Messieur (M Whitaker) clear, 33.59sec; 2 Henderson Gammon (J Whitaker) clear, 36.11; 3 Ancit Countryman (D Broome) clear, 41.58. Fordyce Curry Consolation: 1 Early Edition (P Murphy) clear, 51.04; 2 Blue Bird (J Popely) clear, 52.47; 3 Corella (M Armstrong) clear, 52.96. Daily Mail Christy Beaufort Leading Junior Show Jumper: 1 Strawberry Mojo (N Bingham) clear, 34.23; 2 Flaxlands Major Decision (H Mason) clear, 35.56; 3 CR Operation Charlie (E Edwards) clear, 36.02. Nigerian Ports Leading Show Jumper of the Year: 1 Henderson Gammon (J Whitaker) clear, 32.05; 2 Roby Foulards Shogun (P Lejeune, Bel) clear, 33.45; 3 Faldo (P Murphy) clear, 36.02. Olympic Star Spotters Championship: 1 Legal Venture II (M Edwards) 4 faults, 37.61; 2 Endeavour (A Bradley) 4, 38.56; 3 Katy's Choice (A Saywell) 4, 46.88. Osborne Refrigeration Riding Horse Championship: JCB (A Oliver); Reserve: Brown Sabre (C Scott). Pandiship Speed Classic: 1 Panacee du Thot (A Ledermann, Fr) 52.51; 2 Early Edition (P Murphy) 55.86; 3 Everest Major Wager (N Skelton) 56.35.Reuse content