Jump racing's best-known horse Sprinter Sacre was dramatically pulled up halfway through his comeback outing in the williamhill.com Desert Orchid Chase at Kempton with what veterinary advice diagnosed to be an irregular heartbeat.
Nicky Henderson postponed the gelding's reappearance after ruling out the Tingle Creek Chase at Sandown earlier in the month when Sprinter Sacre had returned an unsatisfactory scope test, but he had been gleaming with health in the paddock and all had been developing according to plan in the early stages.
Simply unbeatable in 10 previous outings over fences, Sprinter Sacre produced a long and flamboyant leap at the fifth but just two fences later jockey Barry Geraghty suddenly applied the brakes and trotted back anti-clockwise around half a circuit before dismounting in front of Henderson and concerned connections.
On stamina-sapping ground, the 2-9 favourite was set for a tricky reintroduction against the Tingle Creek winner Sire De Grugy (4-1), who was left to stave off the challenge of Oiseau De Nuit by four lengths.
Henderson gave an initial debrief before the seven-year-old was taken off for testing, and as he left for his Lambourn stable an hour and a half later he said: "We are still on the heart monitoring side of it, and it's still irregular.
"We were thinking of taking him to Newmarket straight away but I think the horse has had a lot of stress today and I'd rather take him home tonight.
"At the fifth fence he was Sprinter Sacre at his exciting best, and two fences later Barry had to stop him very quickly.
"We have got to investigate this, he's not distressed and he's not going to have a heart attack or anything.
"He is going to go up to Newmarket tomorrow and he has an appointment at 10am with Celia Marr (specialist in equine internal medicine and cardiology) at Rossdales.
"She is the best person for these sort of issues, we used her when Spirit Son was paralysed."
Denman is a recent case of a top National Hunt horse recovering from what is often known as atrial fibrillation.
The 2008 Gold Cup hero had an operation to correct a problem that September and although he was beaten on his racecourse return the following February, he was capable of finishing second to Kauto Star back at Cheltenham and winning the Hennessy a few months later.
Henderson went on: "We don't go around listening to all the horses' hearts - if one worked badly then you might do but Sprinter Sacre has never given us any cause for concern.
"The one everyone has mentioned is Denman, and he went on to finish second in three more Gold Cups.
"If we can get to the bottom of it, then we know what we can do to repair it.
The ante-post market for the Queen Mother Champion Chase, a title which Sprinter Sacre holds and was odds-on for, is in disarray and it is not a race Sire De Grugy's trainer Gary Moore is prioritising.
Moore said: "I'm over the moon for the horse and the owners, and especially Jamie (Moore, son and rider), but it's just a shame he had to do it that way but it's not his fault the other horse has gone wrong.
"I can enjoy it, but it has probably taken a bit of gloss off. That was a messy race, they went off fast then the pacemaker dropped out.
"He'd be better in a bigger field, covered up, and then you'd see his true potential.
"If he comes out of the race OK he'll definitely go for the Clarence House (January 18, Ascot) then he'll have a good break. We'll see if he goes for the Queen Mother then. As far as I am concerned, Cheltenham is not the be-all and end-all."