Alas, poor Zara. It wasn't the serpentine of two loops, each loop to go to the side of the arena, the first loop in true canter, the second in counter-canter (designed to test the accuracy, the balance, impulsion and regularity of the collected canter). No, it was the flying change that followed.

For what we saw did not demonstrate the straightness of the change, the submission, the balance and the engagement of the hind leg. Rather, it demonstrated the fact that the horse was in a right royal strop.

These things happen in sport, these things happen with horses, these things happen with dressage. From a distance, it's just a woman with terrifying blue eyes going round and round in circles on a big ginger horsey; inside it's anguish and heartache.

That's sport, that's horses, that's dressage.

Never mistake calm for lack of intensity, wherever you find it in sport. Dressage only looks calm. Underneath it's a discipline invented to kill and to avoid being killed. It's a cavalry thing: if you happen to be in a battle and riding a big ginger horsey, you are more likely to avoid death - and more likely to cause it - if your horse will jump sideways when you ask, stop here, start there and whirl around somewhere else.

Dressage came from cavalrymen refining their art to a gloriously absurd degree, and so you dance, you and your horse, riding to the silent music that sings for ever in the mind of the horsey person. But yesterday, as Phillips rode Ardfield Magic Star at the Mitsubishi Badminton Horse Trials, a false note was struck and the dancers lost their step. Not much to look at; just a slight tangle of legs that sort of didn't look quite right. But a small disaster for Phillips.

This is not her Olympic horse: that's Toytown and, touch wood, he'll be rockin' and a-rumblin' in China come August. She is riding two of her second-tier horses here. Quite apart from anything else, she is keen to show the world that she's not a one-horse gal: that she can get a tune from any decent horse. It's part of your pride as a horseman.

Phillips was resigned rather than furious or heartbroken afterwards. Ardfield Magic Star has a reputation for various lunacies and perversities. Wildly brave across country, he has never been much of a dressage star and yesterday he was uncooperative and recalcitrant. They've both been there before.

A flying change is a lovely thing to watch, if you can get it right. It happens when you change your leading leg in canter; that is to say, when the horse skips. To ride a good flying change is to feel ever-so-slightly like a centaur, the merging of your will and the horse's in the mysteries of the dance is a wonderful, intoxicating thing. It's a fiendishly difficult manoeuvre for any horse; perhaps especially difficult for a horse whose day job is jumping.

Today, the horses leap obstacles that make the Grand National look like a hack in the park; tomorrow they must throw off their exhaustion after this extreme test and show their accuracy and dynamism in the showjumping ring.

An eventing horse has to be good at absolutely everything, that's the heart and soul of the sport. All sports move towards ever greater height of skill, commitment and fitness, and three-day eventing is not an exception. Thirty years back, you could do a rough dressage test and rely on the rest of the field to fall away after the brutal winnowing of the cross country. Not any more, they're all too good. If you don't do a sweet dressage test, you're not at the races. There's no room for weakness.

Phillips is one of the top riders, and she has two horses here. Her other horse, Glenbuck, performed much better, so the weekend is not a write-off. And, as ever, failure demonstrates how hard all top sports are. Phillips showed yesterday that versatility is in itself an extreme demand.

It's a bit like asking a human athlete to dance an exquisite pas de deux and the next day to climb the North Face of the Eiger. Those that survive will run the 400metres hurdles - track and field's most brutal event - on Sunday. That is the way to find out who is the most versatile, who is indisputably the best.

Here is the immutable law of the horsey world: no matter how great you are, no matter how brilliant you are, no matter how important you are, horses will bring you disappointment and horses will make you look a fool. Yesterday was just a day of disappointment, they come often. It's the other kind of day that is rare and only one rider, only one horse, will find such a day in China this August.