On a hot day in Agra, when the temperature hit 106f and the sun beat down mercilessly from a cloudless Indian sky, the Duke and Duchess of Cambridge did the unthinkable.
They chose to pose at the Taj Mahal on what has become known as "Diana's bench" - the spot where William's mother, the late Diana, Princess of Wales, famously and purposely set out to show the world that she felt abandoned and alone in a loveless marriage.
What were they thinking? For nearly 25 years that image has been seared in the public consciousness as a symbol of Diana's deep unhappiness, caused - according to her own narrative - by her husband Prince Charles's enduring infidelity with Camilla Parker Bowles.
This carefully arranged photo-opportunity in 1992 was her tacit signal to the world that her heart was broken by forces beyond her control. And although she did not know it herself, it was also the beginning of the end.
A few months later the royal couple separated, four years later they were divorced, the following summer Diana was dead. If there is one photograph that reflects the sadness which went before and the heartache that was to follow, it is this celebrated image, so freighted with meaning: Diana the queen of hearts alone and smiling bravely amid the marbled grandeur of the world's most famous monument to love.
It seems to have been a very deliberate decision by Prince William to open these terrible old wounds and pose with his wife in exactly the same spot.
As the Daily Mail's Richard Kay revealed on Saturday, the idea was to lay to rest the ghosts of his mother's unhappiness. Yet what purpose could it serve, except to remind everyone what a miserable time Diana had when she was a wife of Windsor?
F urthermore, it would - and did - encourage further comparisons between the Duchess of Cambridge and the woman whose shadow falls across every facet of her life.
In the end, the couple went ahead with the bench shot because they wanted, said the Duke, "to create some memories of our own".
One can see that he meant well, that his instincts were admirable. He hoped the newer, happier photographs of the loved-up Cambridges would secure their own place in history. However, that is not what happened. Not at all.
For the second Kate's shapely bottom hit the Taj Mahal bench, she was once more in the unenviable position of being judged and assessed in terms of her late mother-in-law.
A hellish enough prospect for any woman, but a nightmare if your mother-in-law just happens to be Princess Diana, one of the most adored royals of all time.
Contrasted to the brilliance of Diana's sheer star power, Kate will always come off second best. That is not necessarily a criticism of Kate - for what woman would not?
That's why I think it is unfair for courtiers to concoct such contrived royal scenarios, which whip up interest and invite the comparison.
Far from setting a happier new agenda, the photo-opportunity gave newspapers and TV stations across the globe a perfectly valid reason to run the old and new Taj Mahal photographs together. Like the flick of a monster's tail in a swamp, it dredged up a lot of old hurts, each of them bubbling to the surface in a most unhelpful way.
For a start, it seems hard to imagine that Prince Charles - or indeed Camilla - was entirely thrilled that the world was reminded yet again of his infidelity all those years ago.
Everyone was also reminded how Charles had promised to take his bride to the great Indian monument to love - then miserably abandoned her while he went to address businessmen in Delhi.
Yet while Prince Charles may have cause to feel aggrieved, it was all so much worse for Kate.
Some might think it would be more obliging to the Duchess of Cambridge if William and his team of courtiers and advisers thought twice before throwing the poor girl into the abyss of always, always, always being compared to Diana.
Particularly in this gilded world of royal protocols and traditions, there is no escape. There are times when it must be suffocating in the extreme. When in London, the Cambridges live in Kensington Palace, to which Diana is so inextricably linked, not least because of the sea of flowers laid outside its gates after her death.
The Duchess is ferried to and from formal events in carriages and limousines that once transported Diana, and is encouraged to wear tiaras in which Diana once dazzled.
Most overwhelmingly of all, on her ring finger she wears the sapphire and diamond engagement ring that once belonged to her mother-in-law, given to her by William when she accepted his proposal.
"This was my way of keeping her close to it all," he once explained. One wonders what his wife really thinks about there being three of them in this marriage, too.
The rigours and constraints of royal life are bad enough, but under the weight of such history, the Duchess of Cambridge is never allowed to forge her own identity.
She is instead sentenced to forever being a shadow of someone who went before, someone whose allure and magnetism she can never match.
Diana is the ghost in Kate's royal life who is never quite laid to rest. Perhaps that is exactly what William wants - but is it entirely fair on his young wife?
In the past, I've criticised the Cambridges for being boring. In my opinion, they trundle around the world as bland as a pair of country mice, terrified of doing or saying anything interesting. Even at the Taj Mahal, William could only mumble: "It's a beautiful place, stunning designs in there." He didn't even put his arm around Kate for the photograph.
Meanwhile, Kate opined it was the "perfect" way to celebrate their forthcoming wedding anniversary. Honestly. You'd think they had just toured a bingo hall in Cheam instead of one of the wonders of the world.
Yet while no one could match Diana's ability to capture the moment, Kate does have a charm of her own. This tour to India and Bhutan has garnered widespread positive coverage, and in many ways proved she is a terrific duchess.
She never looks shy, grumpy or fed up in the way her husband often does. And while the Royal Family was scared of Diana's capriciousness and influence, Kate is entirely different. She is steady and serene and has the rare gift in royal circles of looking genuinely thrilled to meet everyone who crosses her path.
That's why stunts such as the one at the Taj Mahal ultimately do her no favours; a duchess sent to do the job of a princess, an unwilling lamb to the slaughter.
They had washed the bench, put a white cloth on top, made it all as regal as possible. Kate sat there, her knees touching her husband's, her pose an echo of one a quarter of a century earlier, the sun glinting off the sapphire on her hand.
One wonders whether she thought to herself, even for a moment: how can I ever escape from the shadow of this woman? How indeed.