Heavy rain the night before had made the Canadian steeplechase course treacherous.
Goodwill, bogged down in the waterlogged grass, hit the 19th fence and fell sideways, throwing his royal charge clear.
This was nearly 50 years ago, when Princess Anne – competing as an equestrian at the 1976 Montreal Olympics – came a cropper.
Anne – who arrived in France this week for a 12-day stay during the Paris Olympics – won’t have been reflecting on the fall itself, because she suffered concussion and does not remember it.
It is a curious parallel from just a few weeks ago, when she suffered a horse-related injury at her Gatcombe Park estate and was hospitalised for several days. Like in 1976, she is said to have no recollection of what happened.
Watching in Montreal was Anne’s mother Queen Elizabeth, who was pictured nervously biting her nails just moments before she fell.
The Princess remained conscious and, asked if she wanted to continue, agreed to remount, though she has no memory at all of taking part in the rest of the round.
Pulling out was never an option, as she later reflected: ‘I was riding as a member of a team so there was no question of quitting.’
Anne had made history by becoming the first member of the British Royal Family to take part in the Olympic Games.
For the princess, just a few weeks short of her 25th birthday, this was the high point of an already illustrious career as an equestrian.
Princess Anne competed at the Olympics in Montreal in 1976. Above: She prepares to mount her horse, Goodwill, during the show jumping section of the three-day eventing competition
Anne fell from her horse after he hit the 19th fence on the steeplechase course during the Montreal Olympics in 1976
Princess Anne in her team uniform at Heathrow Airport, where she flew off to the Montreal Olympic games with the rest of Britain’s equestrian squad
In 1971, she won individual gold at the European Eventing Championships riding the Queen’s horse Doublet, resulting in her being named Sports Personality of the Year.
Four years later she won silver in both team and individual disciplines at the European Eventing Championship.
But it was not all smooth riding for the Princess.
Her fall in the Montreal came just three months after she was knocked unconscious and cracked a vertebra when her horse fell at the Portman Horse Trials in Dorset.
She also suffered concussion. As with her fall a few months later, she admitted to not remembering anything.
Anne said in an interview: ‘It was going very well and then I don’t remember anything else. Nothing at all.’
In April the previous year she had been thrown into the River Avon when her horse, Mardi Gras, failed a jump.
And in October 1975, Anne fell off her horse after colliding with a steed being ridden by her first husband, Captain Mark Phillips.
In the run up to Montreal she was selected as one of four competing riders along with Richard Meade, Lucinda Prior-Palmer and Hugh Thomas.
Princess Anne seen on the ground after falling from her horse Goodwill at the Montreal Olympics, July 1976
Princess Anne clears a jump on her horse Goodwill during the Montreal Olympics, July 1976
Princess Anne watches proceedings at the Montreal Olympics alongside her then-husband, Captain Mark Phillips. They queued up for food like all the other competitors and Anne played badminton to relax
Captain Phillips, and fellow equestrian Jane Starkey, were the two reservists.
Anne and Mark flew to Canada with the British team before taking the two-hour drive to Bromont, where the equestrian events took place.
The riders had their own mini-Olympic village made up of a series of houses, along with a sports centre for the athletes to socialise.
Here the royal couple queued for food and the Princess played badminton to relax.
While most of the time Anne was treated like every other athlete, there were two noticeable differences.
Firstly, the Princess was protected by a team from the Royal Canadian Mounted Police, a necessary measure given the terrorist attack at the 1972 Munich Olympics, when eleven members of the Israeli team were killed by a group called Black September.
Secondly, Anne was allegedly exempted from the so-called ‘gender testing’ of female athletes, which was mandatory from the 1968 Mexico games until it was suspended by the International Olympic Committee before the 2000 Sydney Games.
The testing had been brought in to prevent male athletes from passing themselves off as the opposite sex during competitions.
Prince Philip talks to his daughter Princess Anne during her participation in the Olympics in Bromont, Canada, 1976
Princess Anne sharing a joke whilst her husband, Captain Mark Phillips, looks on at the Olympic Games, 1976
Princess Anne competing in the three-day Eventing competition during the Montreal Olympic Games in 1976. She was the first British member of the Royal family to take part in the Olympics
Queen Elizabeth and her two eldest sons, Prince Andrew and Prince Charles, watch proceedings
Prince Edward, then aged 12, walks alongside his mother the Queen as they go to watch Anne compete at the Montreal Olympics
Anne, wearing the red and white dress code for female British Olympians, had the surreal experience of parading before her mother, father and brother Andrew at the opening ceremony
The Queen and Prince Philip and their four children posed for the cameras in Canada ahead of Princess Anne’s participation at the summer Olympics in Montreal
The Daily Mail’s coverage of Anne’s fall at the Munich Olympics, July 1976
Anne travelled to France on Tuesday for a 12-day trip related to the Olympics. Above: Anne poses next to diver Tom Daley and the rest of Team GB at the British Embassy in Paris, July 24
Anne’s training was interrupted when she and the other equestrians drove to Montreal to see the Queen lead the the official opening ceremony.
Anne, wearing the red and white dress code for female British Olympians, had the surreal experience of parading before her mother, father and brother Andrew.
A few days later they were joined by Prince Charles and his youngest brother Edward, who was then 12.
It was one of the few times the four siblings were together with their parents overseas.
Sadly, the monarch never got to give Anne a medal, gold or otherwise, as the British team failed to be placed in the top three.
It would be another 36 years before Anne’s daughter Zara Tindall would go one better than her.
It was the Princess Royal herself who presented Zara with a silver medal in Team Evening at the 2012 London Olympics.
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Read more at DailyMail.co.uk