For Mike Tyson, the dreaded moment came in 1990, in Tokyo.
To a disbelieving worldwide audience, the undefeated heavyweight champion absorbed a flurry of punches to the head in the 10th round of his fight against the journeyman James "Buster" Douglas, and for the first time in his career is knocked to the canvas.
As he failed to beat the count while fumbling for his mouth guard, the image of "Iron Mike" as an indestructible beast was vanquished in a few seconds.
He would once again become champion in 1995, but the aura of invincibility left him in Japan.
For Bjorn Borg, it was the Wimbledon final on July 4, 1981, a loss to John McEnroe that halted a five-year winning streak.
Within two years, the Swedish legend had retired, a few months short of his 27th birthday.
Many other individuals have seen their careers spiral out of control once the fear factor was gone.
More often than not, it is personal turmoil that sets the demise in motion
It is a vortex of negativity that, over the past three years, Tiger Woods must have thought he would never escape.
And yet today, having regained the world No 1 ranking following his win at the Arnold Palmer Invitational on Monday, his rehabilitation from disgraced athlete to the world's leading golfer is complete.
Suddenly, his well-documented personal troubles are mostly forgotten, and he now joins a select few who have reclaimed greatness against the odds.
Joe Frazier's astonishing left hook that floored Muhammad Ali in 1971 would have finished off a lesser fighter. Ali beat the count to finish the fight, but defeat and that knock-down looked to have dealt his career a fatal blow.
No longer the fast young man who had declared himself "the greatest" almost a decade earlier, he, too, looked to have lost the fear factor for good.
And yet, three years later, at age 34, he shocked the boxing world by defeating the younger, hungrier and stronger champion George Foreman in the famed "Rumble in The Jungle" in Zaire, to regain his belt.
It remains the sport's most celebrated return from the dead.
Other comebacks from defining setbacks, in hindsight, look less astonishing with the passage of time. When Roger Federer lost the epic 2008 Wimbledon final to Rafael Nadal in five sets, many suspected his era of domination had come to an end.
John McEnroe called that final "the greatest match I've ever seen", and the loss, like Borg's in 1981, denied Federer a sixth straight title.
At the end of the year he had also relinquished his ATP No 1 ranking, and the Swiss maestro seemingly destined to fade into the background.
It turned out the scandal-free Federer was simply taking a break.
A year later he beat Andy Roddick to retain the Wimbledon title, surpassing Pete Sampras's record of 14 grand slam titles (he is now on 17), and eventually topping the world rankings again.
Four years on he is still mixing it with the best, his position as the world's greatest tennis player safe for a while yet.
In contrast, watching Woods celebrate earlier this week, it was easy to forget just how devastating his fall had been, on a personal, professional and commercial level. But after three years of pain, he is on top again.
And it is all thanks to his peerless self-belief.
"It was a by-product of hard work, patience and getting back to playing golf tournaments," he said.
"I've won some golf tournaments in the last couple of years and consequently I've moved up."
Such understatement might seem yet another way in which Woods has matured. The young pretenders for his crown should be worried indeed.
None more so than Rory McIlroy, newly demoted to world No 2. McIlroy's struggles hardly constitute a crisis, yet, and he could even regain the No 1 position if he wins the Shell Houston Open this weekend.
But he has a real fight on his hands now.
Ironically, it is precisely at the same time that Woods has completed his transformation from villain to hero that questions over the young Northern Irishman's attitude have been asked.
McIlroy's struggles hardly constitute a crisis, but it could be a long time before he overtakes Woods again.
And now, the American goes into the Masters at Augusta in two weeks' time as heavy favourite to collect his fifth green jacket, and his first since 2005.
For the first time in several years, the tournament looks far more daunting for the rest of the field.
But for Woods and his fans, it is a long-overdue return to normal service.
As the saying goes, form is temporary, but class, as Ali and Federer have shown in the past, is permanent.
