Now it has happened twice in a blue moon. Winning 6-1 at Old Trafford was, Roberto Mancini had said, a once-in-a-century result but Manchester City have turned the Theatre of Dreams into their private playground in successive seasons.
United, once the disappointment subsides, can reflect that they remain on the brink of achieving their season's objective. Twelve points clear, they are about to reclaim their crown and even if City are champions of Manchester, to use the phrase Vincent Kompany coined last week, United will reign over the country. Yet for City, there is a historic resonance to their achievement.
Mancini was already in distinguished company. Last year, he joined Joe Mercer among the ranks of City's title-winning managers. Now he became the first since 'Genial Joe', the amiable architect of successes in the 1960s and 1970s, to record successive wins on enemy territory. Sergio Aguero, meanwhile, reprised his role as United's nemesis. Scorer in the 6-1, the man who snatched the title from Sir Alex Ferguson's grasp 11 months ago, the nerveless executioner sentenced them to disappointment again. "You don't want to lose a derby game," the Scot said but, thanks to Aguero, United did.
The substitute produced the spectacular, with a speedy, diagonal run, leaving defenders trailing in his wake, the rising, rasping shot into the roof of the net that was reminiscent of George Best's goal against Sheffield United in 1971.
Yet the notion of the best prompts the question of why City, with perhaps the stronger starting 11, languish so far behind United. "We are not so strong we can lose three or four players," said Mancini, lamenting the weaknesses in his squad and indirectly highlighting the depth at Ferguson's disposal.
The enduring question about United, however, is whether they possess quality or simply quantity. Many a setback over recent seasons, whether they have been eviscerated by Barcelona in Champions League finals or overpowered by Yaya Toure 12 months ago, can be attributed to their failings in middle of the pitch. This was another. There was a midfield of pace, power and precision possession at Old Trafford: it was City's, aided by the numerical advantage they had in that part of the pitch.
Ferguson's tactics were flawed but his hand was forced. Phil Jones, who may have been the designated midfield powerhouse, was forced into defence by the absence of three centre-backs. He prospered - "fantastic," said Ferguson - performing a convincing impression of the injured Nemanja Vidic. Yet United missed Jones in midfield, manned by a 39-year-old in his 37th derby. Ryan Giggs set the tone for a fractious evening by chopping down Carlos Tevez in the first minute.
The former pupil at City's School of Excellence was snaffled by Ferguson and has spent a quarter of a century making them rue the one that got away. For once, however, the United icon came to City's assistance. His misguided attempt at a backheel was intercepted by Gareth Barry. He and Samir Nasri combined to tee up James Milner, whose shot was deflected in off Michael Carrick for the opening goal.
Not for the first time, Ferguson had a grievance. "Certainly Tevez is interfering with play," he said. "It was disappointing as he was clearly in the road of the goalkeeper." Eight minutes later, another local hero enabled the opposition to score, Kompany inadvertently applying the final touch to Jones' header. Married to a Mancunian, he was spared the wrath of his City-supporting wife when Aguero ensured his own goal did not matter.
"In the second half we were the better team," insisted Ferguson. "We have to look at ourselves in terms of the goals we lost. We know Aguero is a fantastic finisher." So, too, is Van Persie, the decider of the season's first derby, but the Dutchman's goal drought has now lasted longer than Henning Berg's reign at Blackburn.
He compensated as the creator, setting up Rafael when the Brazilian struck the angle of post and bar and taking the free kick for United's equaliser and a defiant Ferguson insisted: "We had terrific performances: Robin van Persie was fantastic."
Mancini, another of his admirers, has spent much of the season lamenting that his major transfer target ended up in the other half of Manchester. For once, he did not need to. This was an evening to savour the striker City signed 12 months earlier, the razor-sharp Aguero. And yet, half an hour after a final whistle that was celebrated emphatically by his players, the Italian cut a subdued figure, mumbling through his visit to Old Trafford's media theatre. "We can do nothing," he said. "The title is gone."
MAN OF THE MATCH: James Milner - Excellent in the 6-1 win last season, he flourished at Old Trafford again. Brought into the side for Edin Dzeko, in a reshuffle, he delivered the opening goal and provided the industry and discipline Mancini values. A big-game player.
MANCHESTER UNITED VERDICT: Despite Ferguson's assertions to the contrary, there weren't too many outstanding United players besides the precious Jones. Wayne Rooney began well in a compelling start to the game but, as it became frantic, too many lost possession too often. Ashley Young picked up an injury that is set to rule him out of Sunday's trip to Stoke.
MANCHESTER CITY VERDICT: They deserved their win. Mancini's decision to demote Dzeko, bring in Milner and bolster his midfield was justified and his introduction of Aguero worked perfectly. The one downside was that David Silva suffered a hamstring injury that could rule him out of Sunday's FA Cup semi-final against Chelsea.