For the first time in 22 league and cup games, Manchester City did not win at the Etihad, for the first time in 29 matches they tasted defeat, thanks to goals from Bruno Fernandes and Luke Shaw.
But make no mistake, while Pep Guardiola will undoubtedly go on to lift the Premier League title - and probably a couple of cups to go with it - this one victory was the result of a masterful gameplan by his opposite number Ole Gunnar Solskjaer.
The jury has been, if not out then certainly split as to whether the likeable Norwegian is the man to return United to former glories. The club’s executive vice-chairman Ed Woodward even felt compelled to come to his defence on an investors’ call this week.
But now, after inspiring United to their fourth win at the Etihad in five attempts - a third in three visits for Solskjaer himself - the United manager is making a good case for himself to be the man who can finally replace his mentor Alex Ferguson.
“Today we did almost everything perfectly,” said goalscorer Fernandes. And that started with Solskjaer - and his own penalty.
City have stifled any sort of drama or unpredictability out of their last 28 games but here, in almost unfathomable fashion, Gabriel Jesus conceded a penalty with 34 seconds on the clock.
It was a clumsy challenge, the archetypical striker’s 'tackle', carried out in the most ill-advised area of the field as Anthony Martial carried the ball across the face of the City penalty area and blue shirts converged on him.
One of them belonged to Jesus who over-reached from behind and made contact with the United man’s toe and calf as referee Anthony Taylor instantly pointed to the spot and VAR took seconds to concur.
It was a bizarre sight, for a striker to be defending in his own area so early in a game, with a predictable outcome as, for the 18th time in his Manchester United career, Fernandes strode up to take a penalty and, for the 17th time, scored although, having guessed right and got a hand to the shot, Ederson will feel he should have saved.
But, for a United side who had compiled three successive goalless draws coming into the derby, and failed to score in any of their last six games against the 'Big Six', it was a stunning start and unfamiliar territory for Guardiola’s champions-elect.
City had come into the derby on the back of a 28-game unbeaten run - a total of 2,520 hours of football during which they had trailed for a grand total of 22 minutes - and that in an FA Cup tie at League Two Cheltenham.
The City manager had always downplayed City’s dominance and talked of a day in the near future when his side would fall behind, lose even, and here was a real test as to how they would respond.
First, however, things might have worsened as, just a minute after the goal, Luke Shaw dispossessed Joao Cancelo and tested Ederson with a strike that was straight at the Brazilian keeper.
But, suitably chastened, City at least slowly started to resemble something like their normal selves, although it took time and was a painful process to witness at times with Kevin de Bruyne, in particular, looking a shadow of his normally peerless self.
Still, there was a curious lack of end product from a spell of City superiority with Dean Henderson, standing in for David de Gea, who missed his first league derby in a decade due to the birth of his first child, making his best save from a long-range Oleksandr Zinchenko strike.
Raheem Sterling appealed, hopefully, for a penalty after contact with Fred and, in the dying seconds a De Bruyne free-kick was tipped over by Henderson.
The late first half dominance continued immediately after the break, with Mahrez feeding Jesus in the area and the Brazilian laying the ball back for Rodri whose powerful 18-yard shot clipped the City cross-bar on its way over.
But instead of finding themselves level, five minutes into the second half the deficit was doubled thanks to a stunning counter-attack goal from the visitors.
It started with an urgent and powerful long throw by Henderson, out to the left wing almost to the halfway line where Shaw gathered and sprinted to the edge of the City area where his pass found Marcus Rashford.
The striker darted forward, passed back to Shaw and the defender took a touch and stroked a superbly accurate finish into the bottom right-hand corner from 16 yards.
United were in the process of piecing together their best performance of the season, possibly the best of Solskjaer’s reign, and Martial might have netted a third when a Fernandes cross looped off Ruben Dias and the striker’s difficult diving header forced Ederson to save.
Martial had an even better opening in the 68th minute when Scott McTominay was fouled on the edge of the box but a good referee’s advantage saw the Frenchman with only Ederson to beat and the keeper keeping City in the game with a fine diving block.
As tensions heightened, Solskjaer and Guardiola became embroiled in a furious verbal spat on the touchline over when and how United would be allowed to bring on Mason Greenwood as a replacement for the injured Rashford.
But the City manager’s misery was exacerbated by substitute Phil Foden shooting wide on the turn and Sterling mis-kicking horribly from a Kyle Walker cross, just six yards out.
Ederson; Cancelo (Walker 65), Dias, Stones, Zinchenko; De Bruyne, Rodri, Gundogan; Mahrez, Jesus (Foden 71), Sterling. Aguero, Steffen, Laporte, Silva, Torres, Mendy, Fernandinho.
D Henderson; Wan-Bissaka, Lindelof, Maguire, Shaw; McTominay, Fred; James, Fernandes (Williams 90), Rashford (Greenwood 73); Martial 9 (Matic 87). Bailly, Grant, Diallo, Telles, Williams, Tuanzebe, Shoretire.