Paul Pogba spoiled Manchester City's anticipated Premier League title party by scoring twice in two minutes as Manchester United came from 2-0 down to win a thrilling derby 3-2 on Saturday.
Chris Smalling completed the comeback that looked so improbable when City were well on course to round off a resounding march towards the title by racing into a 2-0 first-half lead.
Goals from captain Vincent Kompany and Ilkay Gundogan put the hosts in a commanding position, who were left to rue huge missed chances from Raheem Sterling to extend their lead.
City manager Pep Guardiola added more spice to an already hotly-anticipated clash when he claimed on Friday he had been offered the chance to buy Pogba two months ago by the player's agent Mino Raiola.
Pogba has endured a desperately disappointing season having often been dropped by Jose Mourinho for United's biggest games.
But after so much criticism, Pogba showed why he was the world's most expensive player when United splashed out £89 million to bring him back to the club from Juventus in 2016 by hauling United back into a game that looked lost at half-time.
Guardiola kept his pre-match promise to prioritise Tuesday's Champions League quarter-final second leg against Liverpool with City needing to overturn a 3-0 first leg deficit.
Kevin de Bruyne, a leading candidate for player of the year awards, was left on the bench alongside Gabriel Jesus and City's all-time top goalscorer Sergio Aguero.
Guardiola and Mourinho have a long-running rivalry dating back to their time in charge of Barcelona and Real Madrid respectively.
And the game quickly settled into the usual pattern when the two face off with City dominating possession and United happy to get plenty of men behind the ball.
Yet, for all City's possession, just like when City won 2-1 at Old Trafford in December, it was from a set-piece that United were undone.
City's Abu-Dhabi owners have splashed an estimated 878 million euros over the past decade to assemble a squad that has tilted the balance of power in Manchester from red to blue.
Kompany was one the Emiratis' first purchases, and he was a fitting scorer on what seemed destined to be a historic day.
The Belgian outmuscled Smalling to power home Leroy Sane's corner on 25 minutes.
Five minutes later City doubled their lead in a style that has characterised their season as Gundogan brilliantly turned onto Sterling’s pass before slotting into the far corner.
City should have been out of sight by half-time as Guardiola screamed in frustration after Sterling fired two glorious chances well over the bar.
Unlike in many big games this season, Mourinho played Pogba in his favoured role on the left of a midfield three.
Like the rest of his teammates, Pogba was overrun in the first 45 minutes, but came alive to slot home from Ander Herrera's cushioned pass with his chest to get United back into the game eight minutes after the break.
That goal was the first Mourinho's men had scored away from home against top-five opposition in the Premier League all season.
And they soon had two in two minutes when Alexis Sanchez picked out Pogba's late run into the box and his header beat Ederson low to his right-hand side.
City's Champions League hopes were likely ended by conceding three times in 19 minutes at Anfield in midweek.
And United took just 16 minutes to complete an incredible turnaround when Smalling capitalised on some slack City marking to turn home Sanchez's free-kick.
Guardiola threw on Aguero, Jesus and De Bruyne in an attempt to prevent just a second league defeat of the season.
And City should have had a penalty when Ashley Young somehow escaped without punishment from a knee-high lunge on Aguero inside the area that sparked an bad-tempered finale.
Aguero was denied by a brilliant De Gea save two minutes from time and Sterling then hit the post as United somehow held on for a famous win.