Domenico Berardi secured a sensational win for Serie A strugglers Sassuolo with four goals against AC Milan, who led 2-0 before succumbing 4-3.
Robinho and Mario Balotelli put the visitors two up in the space of 13 minutes, but Berardi netted a hat-trick in a little more than 25 minutes to turn the game on its head.
The 19-year-old added a fourth goal early in the second half and Sassuolo held on to move out of the bottom three, despite a Riccardo Montolivo goal which set up a grandstand finish.
It looked for the early stages as if the game was going to be remembered for the emergence of Milan midfielder Bryan Cristante.
He stole the ball from Raman Chibsah to allow Nigel de Jong to tee up Robinho to put Milan in front in the ninth minute.
Four minutes later, Cristante, who marked his debut with a goal in the 3-0 win over Atalanta a week ago, went one better to set up Balotelli to make it two for the Rossoneri, who appeared well on course for a second successive win - and a perfect start to 2014.
How wrong they were.
Sassuolo's reply was immediate. Berardi beat the offside trap and touched the ball wide of Christian Abbiati before sweeping it into the unguarded goal from an angle.
Still, only the most optimistic of Sassuolo fans in Reggio Emilia's stadium would have truly believed their side could have overturned a two-goal deficit against Milan.
One of those, however, was Berardi. In the 28th minute, he was sent through on goal again and - although Daniele Bonera recovered to block his first effort - he kept his composure and simply had another go, turning in a volley which beat Abbiati at his near post.
Suddenly, there was a belief that the unlikely could indeed be possible, and Simone Zaza tested Abbiati with a free-kick which the Milan goalkeeper punched away at full stretch.
Unbelievably, Sassuolo took the lead in the 41st minute when Berardi completed his hat-trick, showing a true striker's instinct to race in and meet a cross from the left ahead of Urby Emanuelson to score from seven yards.
The Italy Under-21 international carried on from where he had left off in the first half with a shot which took a deflection off Bonera to make it 4-2 just two minutes into the second half.
Milan coach Massimiliano Allegri responded by sending on Montolivo and Giampaolo Pazzini - and the Serie A debut of Keisuke Honda soon followed.
Honda came close to opening the game up again when he hit the post with a left-foot effort from the edge of the penalty area in the 83rd minute.
Milan understandably laid siege to the Sassuolo goal, earning their reward four minutes from time when Montolivo picked his spot in the far corner from the edge of the area, and there was still more drama to follow.
Balotelli's header from point-blank range was saved instinctively by Gianluca Pegolo, who was relieved to see Pazzini head the rebound against the underside of the crossbar with the ball bouncing out to safety.