Juventus missed the chance to move 10 points clear at the top of Serie A as they incredibly let a two-goal lead slip to lose 4-2 at Milan after a remarkable second half on Tuesday.
At the start of a crucial week in which Juve also face in-form Atalanta, the leaders had been boosted ahead of kick-off by news of another defeat for second-placed Lazio.
However, the Bianconeri could not take full advantage despite a sensational opener from Adrien Rabiot and Cristiano Ronaldo's swift second.
All six goals came after the interval as Milan staged a rapid recovery, scoring through Zlatan Ibrahimovic from the penalty spot, Franck Kessie, substitute Rafael Leao and Ante Rebic.
Missing the suspended Paulo Dybala, Maurizio Sarri's side could not rally and so remain seven points ahead of Lazio, while rampant Milan climb to fifth.
Few could have imagined such a chaotic second period when an offside flag against Ibrahimovic ensured the sides went into the break scoreless after a first half of precious little action.
But Rabiot then required only 74 seconds from the kick-off to break the deadlock in magnificent fashion.
The midfielder collected the ball just inside his own half and brushed off Kessie before advancing, nutmegging Theo Hernandez and then, 20 yards from goal, arrowing a left-footed shot into the top-right corner.
There was little the Milan defence could do to deny Rabiot, but Alessio Romagnoli and Simon Kjaer would be well advised to avoid replays of Juve's second, the pair colliding to give Ronaldo time and space to beat Gianluigi Donnarumma.
Just as Juve looked set to coast to victory, a VAR review spotted a handball from Leonardo Bonucci inside the area, allowing Ibrahimovic to convert from 12 yards.
The momentum swung and suddenly Milan were in the ascendancy. A wonderful team move created space for Kessie to shoot inside the area, a deflection deceiving Wojciech Szczesny, while Leao drove a low strike through the goalkeeper for a third goal in six minutes to turn the match around.
Juve threatened to battle back as Donnarumma saved superbly from Daniele Rugani, but Rebic - earlier denied by Szczesny - lashed in Milan's fourth to clinch the points. Ronaldo thought he had netted again in stoppage time, only to be ruled offside.