Domenico Berardi scored a hat-trick, including a so-called phantom goal, against AC Milan for the second season running to give Sassuolo a 3-2 win against opponents who finished the game with nine men on Sunday.
Milan had Giacomo Bonaventura and Suso sent off in the second half as their frustration boiled over, their grievances including Berardi's first goal where television replays were inconclusive as to whether the ball had crossed the line.
Last season, Berardi scored four goals in a 4-3 win for Sassuolo which led to Massimiliano Allegri being fired as Milan coach the next day.
Allegri has this season led Juventus to the Serie A title and Champions League final.
Milan's defeat left them 10th in Serie A and continued a miserable season for the seven-times European champions under Filippo Inzaghi.
Berardi struck after 13 minutes when his 25-metre shot was fumbled by Milan goalkeeper Diego Lopez who clawed the ball away, only to see the goal awarded.
It was the latest ball-over-the-line controversy in Serie A which will introduce goal-line technology from next season.
The 20-year-old doubled Sassuolo's lead before the half hour, breaking clear of the offside trap to fire a left-foot shot past Lopez.
Milan hit back as Bonaventura reduced the arrears with an individual goal and defender Alex headed the equaliser early in the second half.
They appeared to be getting on top when Bonaventura, having been booked for dissent, was given a second yellow card for handball.
As Sassuolo turned the screw, Berardi had one effort brilliantly blocked by Lopez but was not to be denied and turned in the winner from close range after a clever Simone Zaza dummy in the 77th minute.
Milan substitute Stephan El Shaarawy had a late penalty appeal turned down and halftime substitute Suso was sent off for an awful studs-up tackle, Milan's 12th red card of the season, as Sassuolo completed an unlikely double over their opponents.
