Published: 2017-12-03 23:12:47 BdST
With Benevento having lost their first 14 top flight matches, the result was another deep embarrassment to big-spending AC Milan who were playing their first match under new coach Gennaro Gattuso, their fiery former midfielder, after sacking Vincenzo Montella on Monday.
In a later game, Ivan Perisic scored a hat-trick to lead unbeaten Inter Milan to a 5-0 win over Chievo which took them top of the table, overhauling Napoli who lost 1-0 at home to Juventus on Friday.
AC Milan, who spent over 200 million euros ($237.78 million) in the summer transfer market compared to Benevento's total of 20 million, twice led but were pegged back after Alessio Romagnoli was sent off for a second booking in the 75th minute.
Brignoli went up to join the attack after his side won a free kick in the fifth minute of stoppage time and, as Danilo Cataldi whipped the ball in, he met it with a flying header into the bottom corner of the net to spark wild scenes at the modest Stadio Ciro Vigorito.
The 26-year-old became the first goalkeeper to score in Serie A since Massimo Taibi for Reggina in 2001.
"There was nothing to lose. I just closed my eyes and jumped. It was a goalkeeper's goal, not a forward's goal," he said.
A bemused Gattuso said: "It hurts. A knife wound would have been less painful than that goal. I would never have expected to concede a goal in the last minute with the opposition goalkeeper scoring."
Serie A debutants Benevento should have gone ahead just after the half hour but Vittorio Parigini sent a free header wide from Marco D'Alessandro's cross.
Instead, they found themselves a goal behind when Giacomo Bonaventura headed in at the far post after Benevento missed several chances to clear the danger.
George Puscas snapped up a rebound to give Benevento an equaliser five minutes after halftime but, as so often this season, they undid the good work by quickly conceding another, Nikola Kalinic scoring after being allowed a free header.
Milan, however, handed the initiative to Benevento as the pressure grew after Romagnoli was harshly sent off for a tackle on Gaetano Letizia, even though no contact seemed to be made.
The hosts attacked desperately but were finally rewarded with Brignoli's stunning effort on a memorable afternoon.
Inter survived an early scare, when goalkeeper Samir Handanovic parried Riccardo Meggiorini's header from close range, before running riot at the San Siro.
A Perisic volley and Mauro Icardi's 16th league goal of the season put them in control at halftime while Milan Skriniar was on target in the second half with Perisic scoring the other two.
Inter lead with 39 points from 15 games, followed by Napoli on 38, Juventus on 37 and AS Roma on 34 with a game in hand.
Fiorentina drew level on 21 points with Milan in seventh place thanks to a 3-0 win over lowly Sassuolo.