Pierre Kalulu's first-half header gave AC Milan an early edge while Tommaso Pobega's 77th minute effort appeared to have sealed a big win in the title race.
Victory would have moved I Rossoneri second in the standings and just five points adrift of Napoli, who earlier in the day picked up a 2-0 win over Sampdoria.
Instead, Stefano Pioli's side endured a sensational collapse.
Roger Ibanez sparked the comeback with a menacing header from a corner in the 87th minute.
Six minutes later, with the match well into stoppage time, AC Milan were punished by another set piece.
On this occasion, it was Tammy Abraham who hammered home the equaliser from close range as the hosts failed to deal with a free kick.
Speaking to DAZN after the match, Milan chief Pioli said: "We must continue to play like this but matches finish in the 95th minute and more attention is needed.
"We conceded two goals from set pieces which we can't concede. We are disappointed, we played well and we deserved to win."
When asked about the flavour of the defeat, considering results elsewhere, he added: "Bitter. We don't look at how many points we have or haven't sucked away at Napoli.
"We look at ourselves. This was not the draw we wanted. We will have to do better next time."
Pioli's attention will turn to the Coppa Italia on Wednesday as his side host Torino in the round-of-16.
Their title defence, meanwhile, will resume on Saturday with a trip to Lecce who are unbeaten in five games.