Ivan Perisic went through the gears with an extra-time brace as he fired Inter Milan to victory to win the Coppa Italia final in Rome.
Nicolo Barella set Simone Inzaghi's side on their way but two quickfire goals from Juventus appeared to turn the final on its head.
It was not to last, however, as first Hakan Çalhanoğlu and then Perisic netted from penalties to stick Inter 3-2 in front, before Perisic produced a stunning effort to ensure victory and silverware for the Milanese side.
Inter, who are still in the running to retain their Serie A title this term, started on the front foot, racing into the lead after six minutes in the Stadio Olimpico as Barella arrowed a stunning strike into the top corner.
Juventus responded well with Serbian striker Dusan Vlahovic denied by a superb save from Inter goalkeeper Samir Handanovic in a pulsating first half, before defender Alex Sandro levelled five minutes after the break.
Vlahovic then completed the turnaround less than two minutes later to spark wild scenes of celebration among Juve's players, but Calhanoglu's expertly-converted penalty took the match to extra time.
Perisic then took centre stage, firing his side in front once more with another penalty, again emphatically converted, before making sure of a first Coppa Italia triumph since 2011 for Inter with a powerful finish into the top corner.
Allegri, who was sent to the stands for protesting against a decision late on, will become the first Juventus manager since 2011 to finish a season without having won a single trophy, with Juve fourth in Serie A and out of title contention
'It was a good performance against a very, very strong opponent. We did well in the first half-hour, slightly lost our shape towards the break and then had a bad approach to the second half, conceding two really avoidable goals. We should've been more careful,' Inzaghi told Sport Mediaset in victory.
'However, this team never gives up. If I ever needed a response, I got nothing but responses from this side from July to today, it's been one long whirlwind of emotions.
'Now we have two more Serie A matches to play and we want to end the season as well as possible.
'It is also only fair we enjoy these moments, it had been a decade since we won the Coppa Italia or the Supercoppa, so I am happy for the club, these marvellous lads and the fans who stuck by us from start to finish.'
Allegri, who accused a member of Inter's bench of kicking him moments before he was sent off to the stands, was frustrated by the manner of the extra-time collapse.
'Someone from Inter walked past and kicked me. I got angry and quite rightly the referee sent me off,' Allegri told Sport Mediaset.
'Finding out what happened changes nothing. We just have to congratulate Inter for winning.
'We do not win a single trophy this season, we have to retain that anger for next season, when we must get back to winning.
'It was a good performance, but we fell away towards the end, there were players who were not fully fit and did all they could.
'It's disappointing, but we can only thank them for their efforts.'