Lautaro Martinez's missed penalty cost Inter dearly as Bologna staged a superb second-half comeback to seal a stunning 2-1 win at San Siro on Sunday.
Romelu Lukaku had given Inter a first-half lead with his 20th Serie A goal of the season before Bologna's Roberto Soriano received a red card for dissent after 57 minutes.
Martinez then squandered a golden opportunity to make it 2-0 five minutes later, striking his attempt from the spot straight at Lukasz Skorupski.
The visitors took full advantage of that reprieve, scoring through Musa Juwara and Musa Barrow inside the final 16 minutes to seal a memorable away win over the Nerazzurri, who had Alessandro Bastoni dismissed for two bookable offences.
Lukaku looked dangerous in the opening stages and gave his side a deserved 22nd-minute lead, tapping home from close range after Martinez's header had crashed back off the post.
Skorupski pawed away Antonio Candreva's long-range strike moments after the interval, while at the other end Barrow thundered a 25-yard strike against the upright.
Bologna's hopes of getting back into the game appeared to suffer a blow shortly before the hour mark when Soriano was dismissed for dissent after he had been penalised for a foul on Roberto Gagliardini.
Their woes should have been compounded when Mitchell Dijks brought down Candreva in the area, but Martinez's penalty was kept out by Skorupski.
The Argentina international paid a significant price for that miss, Juwara firing an equaliser through the flimsy resistance of Samir Handanovic in the 74th minute.
Bastoni received his second yellow card after 77 minutes, with Inter's misery compounded soon after when Barrow powered through to fire past Handanovic.
Substitute Alexis Sanchez twice came close to salvaging a point in the closing stages but Bologna held on for the most unlikely of away victories.
What does it mean? Shell-shocked Inter throw it away
Quite how Inter conspired to throw away all three points will remain a mystery for some time. They were dominant for the first hour, yet Bologna sensed a vulnerability after Martinez's penalty miss and they took advantage in some style.
The result leaves Antonio Conte's side four points behind second-place Lazio and 11 adrift of leaders Juventus, though they may now be looking over their shoulder with Atalanta so close. As for Bologna – led by former Inter player Sinisa Mihajlovic – they jump up to ninth in the table.
Juwara makes stunning impact
The 18-year-old could not have picked a better time to open his Serie A account. The substitute had already seen a sharp volley kept out by Handanovic before he got the better of the Inter goalkeeper with a well-struck shot from just inside the area.
Martinez pays the penalty
He has been one of the star performers for Inter this season, but Martinez fluffed his lines from the spot. There is little doubt Conte's men would have gone on to register a fourth win in five games since the season resumed had he found the net.
Key Opta facts
- The last time Inter lost a Serie A match in which the opponent received a red card before the 60th minute was in February 2001 (1-0 against Napoli).
- After scoring in three of his first 11 home games in Serie A this season, Romelu Lukaku has scored in four of his last five.
- Lautaro Martínez missed his first penalty in Serie A, having scored with his three previous attempts from the spot.
- At 18 years and 192 days, Musa Juwara is Bologna's youngest player to score since Luigi Della Rocca in January 2003 (18 years and 146 days).
- Musa Barrow has scored in three Serie A games in a row for the first time in his career.
Inter will hope to return to winning ways when they travel to Hellas Verona on Thursday, while a buoyant Bologna host Sassuolo a day earlier.