Riccardo Orsolini's stoppage-time bicycle kick lifted Bologna to a 1-0 victory over the Serie A leaders and left the Nerazzurri level with second-placed Napoli with five rounds remaining.
The defeat brought back memories of a goalkeeping blunder by Ionut Radu that gifted Bologna a victory three years ago, setting up the title for rivals AC Milan. Inter also lost at Bologna the following season in a rain storm.
Inter won at Bologna last season but spent so much energy they were eliminated by Atletico Madrid in the Champions League four days later.
This time, the match was decided when a long throw-in was flicked on by Inter defender Yann Bisseck toward Orsolini, who turned it in with an acrobatic left-footed volley to send the Renato Dall'Ara into delirium.
Inter are chasing a treble this season and also face big upcoming games in the Champions League and Italian Cup, but coach Simone Inzaghi had labelled the Bologna visit as the biggest match of the season.
Now the Nerazzurri are deadlocked with Antonio Conte's Napoli team, who are not involved in any other competitions and can devote all their energy toward chasing a second title in three years.
Napoli had won 1-0 at last-placed Monza on Saturday to heap more pressure onto Inter.
Bologna moved up to fourth, one point ahead of Juventus, who visit Parma on Monday.
Atalanta cemented their third place with a 1-0 win at AC Milan, Ederson scoring the only goal in the 62nd minute.
That left Milan ninth and struggling to gain a place in Europe through the league.
This adds spice to Wednesday's Milan derby in the second leg of the Italian Cup semi-finals (the first leg finished 1-1).
Then after Roma visit them at San Siro next weekend, Inter travel to face Barcelona in the first leg of the Champions League semi-finals.
And there could be another meeting with Bologna in the Cup final, after Bologna beat Empoli 3-0 in the first leg of the other semi-final.
Earlier on Sunday, Empoli and Venezia drew 2-2 – leaving both clubs in the relegation zone.
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