(Adds later games)
Serie A title hopefuls Napoli suffered their first defeat since the opening day when they were stunned 3-2 at Bologna and were overtaken into second place by Fiorentina who eased past Udinese 3-0 at home.
Mattia Destro scored in each half for improving Bologna, either side of a Luca Rossettini header, and while Gonzalo Higuain scored two late goals to set up a dramatic finale, the hosts held on to secure one of the unlikeliest results of the season and raise doubts about Napoli's title credentials.
Goals from Milan Badelj, Josip Ilicic and Gonzalo Rodriguez gave Fiorentina a 3-0 victory over Udinese, extending their unbeaten streak to six matches.
Inter, who beat Genoa 1-0 yesterday, sit top on 33 points, Fiorentina second on 32 and Napoli, who missed the chance to reclaim top spot, have 31 in third.
Napoli boasted Serie A's joint-best defensive record ahead of Sunday's game, having conceded only nine goals, underpinning an 18-game undefeated streak in all competitions.
Bologna, though, have lost only once since Roberto Donadoni replaced Delio Rossi at the end of October and started strongly, taking the lead in the 14th minute.
Destro was picked out by a long ball from Amadou Diawara, and outmuscled Raul Albiol before slamming the ball past Pepe Reina for his third goal of the season.
Rossettini extended the advantage seven minutes later when he headed Franco Brienza's corner past Reina, before the keeper let a first-time Destro shot slip past him on the hour mark.
Higuain revived Napoli when he pulled a goal back in the 87th minute and added another three minutes later with a low strike to set up a tense finale but Bologna held on to move out of the relegation zone.
Fiorentina were not at their best against Udinese, but took the lead in the 26th minute when Badelj's long-range effort took a deflection off Nikola Kalinic and rolled in past Orestis Karnezis.
Ilicic converted from the spot after Emmanuel Agyemang-Badu brought down Kalinic in the 62nd minute, before Rodriguez headed a Manuel Pasqual corner past Karnezis in the 86th.
Disjointed Milan were wholly unable to build on last weekend's 4-1 victory over Sampdoria as they were held 0-0 away at second-from-bottom Carpi.
Atalanta leapfrogged Milan into seventh after defeating Palermo 3-0, Chievo beat Frosinone 2-0 with two goals in the final four minutes, while Empoli defeated Verona 1-0 away and Sampdoria fell to a fourth straight defeat with a 3-1 home loss to Sassuolo.