Leaders Juventus were denied a 16th successive Serie A win when they were held 0-0 at Bologna in a game marred when an object, possibly a firecracker, was hurled at their team bus as they arrived.
The draw means Napoli, who were displaced as leaders by Juve last Saturday, will return to the top if they beat AC Milan on Monday.
"It's always a problem getting into Bologna, somebody threw a paper bomb or a firecracker, I don't know which," Juve chief executive Giuseppe Moratta told reporters before kickoff at the Renato Dall'ara Stadium.
"It's not an act of civility, it goes against what should be a positive spirit this evening. It's an act of hooliganism and violence which has nothing to do with the real values of this match."
Juve kept a ninth consecutive clean sheet in all competitions but failed to win a league game for the first time since losing 1-0 to Sassuolo on Oct. 28.
Bologna had the best chance of the first half when goalkeeper Gianluigi Buffon palmed away a powerful Mattia Destro effort and Godfred Donsah shot wide from the rebound.
Juve rested Paulo Dybala from their team ahead of Tuesday's Champions League match against Bayern Munich and were uninspired until they created a flurry of chances in the final 10 minutes.
Paul Pogba fired one effort wide after Claudio Marchisio's pass released him on the left and Alvaro Morata had an effort turned around the post by Antonio Mirante, the Bologna goalkeeper's first serious save in the 86th minute.
Pogba then forced a fine stop from Mirante with an angled drive and, although the ball flew across the face of the goal, Juan Cuadrado was unable to provide the finishing touch.
Juve, chasing a fifth successive title, have 58 points from 26 matches, two ahead of Napoli, with third-placed Fiorentina on 49.