After Gonzalo Higuain had cancelled out Nene's opener, Rafael Benitez's men felt they had been robbed of victory when the officials chalked Jose Callejon's strike off for offside
The Partenopei fell behind to an early goal from Nene but quickly levelled matters courtesy of a Gonzalo Higuain penalty.
Napoli thought that they had forced a winner with just over 15 minutes to go when Jose Callejon fired home from the edge of the box but the Spaniard’s strike was disallowed for an alleged offside, much to the frustration of Rafa Benitez and his players.
The Partenopei had arrived at the Sant’Elia brimming with confidence after an impressive 4-2 victory over Inter last weekend but they found themselves a goal down inside nine minutes, with Davide Astori teeing up Nene for the opener after Cagliari had ruthlessly exploited a slip by Napoli right-back Christian Maggio in the build-up.
However, the hosts then lost Nene to an injury and things went from bad to worse in the 19th minute when Astori undid all his good work at the other end of the pitch by felling Goran Pandev in the box.
Higuain confidently strode up to take the subsequent spot-kick and sent Vlada Avramov the wrong way to rack up his ninth Serie A goal of the season.
Napoli bossed the rest of the half without creating any clear-cut openings and they were rocked by the loss of Pepe Reina to a suspected hamstring injury just before the break. The on-loan Liverpool goalkeeper looked devastated as he left the field to be replaced by Rafael Cabral given the Spain international had only just returned from a muscular problem.
Reina's frustration was nothing, though, compared to that of Callejon when he thought that he had put Napoli ahead with his second-half strike, after Cagliari had failed to fully clear a free kick.
However, the officials appeared to feel that one of the Partenopei players standing in an offside position when Callejon had let fly had either touched the ball or being standing in the eye-line of goalkeeper Avramov.
Napoli went close again in the dying minutes through Dries Mertens but the Belgian dragged his shot wide after looking set to punish some slack defending from Daniele Dessena.
Consequently, Napoli could only draw to within two points of Roma, and seven of leaders Juventus.