Napoli has dropped points at home for the first time this season after a 95th minute equaliser from Cagliari striker Daniele Conti.
Cagliari went into the match full of confidence after last week's win over Fiorentina and came from behind twice to snatch a crucial away point.
Napoli boss Edy Reja said the result showed up his side's lack of experience.
"It is a shame we conceded so late in the day. It was a lack of experience and we could have been more careful.
"Unfortunately we do make a few mistakes, it happened against Milan and Atalanta. Clearly we cannot always play to our full potential, but we do need a bit more concentration.