Cagliari secured a smash-and-grab 1-0 win away to Napoli on Wednesday thanks to a late goal from substitute Lucas Castro.
The midfielder headed home from a Nahitan Nandez cross with two minutes remaining to stun the dominant hosts.
Napoli controlled large parts of the game but could not find a way past visiting goalkeeper Robin Olsen, while Dries Mertens was denied twice by the woodwork.
Things got worse for Carlo Ancelotti's side when substitute Kalidou Koulibaly was sent off for dissent after the goal.
The result leaves both sides on nine points but Napoli occupy fourth, ahead of fifth-placed Cagliari on goal difference.