Palermo took the lead in the 41st minute through Uruguay midfielder Arevalo Rios who arrived on the edge of the area to open his account for the season by ramming home a deflected shot from a Fabrizio Miccoli cut back through a crowd of players.
Massimo Donati thought he had doubled the Sicilian club's lead when he headed home Miccoli's free-kick with eight minutes to go but it was controversially ruled out for offside.
Cagliari took advantage of their good fortune six minutes later when striker Sau equalised after a goalmouth scramble.
The draw leaves the Sardinian side in 14th spot with two points from three games one place ahead of Palermo who earned their first point of the season.
AC Milan have a chance to provisionally join Juventus, Lazio and Napoli at the top on six points with a win over Atalanta at the San Siro in Saturday's late kick-off.