Minnows Cesena stunned AC Milan 2-0 on Saturday as big-money signings Robinho and Zlatan Ibrahimovic, who missed a penalty, suffered nightmare debuts.
Meanwhile champions Inter secured their first Serie A victory of the season with a 2-1 success at home to Udinese.
But the real drama happened in Cesena where goals from Erjon Bogdani and Emanuele Giaccherini put Milan's stars to shame.
Two seasons ago Cesena were plying their trade in the third tier of Italian football but with this win they proved they deserve their place among the elite.
Milan coach Massimiliano Allegri blamed his team for not dealing with Cesena's counters.
"We didn't start badly and we had a couple of chances at 0-0 to go in front," he said.
"We knew beforehand that they were good on the counter-attack but we still conceded two goals. We made life easy for them on the things they do well."
Cesena opened the scoring with a delightful and simple move as Giaccherini spread the ball from left to right where Luca Ceccarelli brought the ball down on his chest and volleyed it across goal for Bogdani to glance home a header.
Milan had plenty of chances with Ronaldinho bringing a save out of Francesco Antoniolli, the oldest goalkeeper in the league at 40, with a curling effort.
Pato then headed straight at the goalkeeper having got above marker Yuto Nagatomo while his cross-shot then hit Ibrahimovic before falling to Antoniolli.
As Milan pushed forward they were caught again a minute before the break as Stephen Appiah's clearing header found Bogdani, he rolled marker Sokratis Papastathopoulos and released Giaccherini who scampered in on goal and beat Cristian Abbiati with a crisp shot.
Pato had the ball in the net for the visitors after the break but it was ruled out for a handball while Ibrahimovic hit the post with a late penalty to compound his personal misery.
At the San Siro Samuel Eto'o scored the Inter winner after Antonio Floro Flores had cancelled out Lucio's early opener.
But this was a struggle throughout for Rafael Benitez's men.
"Of course the important thing was to win," said Benitez. "We had a very good start but then we lost control in midfield and they had some dangerous counters.
"Then after we went ahead, and we didn't get the third goal, we didn't play well at the end. We were a bit nervous."
The Spaniard sprung to the defence of last season's star Diego Milito, who once again fired blanks and was replaced late on.
"Milito is working very well, he's got quality but he just needs time and he will improve little by little," said the coach.
Inter were off to a dream start as Lucio was left free at the back post to calmly take down a Wesley Sneijder corner and volley home on seven minutes.
But although the champions seemed in control, they created little and the visitors equalised on 31 minutes.
Alexis Sanchez took a short corner, got the return and then crossed for Floro Flores to arrive late and get above Lucio to power home a header from six yards.
The winner came from the spot on 67 minutes after substitute Gabriele Angella needlessly handled a cross from Sneijder.
Handanovic saved Eto'o's initial effort but the follow-up fell kindly to the Cameroonian who slotted home.
Cagliari are the early pace-setters, leading Cesena on goal difference, after thrashing 10-man AS Roma 4-1 as Nicolas Burdisso was dismissed for a horror two-footed lunge.
Daniele Conti opened the scoring for the hosts with a scorcher on eight minutes but Daniele De Rossi equalised 10 miutes later with a header from a Francesco Totti corner.
But Burdisso gave away a penalty and earnt a red card for a terrible tackle, allowing Alessandro Matri to score from the spot.
Burdisso's younger brother Guillermo was brought on to shore up the defence in place of captain Totti but Robert Acquafresca then scored with a header.
Matri added the fourth two minutes after the break and Andrea Lazzari wrapped things up two minutes from time.