Barcelona slumped to a shock 2-1 defeat to Alaves as the champions' hopes of retaining their league title suffered an early setback.
Luis Enrique brought Olympic gold medal-winning hero Neymar and new recruit Paco Alcacer into his side with Lionel Messi, Andres Iniesta and Luis Suarez all on the bench after wins in their opening two games in La Liga.
But they fell behind when Deyverson caught them napping with a smart finish in the 39th minute and though French defender Jeremy Mathieu headed an equaliser a minute into the second period, Ibai Gomez took advantage of some sloppy defending to secure Alaves' first win away to Barca since February 2000.
With Real Madrid winning 5-2 at home to Osasuna earlier in the day, it was a defeat which hands the early momentum to Zinedine Zidane's men.
Alaves, so resolute in earning a 1-1 draw away to Atletico Madrid in round one, made an equally obdurate start at the Nou Camp.
Ivan Rakitic, Barca's match-winner away to Athletic Bilbao prior to the international break, was the first to find space for a shot but the Croatian's left-foot effort from inside the D was high and wide.
Alaves' French full-back Theo Hernandez threw himself in the way of an Aleix Vidal shot to block it and Neymar almost created an opening when he rolled a free-kick square to Vidal with Alaves expecting either a shot or a cross from the Brazilian, with the former Sevilla full-back's shot tipped over by Fernando Pacheco.
Incredibly Alaves forged ahead in the 39th minute when Kiko Femenia's low cross from the right was poked home by Deyverson, who got in front of a dawdling Javier Mascherano to score and silence the Nou Camp.
Neymar should have equalised immediately but headed a great chance wide, and Rakitic was also off-target on the stroke of half-time after Turan nodded the ball down into his path.
The nerves were settled when Mathieu met Neymar's corner with power and precision to beat Pacheco, but Alaves immediately reminded Barcelona this would not be straightforward as Ibai forced a low save from Jasper Cillessen low to his left.
Alaves were frustrated by the initial award of the corner, with the ball having come off Turan last.
Mathieu should have had two in three minutes but blasted a far easier chance wide after a lucky ricochet meant the ball sat up nicely for him to hit following a corner.
Alaves had the ball in the net again after 50 minutes but Edgar Mendez's effort was ruled out for a close but correct offside call as Barcelona played an extremely high line in defence of a set-piece.
The game was now end to end after a cat and mouse first half and Alcacer was the next to threaten as he flicked a low Turan cross wide.
Messi was introduced after an hour to raucous cheers and Iniesta followed in the 64th minute, but before the great Spanish midfielder had even touched the ball Alaves were back in front.
Mascherano made a horrible mess of a clearance and Ibai showed great tenacity to get away from him and Vidal on the left-hand edge of Cillessen's box before steering an angled shot past the Dutch goalkeeper.
Alcacer was immediately sacrificed for Suarez but there was no instant response from Barca, who found themselves chasing the game.
Pacheco had to hold on tight to Mathieu's low cross with Mascherano lurking in the 82nd minute and the final chance passed Barca by when Lucas Digne's cross from the right was sidefooted wide by Messi in the dying seconds.