Celta Vigo stunned Barcelona for the second season running as they pressed and harried their illustrious visitors to beat them 4-3 at Balaidos.
The hosts won the corresponding fixture 4-1 last season and surged into a 3-0 lead before half-time this time around to leave Luis Enrique's men shell-shocked.
Pione Sisto and Iago Aspas both took their chances well and Jeremy Mathieu's own goal, under pressure from Aspas, gave Celta a lead they fully merited.
Barcelona cut the deficit to one via a Gerard Pique header and a Neymar penalty, but visiting goalkeeper Marc-Andre ter Stegen completed a nightmare evening by blasting a clearance into the face of Pablo Hernandez to give Celta a renewed two-goal advantage.
Pique pulled a further goal back but Barca missed their chance to seize the initiative in La Liga as they slipped to their second defeat in domestic competition.
Barcelona started brightly enough and Luis Suarez worked space for a shot after some neat interplay with Arda Turan but his effort was comfortably saved by Sergio Alvarez.
But Celta were squeezing Barca high up the pitch and within the space of 11 first-half minutes had raced into a 3-0 lead.
First Barcelona goalkeeper Ter Stegen gave the ball away in the 22nd minute as the visitors tried to pass their way out of defence. Aspas' angled pass picked out the run of Sisto and the Dane struck a low shot into the right-hand corner of Ter Stegen's goal.
A second followed nine minutes later as defender Gustavo Cabral dispossessed Suarez and cleared his lines to Aspas, who was allowed to advance on goal by Pique before drilling a low left-foot shot home from the edge of the box.
Seconds later the third goal arrived as Mathieu diverted the ball into his own net as he tried to prevent Daniel Wass' low cross from reaching Aspas.
Iniesta came on for Rafinha and made an immediate impact, and it was his inswinging cross from the left which Pique headed in from close range in the 57th minute. The delivery was a carbon copy of the one he provided for Ivan Rakitic to score against Atletico Madrid in round five.
There was far more purpose and urgency about Barcelona now as Celta looked a little uncertain of themselves for the first time in the game.
Iniesta was instrumental in Barca's second too. His incisive pass found Andre Gomes bursting into the box and he was judged to have been tripped by Hugo Mallo as he cut inside. Neymar confidently tucked away the penalty after a stuttering run-up.
Sisto drew a save from Ter Stegen cutting in from the left and Hernandez rifled over a left-foot shot as Celta tried to stem the flow of pressure going the other way.
Denis Suarez came on for Sergio Busquets as Barcelona tried to enhance their attacking options but they proceeded to shoot themselves spectacularly in the foot with 13 minutes to go.
Barcelona worked the ball back to Ter Stegen but as he attempted to clear his lines he seemed totally unaware of Hernandez bearing down on him, and his kick struck the Chilean's head and ricocheted into the net off the post.
Pique headed in a second with three minutes left after a determined burst down the right by Denis Suarez, but he missed a late chance for a hat trick of headers as the game slipped away from Barcelona.