Champions Barcelona suffered a humiliating 2-0 home defeat to Hercules on Saturday when Nelson Valdez struck twice to give the tiny promoted club a shock La Liga victory at a stunned Nou Camp stadium.
Barca had not lost at home in the league since May 2009 when they went down 1-0 to Osasuna having already secured the title but Paraguay striker Valdez scored either side of halftime to help Alicante-based Hercules pull off one of the biggest giant-killing acts of recent seasons.
Pep Guardiola's side, whose sole defeat last term came at Atletico Madrid, looked surprisingly toothless even with Argentina forward Lionel Messi and World Cup-winners David Villa and Andres Iniesta in the starting lineup.
The reverse leaves them on three points from their opening two matches, one behind arch rivals Real Madrid.
Cristiano Ronaldo set up Ricardo Carvalho to score the only goal in Real's 1-0 win over Osasuna later on Saturday and hand their Portuguese compatriot and coach Jose Mourinho a winning home debut.
Hercules have an annual budget of around a tenth of the roughly 400 million euros ($508 million) Barca command but Guardiola's decision to take on the league minnows without players including Xavi, Pedro, Sergio Busquets and Carles Puyol backfired badly.
In glorious sunshine in the Catalan capital, the home side seized early control and striker Bojan Krkic fluffed a chance to put Barca ahead in the 10th minute.
The festival atmosphere for the National Day of Catalunya was spoiled when Valdez scrambled the ball over the head of goalkeeper Victor Valdes in the 27th minute.
Guardiola threw Xavi and Pedro into the fray after the break but Hercules doubled their lead just before the hour when Valdez, who joined from Bundesliga club Borussia Dortmund in the close season, swept a Tiago Gomes cross high into the net.
Messi, Villa, Pedro and Gerard Pique, who had six stitches in a head wound after the match, all went close for Barca but the Hercules defence held firm and keeper Juan Calatayud was in sparkling form.
David Trezeguet should have made it three for the visitors in the 89th minute when he had the goal at his mercy but Valdes scrambled across to brilliantly deny the former Juventus striker.
"It doesn't get any better than winning at the Nou Camp," Valdez said in a television interview.
It was a day to forget for Javier Mascherano, bought from Liverpool last month and making his first start for Barca. The Argentina midfielder was cautioned for the foul that led to Hercules's opening goal and was hauled off at the break.
"The team was trying to the end, we couldn't do any more," Barca and Spain midfielder Iniesta told reporters.
The players would pick themselves up for Tuesday's Champions League clash at home to Greek club Panathinaikos, he added.
After more than a decade in the second division, Hercules had their pre-season preparations clouded by allegations of match fixing, although the club denied any wrongdoing.
Taped conversations, collected by a judge as part of a separate investigation in Alicante, were widely quoted in local media, apparently suggesting money was paid to influence the outcome of second-division matches last season.
In other matches on Saturday, Valencia made it two wins out of two with a 1-0 home victory over Racing Santander.
Atletico Madrid, who thumped Sporting Gijon 4-0 on the opening day of the season, also have two wins from two after a hard-fought 2-1 victory at Athletic Bilbao in the late kickoff.
Uruguay striker Diego Forlan set them on their way on 12 minutes after skillful work from Sergio Aguero and Tiago netted a header shortly after coming on as a second-half substitute.
Spain striker Fernando Llorente pulled a goal back late on after Argentina striker Aguero had been carried off on a stretcher just before the hour with what looked like a serious ankle injury.