Celta Vigo handed Barcelona their first loss of the La Liga campaign and leapfrogged them in the table with an emphatic 4-1 defeat of the defending champs at the Estadio Balaidos on Wednesday.
The result was Barcelona's worst La Liga defeat in 273 games, since May 7, 2008 at Real Madrid. It also snapped Barca's 18-game La Liga unbeaten streak.
Former Liverpool striker Iago Aspas struck twice as unbeaten Celta Vigo shattered Barcelona's 100 percent start.
Ex-Barca youngster Nolito also continued his fine start to the season with a stunning opener and substitute John Guidetti struck for rampant Celta after Neymar had claimed a consolation.
Barcelona created numerous chances in an open encounter but poor defending proved their undoing as the champions were continually caught on the break.
Celta's victory lifted them above Barca in the Primera Division and was reward for an enthusiastic display.
They set the tone early on, pushing Barcelona back in the opening 10 minutes. Fabian Orellana had an early chance but shot high and wide.
Barca responded as Lionel Messi and Luis Suarez both forced saves from Sergio Alvarez but Celta were not intimidated and continued to take the game to the visitors.
They broke through after 26 minutes with a brilliant strike from Nolito.
Hugo Mallo was first afforded too much space on the right by Barcelona and his deep cross bounced through the area to Nolito on the far side. Nolito, also under no pressure, looked up and delicately curled a chip over Marc-Andre ter Stegen into the top corner. Ter Stegen did get his fingertips to the ball but had little chance of stopping it.
Celta maintained the pressure and doubled their lead four minutes later with a breakaway goal from Aspas. Nolito released the striker with a header on halfway and he raced clear to lob the outrushing Ter Stegen from the edge of the box.
Stunned Barca tried to respond and Andres Iniesta set up Neymar with a fine through-ball but Augusto Fernandez got across to block the Brazilian's effort.
Aspas could have had another early in the second half after again breaking clear of Gerard Pique and Javier Mascherano but he drove his low shot across goal.
Barca showed signs of waking up as Iniesta carved Celta open down the left and pulled the ball back for Messi to crack a shot against the post. Pique headed over from a free-kick and Messi had another shot saved by Alvarez.
But just as Barca had seemed to be getting back into the game, they were caught again.
Alvarez punched clear from a corner and Nolito found Aspas just inside his own half. He brilliantly flicked the ball over Dani Alves and broke away before curling a shot beyond Ter Stegen.
Barca continued to show their attacking qualities as Neymar headed over and both he and Messi tested Alvarez again. Alves also missed the target.
Chances also came at the other end and Aspas twice went close to completing a hat-trick before being withdrawn 12 minutes from time.
Neymar finally found the net as he latched onto a lobbed free-kick from Messi and struck from a tight angle with just over 10 minutes remaining.
But it was too little, too late and Guidetti, just moments after coming off the bench, controlled a Mallo cross and finished powerfully to put the game beyond reach.
Celta Vigo last scored four goals against Barcelona in a league game in 1957.