La Liga leaders Barcelona fell to a 1-0 defeat at Real Sociedad on Sunday as they had a Robert Lewandowski goal controversially disallowed, with coach Hansi Flick labelling the decision a "big mistake".
Earlier Julian Alvarez earned Atletico Madrid a 1-0 win at Mallorca to take the Rojiblancos up to third.
Sheraldo Becker's 33rd-minute goal consigned Barcelona to their second league defeat of the season, leaving the Catalan giants six points clear of second-placed Real Madrid, having played an extra match.
Lewandowski's early disallowed strike led to furious debate over whether the Polish forward had been offside or not.
"It was the wrong decision, clearly, it's clear, but we have to accept it," Flick told the club's streaming platform Barca One.
"The referee, it was not his fault, we are humans, we make mistakes and today was a big mistake."
Barcelona were without teenage star Lamine Yamal after he suffered a blow to his ankle in the Champions League win at Red Star Belgrade midweek and badly missed his spark.
The Catalans failed to score in a match for the first time this season across all competitions as Real Sociedad, eighth, held firm, while Barcelona did not manage a single shot on target.
"The responsibility for this defeat is on us, we are responsible, we can play much better," admitted Flick.
"It's a tough place to go, where La Real push a lot... we have to learn from our mistakes and that's it," Barcelona midfielder Pedri told Movistar.
"Normally we score goals but today we weren't accurate, I think it will be a one-off, and in the following games we we'll be more precise.
"I think the team is working well and (not scoring) is not something that worries me."
