Leaders Barcelona fell to only their second La Liga defeat of the season at Real Sociedad with their teenage star Lamine Yamal absent through injury.
The result leaves Barca six points clear at the top of the table with reigning champions Real Madrid in second with a game in hand.
Sheraldo Becker’s first-half effort proved decisive for the hosts, who merited their victory and spurned several excellent chances to make it even more emphatic.
Robert Lewandowski had a first-half effort controversially ruled out for offside for Hansi Flick’s Barca, who endured a difficult evening.
The Catalans, who had won on their previous four visits to San Sebastian looked devoid of ideas going forward and insecure in defence.
The introduction of Dani Olmo for the largely ineffective Frenkie de Jong did little to turn the match in their favour.
And with their 17-year-old Spain international Yamal sidelined, with a knock picked up in the Champions League win over Red Star Belgrade, the visitors failed to register a single shot on target.