Barcelona suffered a hangover from their remarkable Champions League comeback against Paris Saint-Germain as Alex Bergantinos's winner 16 minutes from time handed Deportivo la Coruna a famous 2-1 win on Sunday.
Joselu had fired Deportivo into a first-half lead before Luis Suarez levelled in the first minute of the second period.
However, Bergantinos rose highest from a corner to seal Deportivo's first victory over Barcelona since 2008.
Barca still lead Real Madrid by one point at the top of La Liga, but the European champions now have two games in hand and can go top with victory over Real Betis later on Sunday.
Barcelona boss Luis Enrique made five changes from the side that beat PSG 6-1 on Wednesday to produce the biggest comeback in Champions League history from a 4-0 first leg deficit.
Two-goal hero in midweek Neymar was one of those to miss out due to an abductor injury.
However, Barca still had plenty of firepower available with Suarez alongside Lionel Messi up front and Suarez had the first big chance when he headed straight at German Lux.
It took Deportivo 40 minutes to force Marc-Andre ter Stegen into a serious save as the German leapt to his left to brilliantly turn Joselu's volley behind.
However, from the resulting corner Ter Stegen was found wanting as he spilled Javier Mascherano's miscued clearance and Joselu was on hand to smash into an unguarded net.
Barca got off to a flyer in the second-half as Suarez lashed home a loose ball inside the area to equalise just a minute after the restart.
Enrique turned to Andres Iniesta and Ivan Rakitic for a winner in the final half hour as Arda Turan and Andre Gomes were sacrificed.
But Barca found it difficult to create clear-cut chances against the mass ranks of Deportivo defence, whilst the hosts were always a threat on the break.
Joselu fired a sweetly hit volley straight down Ter Stegen's throat.
Alejandro Arribas then saw his header come back off the post, but once again having survived the initial threat, Barcelona conceded from the resulting corner.
This time Bergantinos got above Jordi Alba to head into the bottom corner at the far post.
Lux made a great save to deny Suarez a second equaliser, but Deportivo held out to give their hopes of survival a huge boost as they move eight points clear of the bottom three and up to 15th.
And Pepe Mel's men should even have capped their victory with a third goal as Ter Stegen denied Faycal Fajr deep into stoppage time before Jordi Alba cleared the Moroccan's follow-up effort off the line.