Barcelona's perfect start to La Liga came to a shuddering halt on Saturday when they were beaten 4-2 by Osasuna.
The Catalans had won all seven of their opening fixtures but Hansi Flick's heavily rotated side came badly undone in Pamplona as Ante Budimir scored a brace either side of Bryan Zaragoza's strike.
Pau Victor had given Barca a lifeline to pull them back into the game before Budimir bagged his second from the penalty spot.
Abel Bretones added a sensational fourth from the edge of the box, before Lamine Yamal pulled one back with a fine effort of his own.
Victory would have matched Barcelona's best start to a league campaign with eight consecutive wins, but they were not able to build on their four-point lead on second-place Real Madrid.
The Spanish champions visit third-placed Atletico Madrid in a derby clash on Sunday.
"In the first half we made a lot of mistakes, it's not normal we play like this – we had a lot of changes in the team and this is maybe one reason for that," Flick told reporters.
"I think it was necessary. We had so many matches, a lot of players had so many minutes, I have to take care about that. This is my responsibility.
"If you want to blame someone for this defeat then take me."
Ahead of a Champions League match against Young Boys on Tuesday, Flick felt he had to shuffle his pack, especially after Barca lost at Monaco in their European opener.
The coach, missing several key players through injury, handed starts to inexperienced defenders Gerard Martin and Sergi Dominguez as well as Pablo Torre in midfield and Victor in attack.
The coach rested Spain Euro 2024 starlet Yamal and Brazilian winger Raphinha among other regular picks, though he did start seven-goal top scorer Robert Lewandowski.
As a result Barcelona failed to ignite in the first half and Osasuna winger Zaragoza took centre-stage.
Last season Zaragoza struck twice while playing for Granada in a 2-2 draw against Barca, a performance which helped earn him a move to Bayern Munich.
On loan from the German giants, Zaragoza created the opener for target-man Budimir with a fine cross, which the Croatian nodded home with ease in the 18th minute after escaping the attentions of Dominguez.
Zaragoza finished the second himself 10 minutes later after being sent through on goal, with the winger bypassing goalkeeper Inaki Pena with a fine ball roll and then slotting into the empty net.
Barcelona complained there had been a foul on Victor in the build-up but the referee waved their complaints away.
The leaders created little themselves, with the best opening falling to Ferran Torres. However the Spanish winger miscontrolled on the edge of the box and was quickly swallowed up by the Osasuna defence.
Victor pulled Barcelona back into the game after Osasuna goalkeeper Sergio Herrera gave the ball away with a poor throw and then could not keep out his early shot from distance.
With Yamal and Raphinha thrown into the fray Barcelona began to threaten increasingly, although Lewandowski was taken off with 20 minutes remaining after a flat display.
Against the run of play Osasuna increased their lead.
Dominguez, 19, scythed down Budimir in the area as he went to shoot and the striker buried the penalty.
Ruben Pena wasted a glorious chance to grab a fourth but Bretones made no mistake with a vicious effort from the edge of the box.
Yamal hit back brilliantly from range and teed up Torres who struck the post, but it was too little, too late.
"It's normal there are rotations, there are a lot of games, it's not an excuse," Barcelona midfielder Pedri told Movistar.
"In the second half we deserved a bit more but we let in goals when we were at our best."
Osasuna rose to sixth and Zaragoza said his team had been determined to beat the leaders.
"We knew Barca were an incredible team, they had won everything and we knew we had to get stuck into it, and that's how we went out there," explained the winger.
"It's a special game and really nice to play in."
Earlier Real Sociedad defeated Valencia 3-0 and Getafe won their first game of the season with a 2-0 home victory over Alaves. Rayo Vallecano and Leganes shared a 1-1 draw.
