Sergio Gonzalez's fourth minute header stunned LaLiga leaders Barcelona as Leganes earned a shock 1-0 victory at the Camp Nou on Sunday.
Barca have now suffered consecutive home defeats for the third time in all competitions in the 21st century, after April 2003 under Radomir Antic (two) and April 2022 with Xavi (three).
The visitors took the lead early on when Oscar Rodriguez's corner kick found an unmarked Gonzalez at the near post to thump the ball beyond Inaki Pena.
Robert Lewandowski came close to restoring parity seven minutes later, but LaLiga's leading marksman was unable to turn Raphinha's cross beyond Marko Dmitrovic.
Lamine Yamal then fired over in first-half stoppage time before Jules Kounde poked narrowly wide shortly after the restart.
But the hosts' best chance of an equaliser came in the closing stages, with Ferran Torres neatly flicking the ball into Kounde's path, but he dragged his effort past the post.
Barcelona remain top of the standings, level on points with Atletico Madrid and a point ahead of Real Madrid, but they have played a game more than their title challengers.
Full Time#BarçaLeganés pic.twitter.com/ywBiim6nOS
— FC Barcelona (@FCBarcelona) December 15, 2024
Data Debrief: All bark, no bite
After Atletico earned a narrow victory on Saturday and with Carlo Ancelotti's side also dropping points, Barcelona missed the opportunity to open a comfortable advantage at the summit.
Hansi Flick's side ended the contest with an expected goals (xG) total of 2.67 from their 20 shots, though only four were on target, while they also managed to restrict Leganes to just six attempts, but the visitors matched their opponents' total for efforts on goal.
Adding to their frustration, Barcelona enjoyed 80% possession against Leganes, the second-highest by a team in a home defeat in LaLiga since 2005-06.