The last time Barcelona lost any match at Nou Camp, Ernesto Valverde was making his debut as manager in a 3-1 defeat to Real Madrid in the Spanish Super Cup to start the 2017/18 season. The last time they lost a league game in front of their faithful was almost another year earlier, a shock 2-1 defeat to Alaves in September 2016.
That run of 42 unbeaten home games in La Liga came to an end on Sunday, 11 November with Real Betis claiming a stunning 4-3 win – despite two goals from Lionel Messi.
Real Madrid took advantage of Barcelona's unexpected slip by winning 4-2 at Celta Vigo to close the gap on the leaders.
Messi did all he could to keep Barcelona's streak going in his first game back since breaking a bone in his right forearm three weeks ago. The Argentina forward scored a second-half penalty, set up substitute Arturo Vidal to reduce the deficit to 3-2, and added a final goal for the hosts late in stoppage time.
But Betis led 2-0 at halftime thanks to opening goals by Junior Firpo and Joaquin Sanchez. Giovani Lo Celso and substitute Sergio Canales then responded to Messi's spot kick and Vidal's goal, respectively, to keep Betis comfortably in front.
Barcelona were left with 10 men in the 82nd minute when Ivan Rakitic earned a second booking for a hard tackle on Lo Celso. A minute later Firpo led another attack for Betis from the left flank, passing for substitute Canales to slide and push in what proved to be the winner.
"They won the match in the first half," Barcelona coach Ernesto Valverde said. "We played a poor first half, and then each time we got closer they hit us right back."
The Catalans remain one point ahead of Atletico Madrid, who they visit next on 24 November after an international break.
Sevilla and Alaves also moved level on 23 points with Atletico after fighting back for victories.
Madrid remained in sixth place, but moved to within four points of leader Barcelona.
Resurgent Real Stay 100% Under Solari
Santiago Solari’s chances of remaining in-charge of Real Madrid were bolstered after he coached his team to a fourth straight win since replacing Julen Lopetegui two weeks ago.
When asked about Solari's future, Madrid club spokesman Emilio Butragueno limited his comments to "what I can say is that we are very happy (with Solari)."
After Karim Benzema opened the scoring with a classy goal, Madrid got an own-goal by Gustavo Cabral, a penalty by Sergio Ramos and a late goal by substitute Dani Ceballos. Hugo Mallo and Brais Mendez scored for Celta, which never seriously threatened to equalize.