Barcelona saw their long winning run against Rayo Vallecano end on Wednesday as they lost 1-0 at Estadio de Vallecas.
In need of a positive result after their 2-1 Clasico defeat, Ronald Koeman's men instead suffered a fifth defeat of the season in all competitions courtesy of Radamel Falcao's first-half goal.
Barca went into this latest contest on a run of 13 consecutive LaLiga wins over Rayo in which they had averaged 4.3 goals per game, yet they paid the price for Memphis Depay's missed penalty midway through the second half.
Marc-Andre ter Stegen made a desperate early save to keep out Oscar Trejo's ambitious long-range chip, but it began to look like Barca would keep up that formidable scoring record, Depay going close and Sergio Aguero seeing vague penalty appeals against goalkeeper Stole Dimitrievski dismissed.
Yet it was Rayo who took the lead on the half-hour mark, Trejo robbing Sergio Busquets and threading the ball to Falcao, who turned away from Gerard Pique before finishing low to Ter Stegen's right.
Sergino Dest missed a golden chance to equalise before the break, turning over the bar from point-blank range after some brilliant play from Depay down the left, and Aguero blasted high and wide from inside the box after the United States international had teed him up.
Depay earned the chance to level from the spot after he was clipped from behind by Oscar Valentin, but Dimitrievski saved superbly to his right before bravely gathering the rebound as the Barca forward closed in.
Dimitrievski was more hesitant with a loose ball outside the Rayo box in injury time, but substitute Luuk de Jong's attempted lob landed on the roof of the net.
In eight minutes of injury time, Aguero glanced a diving header wide and then set up Gavi for what looked a simple finish, but the young midfielder scuffed his effort wide.
What does it mean? Barca title challenge looking bleak
The end of their LaLiga dominance over Rayo feels like a watershed moment for Barca and their hopes of quickly becoming a major force again.
With four wins from 10 games, they are six points off the top of the table and have now gone five away matches in all competitions without a victory.
Falcao still has the magic touch
After scoring in three of his past four league games against Barca, it was little surprise to see Falcao convert his one chance with the kind of poise he showed in his best days with Atletico Madrid.
He had lost all four of those previous meetings with the Blaugrana, so this was a sweet moment indeed for the Colombia striker.
It looked like Aguero would mark his first Barca start with a goal when he lined up the kind of high near-post finish that became a trademark of his at Manchester City. He missed, though – and by a long way.
With just 31 touches throughout, by far the fewest of any starting Barca player, this was not a match to bring the best out of the Argentina striker.
The pressure will be even greater on Barca when they host Deportivo Alaves on Saturday, with Rayo heading to Celta Vigo on November 1.