Four days after the surprising end of the Europa League against Eintracht Frankfurt, FC Barcelona also suffered a painful home defeat in the Spanish football league.
The second in the table lost 0-1 (0-0) against FC Cádiz on Monday.
Lucas Pérez overcame national goalkeeper Marc-André ter Stegen in the 48th minute with a dust collector.
With the victory, the guests left the relegation zone.
In front of just over 57,000 spectators in the huge Camp Nou, Barcelona did not score a goal despite their great superiority.
As a result, the hosts failed to pull away from their equal-ranked rivals FC Sevilla and Atlético Madrid in the race for the Champions League qualification.
However, the Catalans have played one game less.
They are now 15 points behind leaders Real Madrid.
The record champion won 3-2 in Seville on matchday 32 after being down 2-0 and is heading for the next title.
Before the game, supporters of FC Barcelona had protested against the club leadership after the great anger at tens of thousands of Frankfurt fans in the Camp Nou in the 2: 3 defeat in the Europa League.
The media estimated the number of participants at 100 to 200. Club boss Joan Laporta announced a press conference for Tuesday.
There he wants to give a clarifying statement on the events of last Thursday.