Barca went down 2-1 in Andalusia on Saturday to extend their winless run away from home in LaLiga to four matches.
The winning goal, scored by Alvaro Negredo, came after a dreadful mix-up at the back as Clement Lenglet failed to control a simple throw-in and Marc-Andre ter Stegen's clearance was charged down.
The goalkeeper was not blameless, but the decisive strike went down as a Lenglet error, according to Opta, his first in 99 league games for the Blaugrana.
While it was a rare lapse for the centre-back, these mistakes are becoming worryingly common among his team-mates.
In their past three away games, from which Barca have taken just a point, Ronald Koeman's side have conceded three of four goals due to errors.
Neto was embarrassingly tackled in the draw at Deportivo Alaves, before Ter Stegen needlessly charged from his line in the one-goal defeat to LaLiga leaders Atletico Madrid.
Even the opener at Cadiz, although not deemed an error, came after Oscar Mingueza aimed a header at his own goal, allowing Alvaro Gimenez to tuck away the rebound.
It meant Barca lost despite boasting 82.1 per cent of the possession, the second-most for a beaten side in LaLiga since 2005-06, behind only the 2013-14 Blaugrana team at Granada (82.28 per cent in a 1-0 defeat).
"We reacted, but it costs us much more if there are two goals," midfielder Busquets told Movistar. "We made a mistake and then there was half an hour left when we equalized, but we made another mistake.
"Individual mistakes are costing us a lot. We will have to correct that, have more fortune, and continue."
Barca have had six errors leading to shots and three to goals in LaLiga this term, yet their three such mistakes in Champions League matches – including one from Lenglet – have gone unpunished.
Koeman's men lead their European group with five wins from five and can clinch top spot against Juventus on Tuesday.
"In the Champions League, the team seems different," Busquets added. "I do not know why.
"What I can say is that, in recent games, occasional errors turn the game against us. If they score a goal, they close up at the back.
"We cannot blame ourselves for attitude or commitment but this costs you games."
The meeting with the Bianconeri provides a welcome distraction from what is turning into a particularly testing league campaign, with Barca way down in seventh and already 12 points behind Atleti.
Busquets said: "There are many defeats. It is clear that there is a lot of LaLiga left, but the more we lose the more we fall away from the top positions."