Lo Celso blasted a shot towards goal and it seemed routine enough for Ter Stegen but the ball squirmed through his grasp and into the back of the net to deepen Barcelona's misery.
Three hours later, Real Madrid were putting the finishing touches to a 4-2 win away at Celta Vigo, a result expected to earn Santiago Solari an extension to his tenure as coach.
Barcelona's Lionel Messi in action with Real Betis' Marc Bartra. I make assessments that are for the good of the team and I take everything into account when making them. The Blaugrana will play Atletico Madrid after the worldwide break and will then make a trip to the Netherlands to play PSV Eindhoven for the Champions League group stage fixture.
Alaves beat Huesca 2-1 earlier on Sunday, while Sevilla won by the same scoreline at home to Espanyol, Wissam Ben Yedder prodding in an 89th-minute victor. We lack freshness when we have to finish attacks and we have to improve that.
However, Barcelona goalkeeper then let a shot from Giovani Lo Celso slip through his hands and although substitute Arturo Vidal reduced the deficit on 79 minutes, Ivan Rakitic was sent off moments later and Sergio Canales made it 4-2 soon after.
Barca have dropped points in five of their 12 games this season, four times against sides in the bottom half of the standings but Pique said fans should not be concerned by the team's form. "Winning at the Camp Nou requires concentration and doing everything very well".More news: Former Strawberry Farm Worker Charged Over Contamination Scandal
"When you come here, you always expect to suffer but we knew if we showed character and overcame the stress we could play well and have the ball, and Barca are a team that suffers without the ball and struggles to get it back".
Joaquin Sanchez made it 2-0 for Betis before halftime.
His playing time has been limited by a series of hamstring injuries and he has also fallen foul of the club's disciplinary code, including turning up late for a team meeting before a Champions League game at home to Inter Milan in October. Sergio Ramos, however, chipped in a late penalty, Cabral was sent off for a second yellow card and Dani Ceballos wrapped up the points with a superb long-range strike.
Messi added his second in injury-time, tapping a Vidal pass over the line.
The Béticos however finished the first half of the clash with a two-goal advantage heading to the dugout for the break.