Barcelona veteran Gerard Pique made a costly error as Xavi Hernandez's side were left staring into the abyss with their Champions League dreams in tatters.
A 3-3 draw at home to Inter Milan has left the Catalan club facing a costly early exit from the competition having leveraged much of their controversial summer business on a strong run in the competition.
Barcelona will be eliminated from the competition should Inter Milan beat so far point-less Viktoria Plzen in their next game.
Having lost 1-0 at the San Siro last week, Barcelona knew that only a win would do at the Nou Camp this week and they got off to the perfect start thanks to Ousmane Dembele's opener.
However, Pique, making only his fourth start of the season, his first in the Champions League, gifted Inter Milan an invaluable equaliser.
With an aimless ball having been played in Alessandro Bastoni with Inter Milan chasing an equaliser, Pique stepped up and ushered the ball through to - he thought - his goalkeeper, Marc-Andre ter Stegen.
However, lurking behind him and the rest of the Barcelona back four was Barella who gratefully received the gift handed to him by the four-time Champions League winning defender.
Scarcely believing what they were seeing, Gavi attempted to chase back but his sliding challenge was not enough to stop Barella planting the ball beyond a hapless Ter Stegen.
Xavi said of the mistake: 'It was too many mistakes, their first goal was a clear mistake by our defence, we let them have too many chances. Soccer is a game of mistakes and we can't afford those in situations like this.
'This is the Champions League and you pay a high price for each error you make.
'You need to win, to defend well, to score. We needed to play the entire game like we did in the first half. That intensity. Now it's hard because we are not in control anymore.'
Seemingly shaken by the equaliser, Inter Milan sensed blood amid the Barcelona ranks and pressed for the killer blow.
They found it 13 minutes after their equaliser through Lautaro Martinez. Robert Lewandowski's equaliser in the 82nd minute looked to have set up a grandstand finish for the Catalans but Robin Gosen's goal just before the 90th minute appeared to have settled it.
Lewandowski scored again to set up a nervy final few minutes for Inter Milan but they held out to secure a valuable draw and put them within touching distance of the last 16.
The result leaves Barcelona's ambitions in Europe, much of which underpinned their extensive summer spending, lying in ruin.
The club face dropping down to the Europa League for the second time in two seasons. For a club that purports to be the world's biggest and one that has won four Champions Leagues this century, that is seemingly unacceptable.
Xavi told Movistar Plus after the game: 'It's bad, I think this Champions League is being very cruel to us.
'Now it is out of our hands. What is left for us is to turn our attention quickly to Sunday's El Clasico (against Real Madrid), bounce back and focus on La Liga because the Champions League became very difficult and complicated for us.'