A stoppage time equaliser by Thiago Silva ensured Barcelona and AC Milan shared the spoils in a dramatic encounter at the Nou Camp Tuesday.
Barcelona's defence of their European crown got off to the worst possible start when the hosts conceded one of the quickest goals ever scored in the Champions League.
Alexandre Pato opened the scoring with just 24 seconds on the clock, after a static Barcelona defence allowed the Brazilian striker to waltz through their back-line and finish past goalkeeper Victor Valdes.
Lionel Messi turned provider just before half-time with a mesmerising run and cross for Pedro to net the equaliser before David Villa's sublime curling freekick gave Barcelona the lead five minutes into the second half.
However, the hosts failed to convert their dominance into a third goal and Brazil defender Silva headed past Valdes in the 92-minute to ensure six-time champions Milan left Spain with a point.
"There is not much I can say, sometime things like this happen," Barca coach Pep Guardiola said.
"We had a lot of possession, we generated a lot of moves, but that wasn't enough to win this time. But I think we played very well; we moved the ball from left to right; we got in from the second line.
"But Europe is like that - against big teams this can happen. We still have five games; we'll see what happens. I'm not worried."