Holders Barcelona recovered from the shock of conceding a goal after just 23 seconds against AC Milan - only to blow their chance of victory at the death.
Pep Guardiola's men were stunned when Brazilian Pato burst clear in the opening moments to fire coolly under Barcelona keeper Victor Valdes and give the Italians the lead in Group H.
Barcelona stormed back with Lionel Messi clattering a free-kick against the woodwork before the Argentinian brilliantly teed up the equaliser for Pedro after 36 minutes.
With Barca in cruise control a second goal seemed inevitable and it came after 50 minutes through a brilliant David Villa free-kick before both Messi and Pedro came close to adding a third.
But the hosts were punished for their failure to put the game beyond doubt three minutes into injury time when Thiago headed home a Clarence Seedorf corner to earn Milan a point in a 2-2 draw.