Los Leones won what was an absolutely thrilling contest 3-2 having taken the lead on three separate occasions after Barcelona equalised twice.
Iker Muniain fired Athletic into the lead after just two minutes, with Ferran Torres clapping back in the 20th minute to level things up and register his first goal for Barcelona since joining from Manchester City.
Inigo Martinez then thought that he had won it for Athletic in the 86th minute when he made it 2-1, only for Pedri to equalise again for Barcelona deep in stoppage time.
Then, in the first half of extra time, Muniain scored his second of the evening from the spot to make it 3-2 to Athletic.
Aside from being knocked out of the Copa, Barcelona suffered perhaps an even more consequential blow on the evening when Ansu Fati left the pitch crying and injured early in extra time.
Fati is just back from a lengthly spell on the sidelines and the fear around the teenager is that he may have suffered a relapse as nobody was near him when the incident occurred.
It’s expected Barcelona will reveal the extent of the injury in the coming hours and their supporters will have their fingers crossed.