Barca play sweet music but fat lady yet to sing
Worryingly for coach Pep Guardiola, their generosity of spirit extended so far they forgot they are in the business of winning matches as well as drawing gasps of admiration from the crowd.
Given the value of away goals, a 2-2 draw in London put the holders in a strong position to roll their title defence on into the last four but the second leg will be an unnecessarily tense affair after the Catalans let Arsenal off the hook.
"The feeling in the dressing room is one of frustration that we didn't get the job done," said midfielder Xavi, whose perfect pass set up Zlatan Ibrahimovic's second goal.
"We showed that we were the superior team," he told reporters. "It was a great performance from Barcelona and we know we should have closed this one down."
That Arsenal still have a chance going into next Tuesday's return at the Nou Camp is incredible, given the footballing lesson they were forced to sit through in the first half.
The intricate passing and one-touch control Barcelona have made their hallmark under Guardiola earned them a succession of chances they seemed to have to strive to miss before Ibrahimovic finally took them clear with two simple strikes early in the second half.
A place in the semis should have been all but secure given their dominance but they lacked ruthlessness to go with the resourcefulness and goals from England's Theo Walcott and Barca old boy Cesc Fabregas, the latter a debatable penalty, brought Arsenal back from the brink.
"We should have killed them off and we wasted that chance," Guardiola told reporters. "We have to be focused or we run the risk of going out."
Guardiola will doubtless have words with his players for their lack of a killer touch but the man who defined an era as an elegant midfielder with Barcelona, and is doing so again as a coach, could not resist accentuating the positive.
"I'm happy that we came here and showed how football should be played," he said. "It was the best away performance we've put in. I'm happy to have people watch our team playing like that."