In the entire history of the C1 there has been 185 matches were a team lead 4-0 in the home match and precisely 0 failed to qualify after the away match... now there is one: the PSG!
I think the craziest part is that it's not because the Barca played an amazing match, it's a mix of the PSG shaking in their boots and a conjunction of weird calls and situations. I mean you have a clusterfuck in the box with a keeper that should go to the ball but decides to backtrack while his defenders keep their thumbs in their ass for the 1-0. You have a defender that fails to kick the ball out of bound ending in a blind back pass with the heel that a defender manages to turn into an unlikely own goal for the 2-0.
Early in the 2nd half, a defender stumbles and falls head first in the box only to end in the legs of an attacker for a penalty kick (given by the box ref!). Said penalty kick is shot in the worst possible spot with the keeper choosing the correct side, but it still scores the 3-0... The PSG suddenly realize they have to play and score a brilliant goal for 3-1. This deflates the Barca players and the PSG get another couple near goal actions, but one is fumbled and the other is saved by the keeper.
At the 88th minute it's 3-1 for the Barca but they need another 3 goals. That's the moment a defender decides to chose to makes a totally unnecessary foul far from the box that results in a brilliant free kick for the 4-1. Two minutes later, a pretty generous penalty kick is awarded and suddenly it's 5-1 in stoppage time... oddly, the least stupid goal was the 6th, with a brilliant cross over the defense to an area where the keeper cannot really intervene and a player throwing himself just in time to score a brilliant goal that will forever remain in european football history.
It's pretty insane that PSG could play so well in the home game and choke that much in the away game... it's pretty exceptional. More than that: it's a first!