Well that was something. Barcelona’s 6-1 win at the Camp Nou on Wednesday night overturned a 4-0 deficit from the first leg at the Parc des Princes and completed la remontada for Luis Enrique’s side. But however brilliant Barcelona was, PSG’s collapse allowed the Blaugrana an historic victory, giving up three goals in the final ten minutes.

Let’s take a look at just what caused the collapse from the Parisians.

Unai Emery got his tactics wrong

Unless you’re 2012 Chelsea (because really, did anything go wrong for them in the Champions League that year?), you don’t beat Barcelona by sitting deep and letting them come to you. Bayern Munich dismantled the Catalans with their relentless pressure, and Atletico Madrid’s successes in 2014 and 2016 came from of Diego Simeone’s managerial philosophy: if you’re not close enough to blow on your opponent’s ear (Lance Stephenson style), you’re not close enough to defend. Giving Barcelona space to dictate play around your penalty area is the wrong way to go.

Beyond just his errant tactical instructions, the starting XI sent out by Emery was a poor decision, specifically in three areas. First, Maxwell should have started at left back instead of Layvin Kurzawa. The 35 year-old Brazilian used to be on Barcelona’s books and therefore has a wealth of experience in big European matches. In a back line whose only clear leader was Thiago Silva, Emery should have opted for experience and better defensive solidity since he wasn’t sending his fullbacks forward.

Second, Adrien Rabiot was misused as a holding midfielder. Ideally, Thiago Motta would have sat in front of the back four, but with his injury, Emery decided to use Rabiot next to Blaise Matuidi and play Marco Verratti at the top of the midfield triangle. Verratti’s constant energy and willingness to get stuck in would have been better served next to Matuidi. Rabiot would then have been able to find the ball higher up the pitch and send one of the wingers or Edinson Cavani running against Barcelona’s back three. Alternatively, if Emery was so insistent on playing Verratti further forward, he ought to have played Grzegorz Krychowiak in front of the back four, letting Matuidi go win the ball instead of shadowing Lionel Messi. Krychowiak was brilliant for Sevilla last year and has experience playing against Barça. He could have helped PSG’s defense immensely.

Finally, Emery’s biggest mistake was leaving Angel di Maria, the star of the first leg, out of his starting lineup. PSG was instantly more dangerous after di Maria’s introduction in the 55th minute, and he and Cavani each had chances to add a second goal and put the tie to bed. The former Real Madrid midfielder has had success against Barça beyond just the first leg on Valentine’s Day and won the Champions League during his final season in Spain’s capital. Leaving him out, and choosing the inconsistent Lucas in his place, was criminal.

The former Sevilla manager has to take the blame for this one.

Finishing let PSG down

Di Maria probably should have scored his chance in the 85th minute, but that is understandable since even fellow Argentine Javier Mascherano can’t completely deny that he committed a foul. But Cavani’s 64th minute chance, two minutes after he scored his goal, should have been finished. For a striker of his quality, he is far too wasteful in the big moments, and he had a brilliant opportunity to make the aggregate score 6-3 and force Barcelona to score 4 more goals. Instead, Barça still needed only three goals and they got them. Despite his brilliant equalizer, Cavani will rue this missed chance, along with his shot against the post in the 52nd minute.

The defense was horrid

Only Silva had a passable game for PSG, but that’s what expected of your captain. On the left, Kurzawa was let off easy for the first 65 minutes of the match, as Rafinha was always content to cut inside on his left foot and play the ball to Messi or Rakitic. After Arda Turan’s introduction in the 65th minute, Kurzawa was put under increasing pressure, given Turan’s tendency to drift out to the right. Sergi Roberto’s introduction to the match also put increasing strain on Kurzawa. He also had an own goal, but that can hardly be blamed on him.

Marquinhos, a 22 year-old Brazilian who has shown promise, showed instead his inexperience at the Camp Nou. It was his poor defensive header that led to the first goal, although some of the blame there must also go to his goalkeeper. The Kurzawa own goal came from a poor ball over the top from Luis Suarez that Marquinhos tried to shield out of play for a goal kick, a ball that should have instead been cleared away for either a throw in or a corner toward the end of the first half. With a three goal lead, you don’t take risks like that, and especially against a Barcelona team that has exactly one aerial threat, a corner kick is not a very dangerous proposition. Then, in the 90th minute, with Barcelona still needing two goals, why foul Suarez? The Barcelona forward ran across Marquinhos, and when the defender turned his head away from the ball, he reached out for Suarez’s shoulder and his knee clipped Suarez’s trailing leg. It wasn’t a lot of contact, but it’s not incidental. You know the top strikers are going to exaggerate that contact, and while Barcelona has a reputation for diving (maybe because of this), it’s more than just Barcelona players who do it. You know this if you are a professional center back. It’s a stupid penalty to give away in a moment of panic, and he’s lucky that the rules for denial of an obvious goal scoring opportunity in the penalty area have been changed, or Marquinhos would have seen red too.

But even Marquinhos wasn’t as bad as right back Thomas Meunier. While he was brilliant going forward in Paris, Neymar had his way with him offensively in the first leg, and that was only a preview of what would happen in the return leg. For every late clip at the heels Meunier left on Neymar, there were two runs from the Brazilian that left the Belgian right back exposed. It’s still baffling that Meunier avoided a booking in the first half, but even more of a shock that he wasn’t booked after giving away the penalty that Messi converted to put Barça up 3-0. This time, it was Andres Iniesta’s reverse pass for Neymar that made Meunier slip, but instead of just falling, he stumbled for a few steps before eventually giving up on the ball and collapsing to the ground. His forward momentum took him into Neymar’s path, which made it all too easy for the Brazilian to just keep running and take the easy penalty. Neymar continued to torment Meunier all night, and although the right back escaped Spain without a card, he did little to cover himself in glory.

For Roberto’s winner, the entire PSG defense froze, seemingly playing offside and letting runners into the box. It was Serge Aurier who was tasked with marking Roberto, but Gerard Pique also got off the back shoulder of his marker, and a shorter ball from Neymar would have fallen to Pique instead. Both players were onside. PSG should have known better than to leave a close decision like that in the hands of the assistant referee, even if they were correct. They fell asleep after clearing the initial free kick away and certainly deserved to lose based on that mistake.

PSG has a goalkeeper problem

Kevin Trapp was partially at fault for four of the six goals allowed by PSG on Wednesday, the exceptions being Kurzawa’s own goal and Messi’s penalty goal. Trapp is a better than average shot stopper, but his lack confidence during the game against Barcelona was obvious. On Suarez’s opener, a poor defensive header from Marquinhos floated into the air and bounced on the edge of the six yard box before Suarez beat Trapp to the ball two yards from the goal line. There was more than enough time for Trapp to see the floating ball and get to it before Suarez even had a chance at a header.

The next ball served into the box by Barcelona found no attacker, and Trapp was able to easily claim the ball under no pressure, or so it seemed. Instead, he bobbled it before eventually collecting the cross into his chest. On another cross from a free kick, Trapp decided, correctly, to come off of his line, but with no Barcelona player within two yards of him, he elected to punch a ball that was easily catchable. Throughout the first half, Trapp’s distribution has poor, doing nothing to allow his team a foothold on the match.

By the time it looked like Trapp had finally quelled his nerves, Neymar’s free kick from ten yards wide of the near post found the top corner. Yes it was a great shot, but Trapp didn’t dive, nor did Neymar hit it particularly hard. Barcelona needed three goals to advance, so there was no reason for Trapp to gamble that the ball was going wide. If Trapp had made absolutely sure that the free kick stayed out, something he could’ve done easily, Barça would have needed three goals in stoppage time instead of two. Three in that time seems insurmountable, but two had been done many times before, most famously by Manchester United in the 1999 Champions League final.

The first of those two came from Neymar’s penalty after Marquinhos’ foul, but the ball played over the top to Suarez wasn’t a driven long ball (like this one). It was a long, floated ball by Lionel Messi from nearly midfield that eventually landed around the penalty spot. Suarez made a 15 yard run in front of Marquinhos to get in position to play the ball; Trapp moved backwards to just in front of his six yard box. If Trapp came for the ball, Marquinhos wouldn’t have needed to try to stop Suarez. Trapp could have taken a couple steps forward and plucked Messi’s ball out of the air, preventing a penalty before there was even a chance.

For the final goal, Trapp began too deep with Neymar cutting in on his weak foot. A left footed Neymar shot from 35 yards out isn’t going to beat Trapp, so he has to know that Neymar wants to clip the ball back into the box. He was only a step off of his line when Neymar’s ball inevitably came, and hesitantly took two steps forward as Roberto slid to put the ball into the net. This is 5-foot-10, 157 pound Sergi Roberto up against 6-foot-2, 194 pound Kevin Trapp that can use his hands. It is this exact situation where a confident goalkeeper is coming out for the ball and being as large as possible to either make the save or intimidate the attacker (a Joe Hart specialty), and in this case the attacker is a center midfielder turned right back. Trapp instead came out slowly, never fully set his feet, and hardly fought the momentum carrying him to his right, leaving the goal open as he simply flapped his arm in desperation.

Kevin Trapp is not good enough for PSG to win the Champions League. Wednesday’s match at the Camp Nou made that painfully obvious.

PSG’s transfer strategy is ineffective

Given the defensive frailties of this PSG team, it’s a mistake to continue to look for attacking talent in the transfer market. It will sell tickets, and Julian Draxler was magnificent in the first leg of this tie, but it doesn’t matter how many goals a team can score if it’s liable to leak more in. That’s the problem PSG has, and their next step in the transfer market has to be to shore up the back line and defensive midfield. Thiago Silva can’t be the answer to all of the team’s defensive issues, the same defensive issues that lead to PSG’s demise against Barcelona.