#1 Barca's away-day blues
Barcelona recorded a domestic double, with their only defeat in La Liga coming after they had already wrapped up the title.
The 2017-18 season must go down as a success for new coach Ernesto Valverde, who helped the Catalan club recover from the shock loss of Neymar to PSG, and a resounding and concerning Supercopa defeat by Real Madrid.
However, Barca once again came up short in the Champions League. Their second-leg collapse at the Stadio Olimpico was undoubtedly shocking but that they were beaten away from home was wholly unsurprising.
Indeed, Barca have lost four of their past five away games in the knockout stage, by a combined score of 0-12, with their only goal coming in a 1-1 draw at Chelsea in this season's last 16.
The Catalans, thus, have serious issues to address, both in attack and defence, if they are to challenge for a sixth European Cup next year.
#2 Buffon lets himself down
One can understand why Juventus Gianluigi Buffon reacted so emotionally to Michael Oliver awarding Real Madrid a contentious penalty in the dying seconds of normal time in an absorbing quarter-final second-leg clash at the Santiago Bernabeu.
After wiping out a 3-0 deficit, the Bianconeri deserved extra time and were understandably crushed when it became apparent they would instead be eliminated by a penalty from the nerveless Cristiano Ronaldo.
However, Buffon nonetheless stepped over the line with his furious outburst at the match official. Even then, it could have been dismissed as 'heat-of-the-moment' stuff but Buffon made matters far worse by launching into a very public tirade against Oliver after the full-time whistle.
It was most unbecoming of a man who has always been renowned for his sense of sportsmanship and fair play, and the hypocrisy of the Juve captain complaining so furiously about a referee's decisions was not lost on the supporters of Old Lady's many rivals.
Thankfully, the goalkeeping great belatedly apologised for saying Oliver had a "dustbin for a heart" but by that stage, the damage had been done, both to Oliver, who was subjected to death threats, and Buffon's reputation.
#3 Paris Saint-Germain
This was meant to be Paris Saint-Germain's year.
Last summer, the Qatari-funded club obliterated the transfer fee world record by paying €222 million to secure the release of Neymar from his Barcelona contract, before then taking Kylian Mbappe on loan from Monaco on the proviso that they would sign the teenage sensation at the end of the season for approximately €180m.
It was clear that PSG, more than ever before, had the players to win the Champions League. Unfortunately, it ultimately transpired that they still don't have the requisite character.
A year after their embarrassing capitulation at Camp Nou, they suffered a meek exit at the hands of Real Madrid at the same stage.
Neymar's injury played a part of course but, during that dismal second-leg loss at the Parc des Princes, it was clear that PSG still had neither the intelligence (see Marco Verratti's stupid sending-off) nor the bravery (see Unai Emery's negative tactics) to win the club game's most prestigious prize.
With Neymar having already grown tired of Paris, and Thomas Tuchel feeling like a rather uninspiring replacement for Emery, PSG's Champions League-obsessed owners have a tough job on their hands convincing the footballing world that their plans for world domination can still be realised.
#4 Ill-disciplined & inconsistent Dybala
Paulo Dybala is a lucky boy.
The Juventus striker may have netted a career-high 22 Serie A goals this season but he did not play with anything like the same consistency as Mauro Icardi, yet he made the Argentina squad for this summer's World Cup while his compatriot did not.
Furthermore, while Inter captain Icardi inspired his side to Champions League qualification, Dybala deserted his team when they needed him most, in the first leg of the Champions League quarter-final clash with Real Madrid.
Dybala's senseless dismissal for a second yellow card paved the way for a rampant Blancos side to rack up a third unanswered goal in Turin, which ultimately proved decisive.
Argentina boss Jorge Sampaoli has called Dybala an "anarchic talent" and it is a fair point: it is high time that the 24-year-old complements his exceptional skills and wonderful left foot with greater composure and consistency.
#5 VAR still MIA
VAR is by no means flawless. During Saturday's FA Cup final, Ashley Young got away with a clear handball even though the Manchester United defender had clearly motioned towards the ball with his left arm.
However, thanks to VAR, officials in Serie A benefited enormously from video assistance this season, righting a number of wrongs.
It was, thus, easy to understand the frustration of Roma president James Pallotta after seeing his side narrowly fail to overturn a 5-2 first-leg deficit in a game in which his side were denied two clear penalties, one of which would have resulted in a red card for Liverpool's Trent Alexander-Arnold.
The American opined, "If we don't get VAR in the Champions League, it will be an absolute joke."
Improvements still need to be made - particularly in terms of transparency and the time involved - but it's hard to disagree with Pallotta's opinion.