The loss they suffered against Chelsea was not about mistakes, PSG did not make too many or even enough mistakes to say there was any in that game, for me it was more about the mentality, the mentality of the team was one of the factors that gave them such loss and the coaches tactical approach to that game. The mentality and reluctance they played that game was too much, one they never displayed against Madrid so it was not at all about any mistakes but they will have definitely learn and will be poised towards making up for such and hopefully in the champions league they do not repeat such with any team.
I will say this defeat was very much needed for PSG. Because we have seen that PSG has been performing well since last season. Also PSG wins by big margins against every elite class team like City, Liverpool, Arsenal, Inter Milan, Bayern Munich and Real Madrid. So the team was utterly overconfident, after this defeat, Luis Enrique must have perceived that PSG is not Unbeatable at least. They have to know there are still a number of weaknesses in his squad, so this is the right time to fix those weaknesses. I hope that before the start of the season, Luis Enrique will identify those weaknesses and try to strengthen overall. Actually I am looking forward to seeing PSG's comeback in a stronger form next season.