#2 Failure to seize the initiative after a miracle comeback against PSG
If there was any instance that perfectly encapsulated Barcelona's aforementioned tendency to go missing at the most inopportune times, this was it. Barcelona had just won 6-1 against PSG; the greatest European comeback in history. Ahead of Real Madrid in La Liga by a point at the time (having played a game more), Barcelona seemed destined to turn their season around at that point.
Two days later, they lost away to Deportivo in the La Liga. There it was, all the optimism was quashed, the old woes still glaringly on the surface, a thumping crash down to Earth after their historical achievement.
Barcelona never really recovered from that; the feeling of invincibility had been irreparably damaged. Barcelona owed themselves and their fans at least a run to the semifinal stage after the PSG miracle but a strong Juventus side stopped them from doing that as well.