It wasn't anypony's fault, there was no mistake. When Twilight Sparkled gets word that Trixie has been accepted into Celestia's College of Advanced Magics, she also discovers the reasoning behind it. Much to her disappointment, she becomes a lesser unicorn than Trixie, and helps Trixie learn that the power of friendship is something earned, not achieved, as they fight an upstream perilous journey to fulfill and ancient destiny.

The Reckoning.