The day Twilight earns her cutie mark, something unexpected happens that no pony saw coming. Twilight undergoes a transformation due to an absurd amount of magic and literally becomes the spirit of magic.
A changeling suffers from an embarrassing affliction that causes him to shape shift whenever he has the hiccups. Dealing with it is half the problem, when Chrysalis also sends him on an infiltration mission as well.
After returning from the moon, Luna discovers the wonderful new invention of coffee. Naturally, she takes it way too far and loses her mind. Or maybe she finally sees things as they really are.