It's a bit of a chore having to hide a part of ones body,regardless of the circumstances. Be it an injury or the side effect of a sickness, to a birth defect, or even just part of a costume or disguise.
Some ponies have to do this for their whole lives.
Rarity is just starting to understand this, it was…quite hard at first,but it's become almost distressingly routine now.
So routine that one day she forgot to cast her glamour spell…