Previous to her redemption, Sunset Shimmer was truly fearsome, specially for Fluttershy. This ended in more than one embarrasing accident for the shy girl. Fluttershy was so afraid of Sunset that she even had nightmares, whose ended in bedwetting, and eventually, in diaper usage.

Obviously, after her redemption, Sunset feel terrible when she learned about Fluttershy's bedwetting problems, even when Fluttershy enjoys to wear diapers.

Bad edit of >>527146 (Come on, that "pee" looks like mustard)