Quote Originally Posted by fizmat View Post
Maybe there is just a strict distinction between magic and curses? Magic is always glowy and immediate. You horn glows - Rarity's mane turns green. Curses are underhanded and delayed. The witch doesn't need to even have a horn to curse you, and the result does not manifest until later. If curses are established as a staple impossible spell example since "magic 101" (like perpetual motion in non-pony physics), then Twilight's response is quite understandable.
I still prefer to think that Twilight's just picky about her terminology.