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 think that's basically my point, yes. Except perhaps the last part.

I don't think curses or at least curse-like effects need to be impossible, if you're a unicorn. Seems obvious that Twilight could do a lot of things that would seem like a curse to the recipient.

I think when the ponies talk about curses, they're talking about magical-like effects with a non-magical origin, and that's what Twilight is or should be objecting to. If she's objecting to the effects themselves being impossible, then she's just plain wrong.