My Little Pony XIX: I Emptied Your Thread

[darthbobcat]

That came off as arbitrary to me, too. Both of those episodes would have been better had they done more world building early on and explained how unicorn magic is supposed to work.

Ah, but they don't dare. Much like they'll never publish an official map of Equestria so they can add, say, a Russia equivalent for an episode when the fancy strikes them, they don't dare establish real rules for Unicorn magic because then in a later episode, when they need Twilight to do something they said Unicorn magic can't do, they're sunk.