(For My Little Pony Friendship is Magic Fans ONLY) What is the Dark Secret?

So, there is an episode coming out that's called A Canterlot Wedding. It will have a new Alicorn named Princess Cadence(shown in picture). In something I read about the upcoming episode, I read that Twilight Sparkle will discover a dark secret about Princess Cadence. What do you think the dark secret is?



(only fans of My Little Pony Friendship is Magic will understand what in the world I'm talking about)