Bah, she probably was simply tested on something related to frienship and magic, and since she doesn't have any hard facts on that kind of magic, only her own experience is that she is so aprehensive. Also we know Twilight is a worrysome worypants of first magnitude...
Celestia definetely trolls Twilight... but for her own good. I'm certain of it...
So should we start a betting pool on what Twilight believes that she failed at.
My vote is as I said something like "Friendship and it's effect on magic" on something around that.