I just about laughed up mah apples when they sang this line, because it was just so hilariously WRONG that it sounded like the writers deliberately calling out all the fans who try to slap a Mary Sue bumper sticker on Twilight’s haunch. For someone who “never fails”, she certainly does spend a lot of time effing things up, running around doing the headless chicken dance trying to fix them, and learning something important from the self-inflicted weekly catastrophe.
Twilight's failures are uniformly based on her own poor decisions. The accusations come from the absurd levels of fortune and success that come from without. In some ways, a character who consistently errs only to have everything still turn out beyond wonderfully for them is rather more of a Mary Sue than ones who are perfect themselves. At least the ones who never fail typically don't destroy dignity by getting wings and political power as a direct result and within a day of forgetting rule number one of magic safety and tearing apart and switching around her friends souls.
Have a fave and a new watcher who will follow you to the ends of the earth.