Twilight is sure of what she's doing, every calculs, every part of the spell, every has been checked, and even in case of failure everything says that her going back in history is part of it, she just don't know about it! 0% of chance of faillure in this!

or maybe the theory isn't true and in that case a failure would be mor that a problem...

/)^^(\

inspired by pretty much everything in this but probably more by the time-travel story I read, some may not be finished writing so they won't be in there