Everypony makes mistakes, from the youngest foal to the wisest elder. Ponies do not get much elder or wiser than Princess Celestia, so much that most would say she's an exception to the rule. She has led the ponies of Equestria for over a millenium, bringing about an unprecedented era of peace and prosperity. Some ponies think of her as a god, an utterly infallible being who they can always count on to make the right decision no matter what. Others beg to differ, but reason that a thousand years on the throne give her a foresight which others lack.
In truth, she is neither infallible nor omniscient. She makes mistakes like any other pony. Sometimes her plans go awry, sometimes she simply doesn't see danger coming. Her errors are few in number and often work out for the best, and the ponies closest to her tend to forgive her. They know her, after all, and unlike the masses they know she's only a pony.
The Princess, however, is not so easy on herself. For when you are both the absolute ruler of the land and the one everypony relies on to literally keep the heavens up, any misstep can have grave consequences.
