It plagues authors with doubt, especially when they're inserting an OC into a fanfiction or writing a female protagonist.
And it can dampen a fan's enjoyment of a story, when they find themselves questioning whether they're looking at a good and competent character or a Mary Sue.
Since I'm human and not-Mary-Sue enough that I've been slowed down by a cold today, I'm going to let another author cover this topic for me. In this article, she'll explore the criteria that define a Mary Sue, and also reveal a double standard that has appeared in modern fiction.