- A Possession Sue. It's more frequently referred to as Canon Sue, but it has its own article.
- A professional work in which a canonical character already has the attributes of a Mary Sue.
In the second version, this can sometimes be tricky to identify; creators often draw on their own personality, appearance and/or history for inspiration, since it's easy and convenient to write what and who you know, so the appearance of these doesn't automatically mean a Canon Sue. However, if the character that appears is very obviously an Author Stand In (especially to such a degree that the character's back-story and the author's biography are practically interchangeable), believes in everything the creator believes, does everything the creator does (or wants to do) and is absolutely correct and perfect in both, is over-idealized and perfect to a fault, and/or (most importantly and required) seems as though the story exists only to further this one specific character (most often to the detriment of any other characters present, who simply cannot measure up to the author's clear favourite), you're in Mary Sue territory.
Ship-to-Ship Combat also plays in, as the Canon Sue accusation is often leveled at characters who hook up with the handsome male lead (or the Ensemble Darkhorse) in a blindingly obvious form of Die for Our Ship. Also, with the general trend of Mary Sue being used as an all-encompassing insult for any positively portrayed character, this accusation gets thrown around a lot without much basis in fact.
In general, Canon Sues are significantly less common than Fan Fiction Mary Sues, since most professional authors have more skill, talent and accumulated experience than the amateurs who write fanfiction. However, this means that when they do occur they are significantly more visible (who's going to remember the hastily typed out daydream of a 12 year old girl in a year?). Due to the author creating an entirely different template for the work of fiction, they can set up the rules of The Verse that allow or disallow particular tropes or characteristics. As such, they can choose what Common Mary Sue Traits are allowed, making it a bit harder to see when plot bias is in effect. However, they're hardly immune to this trend.
The Magical Girlfriend, Tsundere and Yamato Nadeshiko, while occasionally possessing similar traits, are not usually viewed as such; they are seen more as targets of enticement instead of ones to emulate; whether they're still Canon Sues is left to the viewer. Also see The Ace, which puts a fairly blatant Marty Stu in a supporting role for comedic purposes.