- 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/Marty Stu.
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.
On the other hand, this can feel very appropriate in a franchise's spin-off—to keep the series 'fresh', a new character is introduced, and in order to make an imprint on the viewer they are just AMAZING. They're clever, they're strong, they're authorative. If they're in the military, the uniform doesn't necessarily apply, and neither do regulation hair-dos. Frequently we can see characters' attraction to them—even if this requires dumping a former love interest. And of course, Sue/Stu shows that they doesn't take shit from anyone by being dismissive or outright terrible to the people fawning of them. This fault either goes unnoticed, making them impossible to like, or fades within an episode or two, and their personality evaporates with it because that was all there was to the character.
Ship-to-Ship Combat also plays in, as the Canon Sue accusation is often blatantly leveled at characters just because they hook up with a lead (or the Ensemble Dark Horse) who the accuser ships with somebody else. Also, with the general trend of Mary Sue being used as an all-encompassing insult for any female character considered to get "above herself", this accusation gets thrown around a lot without much basis in fact.
In general, Canon Sues are significantly less common than fanfiction Sues/Stus, since most professional authors have more skill, talent and accumulated experience than the amateurs who write fanfiction, not to mention the help of professional editors. 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. Nor is this a new element; George Eliot's Silly Novels by Lady Novelists castigates many original novels of her day for obvious Mary Sue traits (among other flaws).
For the most blatant and infamous examples see Creator's Pet. 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 an idealized character in a supporting role in the story, often serving as a rival, a foil, a source of comedy, or as a mentor (or possibly just the Hero of Another Story). Suetiful All Along can also play a part as well. See Her Code Name Was "Mary Sue".