Loose Canon

This material is separated from the main continuity, usually in the form of special episodes or even Bonus Material. It's not out of Canon. But it's not treated as if it was fully in Canon either. Its continuity status is "No Except Yes": not quite accepted, and not quite rejected. None of this will be referenced in Canon, but it will not be contradicted outright either.

As opposed to the Canon Discontinuity which was explicitly removed from Canon later, this "Optional Continuity" implicitly was not firmly tied into Canon to begin with. Unlike Big Lipped Alligator Moment, it's not just a random throwaway gag that pops up unexpectedly only to sink forever, it's a complete piece of continuity not thrown away, but still set aside. While it cannot participate in over-arching storylines, it serves to highlight characters or setting that do belong to the Canonic continuity without interrupting the main story flow with expositions, up to becoming dedicated Exposition Bonus Material.

If the events are not inconsequential, such an episode may contain Reset Button, Or Was It a Dream? or Perspective Flip to Unreliable Narrator, forming a weakened link with the Canon. Compare Broad Strokes. Unrelated to Cowboy Cop.

Anime and Manga

 * Darker Than Black: OVA is a Deconstructive Parody on the series... with a Reset Button at the end.
 * Detective Conan has an ongoing Spin-Off manga series called Detective Conan Special Edition that is going on on a children's manga magazine. While it agrees with most canon elements of the main series, it's not drawn by the original mangaka (and hence has some Off-Model issues) and The Syndicate never appeared outside of the First Episode Spoiler. It's lack of firm link with the canon can be demonstrated by... despite being a Long Runner itself (>30 volumes), its story were very seldomly adapted into the Anime, despite it may help to fill up "original" fillers.
 * Many Pokémon movies - while a few of them are shown to be part of the canon, one wonders how the Victini movies would fit in, due to them being the same movie, but with different twists.

Collectible Card Games

 * Magic: The Gathering's original dozen spin-off novels were published by HarperPrism. When the Weatherlight saga began and Wizards of the Coast started its own novel line, the continuity was revised (This is commonly referred to as "The Revision"). Any material in the old novels is considered Canon unless new material directly contradicts it.

Comic Books

 * This is the status of the Vertigo Comics line of DC Comics: it started as part of the DC Universe, but has grown progressively separate. Basically, a Vertigo story only applies to the main universe IF a story in a DC comics says it did. For example, we know that Dream of the Endless exists in the DC Universe because he (or rather his replacement) has appeared in issues of Justice League of America and Justice Society of America. However the versions of Hell from those universes do not match.
 * Though even those interpretations of hell remain as Loose Canon due to the fact that the Sandman series posits that there are multiple parallel afterlifes.

Fan Works

 * The Pony POV Series has several chapters which Word of God has specifically said are optional for readers to consider canon or not, depending on their personal preference. This includes Luna's sidestory and the "Battle Pros" chapter. The former is refrenced somewhat in the actual canon, but never truly confirmed.

Live Action TV

 * The Lost tie-in books and video game aren't canon (except for the Incident Room in the game...) but they don't interfere with canon by involving background characters and just mentioning the canon events as happening elsewhere. (The one trip-up spot here is really the part right before the end of the video game where you have to . There's no reason why that wouldn't come up again in the series...)

Theatre

 * The Drowsy Chaperone: In-Universe, Man-in-Chair never elaborates on the actors playing George and Trix. However, the booklet that comes with the 2006 Cast Recording contains fake liner notes from the record of the Show Within a Show, naming both of them.

Video Games

 * Metal Gear Solid Mobile. It hits the Reset Button at the end by erasing Snake's memory so it doesn't interfere with canon. (Though that doesn't explain why Otacon doesn't remember any of the events either...)
 * In The Darkness Of Shadow Moses and The Shocking Conspiracy Behind Shadow Moses also qualify. It's likely they were both intended to be canon at the time, but have been ignored for convenience. That said, not much is specifically contradicted between the games and the books.
 * Half Life expansion Opposing Force (note that the expansions were made by a different company) introduced the main character (marine Adrian Shepard) who proved quite popular among the fanbase; and the mysterious "Race X" from another, unknown world who happened to arrive at Black Mesa during the events of Half Life and made a weak attempt to conquer Earth. Their canonicity is pretty much a gray area: it was never completely exiled from canon, but they did not appear in the series again. Same goes for Blue Shift; though Barney Calhoun made his way into Half-Life 2, the supporting characters (Dr. Rosenberg) and the exact events are semi-canon at best.
 * A generic, unnamed guard in the original Half Life offers to buy Gordon Freeman a beer if they leave Black Mesa alive. While Barney Calhoun appears in Blue Shift, he comments in Half-Life 2 that he still owes Gordon that beer. Calhoun never directly encounters Freeman during the course of Blue Shift, though it is conceivable that many guards (perhaps even all guards) owe Gordon Freeman a beer for some reason or other.
 * The only detail from expansions that Half-Life's writer, Marc Laidlaw, has explicitly declared canon is Black Mesa's destruction in Opposing Force's Deus Ex Nukina ending.
 * It is somewhat unclearly stated whether the Team Fortress 2 tie-in comic “Loose Canon” is in fact Loose Canon or not.
 * In the Tales (series), the Tales of Fandom games are treated this way, at least by the parts of their audience that don't speak Japanese, due to No Export for You. Details may gradually trickle through the fandoms as Fan Translations are made, but since these can be unreliable, most fanfic writers consider them optional at best.

Web Comics

 * The Gunnerkrigg Court bonus pages at the end of every chapter represent two different varieties. Some of the pages just show brief scenes which are officially canon, but are rarely ever mentioned again. Other pages feature a white-haired girl named Tea who pops in to describe background details to the audience. The contents of her exposition are canon, but Tea has yet to appear in the comic proper, and she has interacted with a cartoon representation of Tom Siddell (a character that the real Mr Siddell insists is non-canon).
 * The Order of the Stick comics that appeared in Dragon (magazine) might fall into this. In one online strip where Haley mentions Belkar claiming to be a gourmet chef, he responds that it was in Dragon so he's not even sure if it's the same continuity.
 * Templar, Arizona: Bonus comics.
 * Unwinder's Tall Comics: The Rant below page 32 specified that "It's not Tall Comics canon, but it IS Marmaduke canon." But fallout from the events of that page pop up again thirty pages later, anyway.
 * After Homestuck ran an intermission featuring the Midnight Crew from Problem Sleuth, one member of the MS Paint Adventures forums began a forum-based adventure serving as a prequel to the intermission and starring the Midnight Crew's rival gang, the Felt. Although the author was eventually forced to cancel it due to accusations of forcing his fan fiction into canon, it was generally seen as this, especially due to the author getting permission from Andrew Hussie to use plot points and character designs that hadn't yet been featured in the main comic, and it was even given a Shout-Out by having Hussie slip a necklace resembling one worn by an Original Character from the forum adventure into the actual intermission. Andrew eventually declared it to be non-canon, however.
 * Slapdash Application of Verbiage alternates between regular comics and "Tales of Dubious Canonicity", pieced together from previously-drawn artwork. Whether the events and dialogue of these comics have any bearing on the plot or characterization in the main comic is unclear.
 * Questionable Content has a handful of characters who show up only in filler strips that the author writes when he doesn't have time to do a regular strip. However, he did write a multi-part New Year's comic with them, and one of them had a cameo in the comic proper. Oh, and there was also the birthday comic.

Web Original

 * The Fine Structure story "Marooned" is Optional Canon by Word of God. A number of details, mostly Alternate Character Interpretation on the Big Good, are left to the opinion of the reader.

Western Animation

 * Zuko's Story is a tie-in Prequel comic to The Last Airbender, the live-action film adaptation of Avatar: The Last Airbender. However, the writers went out of their way to incorporate details from the animated series. Except for Zuko and Iroh being drawn as their movie counterparts, the story would fit right in with the animated series, which was in fact the authors' intent. There is however no word on whether the series considers it to be canon.