Incidental references are extremely small appearances of elements from a series in another series without being actively intended as references. These are in general not notable to this Wiki.
In most cases incidental references happen when elements from another series are used in the background to create a specific enviroment, for example a scene taking place in an arcade will most likely feature various videogames in the background, or a scene taking place in a comic store will feature various comics for sale. Even if these elements are actually from other series, we don't consider them fictional links, since they're usually not intended as references to those series. Basically, if it feels that the creators of the work just picked some generic elements and used it to create an enviroment, that's an incidental reference.
Note that when one of these elements is higlighted either by the characters or by any other mean, that is no longer considered incidental and starts being an actual link, since we can assume that the creators wanted it to be a specific reference (for example a character in a TV series entering a comic book store and buying a specific comic, say a Batman comic, counts as a reference to the Batman series, but all other comics shown on the shelf are incidental references). One of the other mean of highlighting an element might be a directing choice (for example if in that scene the camera directly focuses on a comic on the shelf, that might be intended as a direct reference).
This usually only applies to live action works, since in animation even background appearances are most times considered actual refereces because the creators had to specifically re-draw those elements rather than simply picking them up.
A different type of incidental reference happens when elements from a series are accidentally referenced since the creators actually wanted to reference elements from another series. Aside from common Undirect links, this specifically happens when elements from a series are mistakenly associated to another series (for example a series might reference the Donkey Kong series by featuring a parody of the original game in which a moustached plumber fights a giant ape. Actually in the original game Mario was a carpenter, he was changed to plumber in Mario Bros. Despite this, we can assume that it was a mistake by the creators, and even if that's due to Mario's popularity in his own series, that doesn't count as an intentional reference).
By definition, however, these unintentional references have to be small. Notably large references, even if unintentional, are still notable to this Wiki. (For example the creators of Era's Adventures declared to be unaware of that game's character to be based on Yoshi, but despite this reference to be unintentional, it's still very notable, since it concerns the game's protagonist).
As already stated incidental references are not notable to this Wiki, however they might be cited if they happen along with actual links between the same series (returning to the comic book store example, if other, different Batman comics appear on the shelf they might be noted along with the one the character chooses to buy, even if they're only in the background).