No other fiction genre comes close to comic books when it comes to the concept of canon. The first reason is that most comic books, a 70 year genre, come on a monthly basis and can continue indefinitely. This raises many temporal problems in canon: Is Lois Lane actually over seventy years old? Did Captain America really fight Nazis during World War Two?
And over time, a character can go through different changes and have different writers. As anyone who has followed a character for a length of time would know, this poses problems, especially since quality or vision of the character may vary, and sometimes the plots can become so bad or so outrageous, people sometimes ignore a story in regards to canon. It could just be a technical issue like a character before could lift only 500 pounds, but in the latest issue, they're lifting something heavier than that. Was it an oversight? Will it be revealed later why that character has more strength?
Also, in comic books, there is some consistency and coherency expected within that comic universe. This is a problem in comics: take Batman for instance, he stars in multiple ongoing and limited titles, and can guest star in various other titles. All these have different writers with different ideas. Batman is also problematic because he's been in superhero titles like JLA, yet stars in more mundane detective stories. Also, as with Wolverine, people ask "how does he find the time to star in all his titles and do all those guest shots?" Some aspects of "reality" are ignored when it comes to canon, especially time aspects.
Another relatively unique feature of comics is that they re-re-re...tell origin stories. In each retelling, they can add or take away details which can contradict previous tellings. They can even give a fresh new origin without mention of the previous origin stories, and the reader has to infer that the latest origin story is the "correct" one.
A problem also lies, within the concept of canon itself, that is people have to agree what is canon, and there can always be differing opinions on what is canon or not.
Star Wars Canon
In Star Wars canon, there are five different classes. If a higher class of canon contradicts something in a lower class, the events of the lower class are nullified. An example of this happening would be that in the novelization of The Empire Strikes Back, Hobbie Klivian dies when he crashes into an AT-AT. Because this didn't happen in the film, Hobbie was available to be used in later fiction such as Luke Skywalker and the Shadows of Mindor. The canon types are:
- G-Canon. This is the canon class for the films and is named for George Lucas. An example would be: Star Wars Episode IV: A New Hope.
- T-Canon. This is for television shows. An example would be: Star Wars: The Clone Wars.
- C-Canon. This is the canon for the Expanded Universe, it is for the comics, novels, video games etc. Star Wars: Dark Empire is an example.
- S-Canon. Any thing Star Wars that has not been officially confirmed whether it is canon or not falls under S-Canon. Some Star Wars: Tales comics are examples of this.
- N-Canon. N-Canon is confirmed as not canonical. Star Wars: Infinities - A New Hope is an example of this.