There's a couple of possible scenarios I can think of which fall under the situation you describe.
The question is about a fan-fiction based on some original work.
For example, consider my question Identify non-Doylian Holmes short story: Watson outstrips Sherlock and Mycroft. This question is about a Sherlock Holmes story, just not one written by Arthur Conan Doyle; thus I used the sherlock-holmes tag but not arthur-conan-doyle (which led to the odd fact of the former tag having more questions than the latter). There are a couple of ways to justify this:
- The story being asked about is in some sense part of the same series, thus gets the title tag, but not by the same author, thus doesn't get the author tag.
- People interested in Sherlock Holmes stories may also be interested in non-Doylian ones, so it makes sense for them to find a list of all questions about any Holmes story by a simple tag search. If they're purists and only want to look at questions about the original Doyle stories, that list can also be found by a simple tag search, just with two tags instead of one.
For the same reason, I would say that this question should have the harry-potter tag.
The question is about a story which is about another story.
This is the situation with the Canterbury Tales question you mentioned, and it's a little harder to deal with. There are (at least) two ways of looking at this, with different conclusions.
Do we want to tag questions according to what the story is about? The answer to this seems to be no, judging from our current tagging system. We're not using a world-war-two tag for questions about WWII literature, or an animals tag for questions on books about animals, so why should we use a geoffrey-chaucer tag for asking about a story about Chaucer, even if that tag does already exist in the system?
Is this question relevant to Chaucer experts? I'd say the answer to this is yes. Someone who knows a lot about the real Chaucer and his works might also know a lot about portrayals of Chaucer in fiction, or at least be interested in such stories. Imagine if the question was about a non-fiction biography of Chaucer; I doubt there'd be any objections to putting the Chaucer tag on it in that case. And fiction or non-fiction doesn't make a difference to the question's Chauceriness.