Occasionally, particularly in Harry Potter questions, I see people give answers which cite fan fiction instead of canon, in response to questions that aren't specifically about that fanfic.
We see a fair number of speculative answers along the lines of "In canon, [X] is true, and it seems like that would imply [Y]." These are not the answers I'm talking about. I like those answers. They're fun and interesting. I'm specifically concerned with answers like "In fanfic [W], [Z] is true, and canon never says [Z] is false, (so I'm going to assume that [Z] is true)." The part in parentheses is often left out, but it is the natural implication of these answers.
Do we have a formal policy against this? Should I downvote such answers, flag them, or just ignore them?