Know your characters--but not someone else's!

I've been reading several different threads that got me thinking about how wow, as fans we really seem to walk this odd line between wanting to be able to be creative and interact with the a canon on our own, and also needing that canon to be...what's the word? I was going to say "untouchable" but I think I really mean "clear and authoritative." For instance, it's very common for fans to lament "why do people write characters OOC/not get canon right/change basic facts" when those same fans, if they write fanfic themselves, are always also doing that to some degree. And also, this tends to get into the side discussion of how arrogant it is for any fan to think they ( they know the world/characters better than the author himself/herself. )