Something I think too much about is the foremost “error” a majority of (western) fantasy books and RPGs do. They have gods and magic and all sorts of stuff and when someone, for one reason or another, scratches on the surface the author tries to present an “scientific explanation” in parallel to our IRL world view.
Not only do they usually bungle the science (but that is another question), but it is completely unnecessary and immersion breaking! (I put some examples in comments).
#worldbuilding