This was 'ported over from the Paksworld blog, on the grounds that it might interest others who don't read fantasy. If you read it there, it's pretty much the same.
Ritual disclaimer: nobody gets through a long writing career without some mistakes. You will sometimes trust the wrong research source (even if it’s someone who should have the knowledge you’re looking for–say a fire department veteran you’ve asked about a procedural point in managing a multi-alarm fire…and no, this isn’t a problem I’ve had.) No writer knows everything, and every writer must, at some point, trust a map, or a reference book, or a person who seems to have first-hand knowledge.
But there’s a huge difference between occasionally trusting the wrong source and not looking something up at all. Writers should look things up in the best source they can find or beg/borrow/get via Interlibrary Loan before they plan a book or a chapter–and should let the facts dictate how the story goes, rather than ignoring the facts because they already have a fantasy-version in mind.