I'd just like to add that, even if you do all the research (from books or hands-on) that you are physically and mentally and financially capable of, there are still things you're going to get wrong. Most Writers Are Human, after all. And human = fallible. A lot of things that people in older eras would have considered common sense were never written down, and thus lost to time. Not to mention all the libraries that have been burned down in crusades and such. Basically, nobody's going to have 100% knowledge of everything.