Basic facts are basic facts. Details tend to be basic facts, and if a writer does superficial research, and misses the basic facts, then there will be reader problems.
True, true, although is it not worth mentioning that, sometimes, even those we take for granted as "basic facts" can change over time? Landscape, weather patterns, language, attitude, mores.... I had American beta readers gently correct my American character who said he had just gotten back from a week of skisport, by letting me know that it's just called "skiing" in the U.S. But it
was "skisport" at the time and place my character wrote that line. Still, it was a perfectly understandable thing for them to have tagged.
I'm sure I'll get disagreement here, but it's a question worth considering. Natives or those otherwise firmly enmeshed in a particular culture carry assumptions with them about "the way things are" and may not always be aware of "the way things
were". Yes, there are plenty of examples such as the ones
pdr is talking about, where very common errors are made by those outside of a particular culture or setting. However, as a reader, I don't automatically dismiss a book written by an "outsider". I argue that, for all of those "outsiders" who do shoddy research, there are even more who do extra research, not having the same assumptions as a "native". They meticulously research details that a "native" wouldn't think of (but, in some cases, probably should).
I think that's why this old argument of "don't write outside of your own little world" always rankles so when it comes up on here, because I suspect most of us on this forum fall under the second group. When writing about a place or people different from our own, we apply the same zealousness in research when writing about a
time different from our own.