As I have mentioned a time or two, I have been reading paranormal romances in an effort to get a feel for the field since I'm developing a story concept for one. In particular, I've been reading the finalists for the most recent "Rita" awards.
One thing I've noticed is a decided lack of concern for "fact checking." In one, we have a person doing a "practice form" with a Western style sword that is apparently a long sequence of moves against notional (imaginary) opponents. There's just one problem with that. The use of long, stylized forms like that are an Asian approach, not one used in Western swordplay.
Another writer had an extremely experienced diver apparently confused between air embolism and the bends (two different problems that can occur from among other things, ascending too fast) and got durations and the like wrong. A more subtle error was someone suddenly appearing having been "swept" there from a just sunken boat when no such boat was within sight beforehand. But how far could the person have been swept while still being not only alive but conscious?
The same thing happens in the "traditional" SF and Fantasy genre too. One writer recently had a ship leaving a station apparently in Earth orbit and taking weeks to return to Earth (the journey was interrupted before completion but the expected duration was weeks). Even if the ship was as far as the moon it would take less than 5 days on a minimum energy orbit. Anything longer than that would take more energy. There are reasons why the outbound leg might take that long (high efficiency, low thrust engines) but they won't apply for a vehicle that returns from high orbit and reenters the atmosphere.
These things dropped me right out of the story for at least a moment. I managed to get back into it but I should never have been dropped out in the first place.
A lot of people will say "but it's fantasy" or it "uses future tech" or something like that. And that can be the case if a fantastic element is involved. If, for instance, the story is in an alternate world where Western martial arts took a different turn becoming more Asian in approach, that's fine. But that needs to be explicitly part of the world and not "it's a world identical to our own except some short time in the past people became aware of the existence of 'supernatural' creatures." But the existence of the supernatural or "future tech" just increases the need to get the parts that aren't part of that invention right. By showing the reader that you get the "mundane" aspects right you give them confidence to accept the fantastic.
Perhaps you, as the writer, think you can just get away with it. "Nobody will notice." And there's some justice to that. After all, these writers did get away with it. They were all finalists for a major award. (The non-romance in the above, was not only a finalist for a different award but actually won it.)
But somebody did notice. I noticed. And if I were not particularly motivated I might well have not taken the effort to get back into the story after those glaring (to me) errors. How many other people did stop after seeing those errors? How many people put down the book and wrote that author off? How many sales did the author lose? We'll never know.
Now, on the other hand, whatever you do, you're going to make mistakes. That's part of being human. And maybe, someday, when you're famous (or not so famous) you'll get a letter from a reader saying "you got xxx wrong in your story." It happened to me in my story "EMT" (Analog Science Fiction & Fact, December, 1993). It involved an ambulance service on the moon. For that story I spent a lot of time researching how ambulances operate, the effects of severe blood loss on the body, how CPR is performed and, especially, under what circumstances professionals performing CPR say "we're done." Despite all that effort, Analog forwarded a letter from a reader to me from a physician who, basically, said "you got that wrong."
Does that mean one shouldn't try? Of course not. You're probably not a professional in every field you might have a character engage in. You do the best you can and realize that it won't always be perfect. But you can hope to get things close enough that when a professional in the field calls you on it he qualifies things with "although it's unlikely, I suppose it could happen" and "or course, protocols might change in the future."
And the fewer and smaller mistakes you make, the fewer readers will lose that Willing Suspension of Disbelief that allows them to enjoy your work.
And that's what it's all about.