I realize this question may sound broad or vague, so allow me to explain.
I never knew much about combat or dealing with injuries until I started gathering research for a fantasy novel I'm planning to write, which will have a lot of action and fighting. Prior to obtaining my research, I thought that you could get hit in the head and fall unconscious for a few hours. I see it all the time in stories. Somebody sneaks up behind Joe while he's in the woods -- WHAM -- the next scene shows Joe waking up a few hours later tied to a chair in the villain's basement or something.
After doing some very basic reading on concussions, I learned that this situation is not realistic. In fact, when people are knocked out, they're usually knocked out for less than 30 seconds, a few minutes at most. Any more than that and they're dead.
It's not just this specific case. I learned that a lot of other events that are so frequently portrayed in fiction simply would never happen in real life... I never had a problem with it before, but now I am the writer. I am afraid that if I write a scene like the one with Joe above, people might criticize it for being unrealistic, even if I was writing a fantasy. So, in general, where is the line drawn between what's believable and what's too unbelievable (especially if I'm writing a fantasy)? How accurate is too accurate? Should I stray away from writing a flying scene because, realistically, my character would die from increasing in altitude too quickly?