It doesn't take many careless slip-ups to make you look like a knownothing. For some readers, the slightest mistake screeches and jangles like a first-day violinist playing along with the orchestra, and they won't give you another chance to let them down. An author is the architect of a window onto a world, and one of the author's jobs is to ensure that the glass is polished and clear and without so much as a crack.
But mistakes happen. No matter how detailed your research, no matter how familiar you are with an aspect of the story, some nit will inevitably slip through. That's why prereading - reading over the story with an eye to content, before it crosses the desk of someone who might be inclined to buy it - is so important. It's a lot harder to take an author's world seriously when the author bends or breaks reality without realizing it.
When I was younger, I always wondered how writers could let contradictions and errors scuttle in from the cold. Weren't they as interested in the story, the background, the nature of the world as I was? Didn't they care to take care about what they were putting together, to make sure it was solid? That was, of course, before I started writing in any serious way. Now I know better, because even though I am interested in the story, the background, and the nature, that doesn't mean it will be perfect on the first pass, or the fifth.
Part of it's just down to the way writing is done. In a good story, the seams where the writer kicked off for the night and picked up the next day won't be visible, but psychologically they're there. Sometimes a writer might establish something offhandedly in one paragraph, but by the next day it's slipped from the brain. Other times, it's the result of a subconscious tug-of-war as to how a scene should be presented. I've run into this error myself in a story that's still searching for a home. One scene I'd vaguely envisioned as wintry, though I'd never specifically established that, and additional description written the next day - people lying on the grass, a man taking his miniature horse for a walk - suggested, upon later reading, something at least autumnal.
What made this worse was that I didn't discover this until the story had already been rejected from the first place I sent it. While I know that there's no way a story would be bounced if that was the only consideration - editors, after all, change words all the time - it left me with a bad taste in my mouth.
That wasn't the only time I made that mistake, either. In another story, two characters have no choice but to escape a stricken space station without spacesuits, or any other kind of protection. As neither of them are Batman, and thus cannot breathe in space, I spent a while gathering information on vacuum exposure in order to make it as accurate as possible for someone who's never been more than 40,000 feet above sea level. One of the key facts of vacuum exposure, and one that I didn't want to flub, is that the "common sense" maneuver of taking a big, deep breath before jaunting into the vacuum is actually a really, really bad idea, as rapid decompression would create pressure differentials that would seriously damage the lungs. To say the least.
So, if rapid decompression is going to happen, the best thing to do is to totally empty one's lungs of air. This is what I had my characters do. What I also had my heroine do, unfortunately, was count out the seconds until she triggered the airlock and created the rapid decompression event. This didn't strike me as off at the time, but a few days later I tried it myself. (Talking after emptying my lungs, that is, not experiencing rapid decompression.) The result wasn't even so much as a croak - croaks, at least, are audible. I edited the scene slightly so that someone else was giving the countdown over the radio.
Errors can slip in anywhere. You've got to always be vigilant if you want to get ahead of them. So after you've finished something, leave it for a while and read over it again with fresh eyes. The nitpickers will always have things to complain about, but it's not an author's job to make things easy for them.