It's what you know that ain't so that bites you. There is a rather sad example of this on Baen's Bar right now, but rather than embarrass the culprit I'm going to tell a few tales against myself instead. I figure after I subbed for Sarah yesterday, she being somewhere in Portugal right now, you didn't need pointless pontification from the pulpit (actually it's a soapbox, and I'm sure it escaped from Terry Pratchett because it has lots of little legs and it just keeps ambushing me).
Anyway, writers need to keep an open mind because what we write gets to all kinds of people, and a lot of them get upset when we make a mistake with something they're very familiar with. This is where that big time-sink known as research comes in. There's just one problem. You know to research things you know nothing, or very little about. But how do you find the things where you think you know it but you really don't?
Google is your friend. So is listening to and reading just about everything - and being willing to re-evaluate what you think at any time. Take me (actually, don't. My husband would be a little unhappy about that). One of my more interesting exploits was a 1500 mile drive with a broken right ankle. How did I manage that?
To start with, I didn't realize it was broken. After all, you can't walk if you've broken anything more serious than a toe, right?
Yes, you can. The position of the foot has to be exactly right, and it certainly doesn't work for every broken limb, but there are times when you can actually put weight on a broken ankle, and walk. Well, hobble. And yes, drive.
It does hurt. Rather a lot, I might add. And that's another one I thought I knew and it weren't so - extreme pain does not necessarily cause screaming. In my cause it mostly caused whimpering, but I could still function, more or less. Towards the end of the drive - it was a 2.5 day trip - I was pretty much mono-focused and on a double-dose of painkiller to stay more or less functional, but I was still on my feet. Yes, including the broken one, which by this stage had reached something more than twice the usual size, attained a truly spectacular range of color in the blue, black, and purple range, and was blistering the skin.
When the pain stopped - thank you IV painkillers - I went to sleep, which was another what I knew what ain't so. I hadn't realized until then just how exhausting prolonged pain can be, and how the first reaction when it stops is to pass out because up until then you hurt too much for that. Of course by this time I had Emergency Room nurses telling me they'd personally come and kill me if I put any weight on that foot before an orthopedic surgeon said I could (something like that, anyway. I was a bit hazy, for some reason).
When I was doing research on trauma injury for a work in progress and stumbled across this site, I read through all two thousand plus on-topic posts half in horrified fascination, and half because I expected to see my broken ankle there. I recommend it for anyone who's interested in the many and bizarre ways people injure themselves, and what they do afterwards. Fair warning: sometimes it's spray worthy, and others kind of gross. It's very educational. If the experiences of American city ER staff are any guide, the most dangerous occupation in urban USA is "standing on the corner, minding my own business". And watch out for "some guy" or "that dude". They're trouble. Reading your Bible increases the risk factor.
The point here, of course, is that if you don't go looking even for the things you think you know, you might get smacked with them later. Hopefully not in quite such a vivid and painful way as I learned about walking on a broken ankle, but usually the experience isn't fun. There's nothing quite so deflating as the person two rows back loudly telling everyone that he's a professional horseman and what you describe on page X is physically impossible because horse's legs don't bend that way. (No, this hasn't happened to me. Yet. I'm hoping to avoid this fate).
What are some of the ways what you thought you knew has smacked you in the face? Feel free to conceal names to protect the guilty.