Back to Front Research
I always fall into this trap. Really, if you want to make your novel convincing, you should go where your research takes you. But instead, I usually find myself trying to make my research follow where I want the novel to go. Partly this is because I can't hold huge amounts of information in my brain for long periods of time. I thus have to do a lot of the research at the point of writing, so I often find myself spending an afternoon scrabbling around on the internet, trying to find enough information to back up an idea I've just had. This is basically what's happened today; unfortunately, this time, I've turned up very little in the way of useful information. So rather than break flow and let the problem fester, I've made Common Mistake Number Two, which is to try and bluff my way through it. My reasoning is that I can go back and fill details in later; but the problem with doing this is that you often make structural errors that can never be entirely untangled.
Basically, it's an approach that's going to come back to bite me.
I never learn.