We all do it. Everyone has one thing that they always screw up when they say it. In my case, it's "de-thaw". I say it all the time and it makes no sense. "Can you take the chicken out of the freezer to de-thaw?" Yeah, "de-thawing" would be freezing. For some reason I get caught between "Take it out to thaw" and "Take it out to defrost" and end up creating a new nonsensical word which ends up making everyone doubt my general intelligence.
It's not just me though, I've seen perfectly bright people misuse other words, probably the best example is "irregardless". Yeah, the word is "regardless".
It's one of those things that I think comes with typing transcripts. I spend a lot of time listening very intently to what people have said because I then have to reproduce it exactly. I'll never forget the time that a coworker of mine, a very well respected lawyer, shot this beauty at a witness - "Well, that's a new relevation." Um, I'm pretty sure the word is "revelation". And then you're caught in the situation of do you fix the word, or leave it as it is and make them look like an idiot?
The point is, we're all the same. It doesn't matter how long you've been in school or how smart people might think you are, I'll bet there's some word that you mess up all the time. A famous one in my family, and one I've heard other people use is "lip chap" for chapstick. Why would you want to rub something you seem to think is called "lip chap" on your chapped lips? Doesn't make sense.
What about you? What's the one word that trips you up consistently? It is something that you totally screw up the meaning, or just something you can't pronounce quite right? (Damn you, "rural", you get me every time!)