I read a lot of student papers and exam essays. I see a lot of repeat errors. It gets really annoying after a while, but one interesting fact about language acquisition becomes pretty clear after a number of instances of the same error. Some of these errors seem to be a result of people learning vocabulary by hearing without ever reading the term in question.
One common error I see (and I see it online quite a lot also) is referring to a transition between one subject and another as a segway. No, a segway is a two-wheeled device that moves around while you stand it. A transition between two subjects is a segue. But you don't have to be a non-reader to make this mistake. It took me until well into my undergraduate years to figure out that the word that I thought was pronounced "seeg" was the same word that people kept pronouncing "segway". I don't know how anyone ever does figure this out, in fact, including me.
But not every instance of what I have in mind is like that. For instance, I sometimes see people referring to the "rank-in-file", which is not a normal English expression at all. They meant to be talking about the rank-and-file. One of the most annoying is one I see extremely frequently in philosophy papers, especially on issues in metaphysics. Spiderman and Peter Parker are one and the same person. It's sort of an old-fashioned expression (which I sometimes see written as "old-fashion", a bothersome construction in itself). I would never use it. But if I were to use it, I'd get it right and not speak of Spidey and Peter being "one in the same".
I'm convinced that these mistakes result mostly from people who never read (or perhaps only read people writing on the internet who never read). There are certain mistakes that people who read would never or almost never make. These students are basically signaling to me that they hardly ever read anything when they do this sort of thing. Yet they have no idea that they're doing it, and they wouldn't be in a position to know that unless they read more.