Why Do Native Speakers Make "Mistakes"?

Also, how do you pronounce "often" anyway?

I ask you, have you ever known what it is to be an orphan? Often!

J. David Barker has a question that neatly sheds a light on the difference between the way language is traditionally taught and the way linguists see it. I mean really, neatly. He writes:

I was wondering if you might be able to talk about the reasons that native speakers make mistakes with some elements of grammar, but not others. For example, it would be unlikely for a native English speaker to make a mistake with basic tenses (save for an occasional irregular past tense form), but we do occasionally slip up with the present perfect (“Have you ate yet?”) and the past perfect.

I don’t have a background in linguistics, but I feel I’ve heard linguists (probably Chomsky) talk about how native speakers have internalized the rules of their native languages and as a result, don’t make mistakes — yet it seems there’s a limit to this.