I have another example, but I wanted to get the person's permission before using them as an example. I now have permission.
I think we will all agree that Jiro has a proficient command of the English language. He has a large vocab, he structures his sentences well, on the surface everything looks great. Jiro was raised orally with English as his primary language. Here is how I know, for example, without ever having been told, that Jiro is not a natively fluent user of English, even though it is his primary language. I know that he did not acquire it, but learned it.
Last week Jiro started a thread about officers killed in the line of duty. His thread title said something about "More Police Officers Slained." This is an example of the overgeneralization of grammatical rules that happen when children have not been able to internalize language function. Actually, there is no -ed needed to indicate past tense in the word "slain". That is the past tense. The present tense is "slay". But because Jiro was taught that -ed signifies past tense, he overgeneralized this learned rule, and put an -ed on slain.
This is not to criticise Jiro's language use in any way, and I would never have pointed it out except that it is a perfect example to illustrate what I am talking about.
Thanks for letting me use that, buddy.