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In one sense, native speakers literally can't make mistakes (unless they're drunk/tired or stumble over a word or something like that). For example, misconjugation is not a mistake-- it could just mean that the word wasn't popular enough in their dialect to have a rigid conjugation, so usually this means that an irregular verb is falling out of use. A verb like abide is uncommon enough that its past participle form abidden has fallen out of use, and the simple past abode also has the acceptable abided, mostly because not enough people use it to maintain the same conjugations. So in a certain group of people with the same dialect, using abode is actually less effective at communicating than abided (I had to look up the conjugation for this, as I'd never heard abode outside the noun meaning home).

As for the direct and indirect pronouns (I assume you mean subject vs object pronouns), natives often use different pronouns in coordination (using an "and" to join two pronouns) than they would alone. It's very common to hear "jim and me went to the store" whereas only a tiny fraction of those speakers would say "me went to the store". Although no one would misunderstand (like they might with the verb conjugation above) a "jim and me" vs "jim and I", people can hear the difference in register, and in certain situations a less formal register is more appropriate than informal. This doesn't make those speakers wrong, and I don't think it even changes the communicated meaning, as you said with "what I understand is not what you mean."

Because no one has ownership of what's absolutely correct, a lot of this stuff falls under the purview of register, and therefore we have prestige dialects. So language "mistakes" just become another way to separate classes of people, because for a long period of history the only people with any power sounded one way, and they decided it sounded "better" than what other people sounded like. Those distinctions are less rigid these days, but I don't think we can say they've gone away entirely.

As an aside, if you'd like to discuss countable vs mass nouns, I had a bit of a dive into those when I tried to teach them to an EFL class and I'm still not sure how to explain a lot about certain aspects of the topic, but things weren't nearly as simple as I thought before I started talking about it at the front of the classroom.