proper usage of "she and I" vs. "me and her" through exposure to people who speak correctly (either in writing or just talking)
Even when you know which one is correct, constant exposure to people who use them incorrectly erodes your ability to use the correct one without conscious effort.
Particularly for words that most people think are homophones ("there" and "they're" do *not* sound the same when I say / read them; nor do "your" and "you're"), seeing other people use the wrong one (in which case I have to put the wrong sound with the spelling, in order to get the right meaning) means that I make far more errors now than I did as a younger person (in the pre-Internet era).
Plus you're assuming that the person in question has enough formal grammar training to know which one is correct, so recognizes that it's being correctly modeled, rather than thinking that the other people are wrong. "Between you and me" sounds wrong to people who learned the "you and I" rule by rote, rather than understanding the underlying logic.
