Science

"Should of" instead of "should have."

"Me and her went" instead of "she and I went."

"Flustrated" instead of "frustrated."

"To who" instead of "to whom."

"For all intensive purposes" instead of "for all intents and purposes."

"Aks" instead of "ask."

"Literally" to mean "figuratively."

"Shoe-in" instead of "shoo-in."

A semicolon instead of a colon.

Using a preposition at the end of a sentence.

Splitting infinitives.

Starting a sentence with a conjunction.

Each a simple "error" to remember. But there are thousands of them. None make an appreciable difference in understanding. None would ruin a business deal or a meeting except in terms of lost social standing for getting it "wrong." This category of errors is what I believe to be meant by "improper English." This is in contrast to "incomprehensible English."

As I said, successful transmission of the message is the only true test of linguistic legitimacy. You're absolutely right. People are instinctively aware of when their dialectical quirks are going to cause a problem communicating with outsiders, and they code switch. They simplify. Ironically, the less familiar the interlocutor is with English, the more "improper" a native speaker's English might become. "My name? John. Your name?" Yet in so doing, they become more compensable because they've dropped the complex cultural dance which they are so often required by the powerful to perform.