Hey, it's been a while since one of these! I'm kind of out of practice, I think.
So, once upon a time, there was a guy who was always correcting other peoples' grammar and speech practices and what not. But he was really only capable, or so it seemed, of doing so in formal situations—vetting speeches or looking over writing to be submitted for publication, or the like. As a result everyone assumed that this individual would be utterly embarrassed if he ever tried to interact with someone who spoke in a very slangy style, much less to correct such a person. However, once, at a party, in full view of all the doubters, he not only engaged in a lengthy slang-filled conversation, he even managed subtly to correct his interlocutor. It went down in history as the vernaculate correction.