You know it – it’s that “optional” comma at the end of a list. “I went to the bar last night with Jim, John, and Sue.” It’s that comma between “John” and “and.”
There’s not a “correct” answer as to whether it’s required. Generally, as I understand it, the comma is optional, and its usage is a stylistic, rather than a grammatical, question. But it’s a stylistic question about which people have quite strong feelings. Feelings that, for the most part, they take very seriously.
And some friends and I have something of a hypothesis: the Oxford comma is, in fact, the closest thing to a “secret handshake” those of us in the “monogamish” club have. If you see mention of it in an OKCupid profile? Chances are, that’s a person who’s a swinger, poly, or otherwise monogamish. At a party? If someone brings it up? Likely she’s fair game for an advance, even if her husband is with her.
Have you noticed this? Are we alone in this?