I ordered an Oatmeal Stout at a brewpub recently. The server said "the Porter?" And I said, "No, the Oatmeal Stout." And she said, "Right." I got the beer, and there was no detectable oat body to it, much sweeter than most OS I've had. It was an ok beer though, although I didn't want another. When she came back for round two, she said, "you had the porter, right?" I told her I ordered an Oatmeal Stout and she told me "Yeah, it's the same thing. We switch them each month and this month it's the porter." I was bit pissed, but didn't want to make it an issue because of my friends.
It's one thing to wonder about the dividing line between porter and stout, but wholly another to give you an entirely different beer and just assuming that they are "the same thing."