Specifically, is there a term for the situation where both parties agree not to discuss a certain subject and in so doing end up engaging in a discussion about the very topic they said they wouldn't?
Person A: "What do you want to talk about?"
Person B: "Anything but the weather."
Person A: "Agreed, I find it boring to discuss the weather."
Person B: "Me too. Oh, that reminds me -- did you hear it was supposed to snow tomorrow?"
Person A: "Wow, really? I didn't know that. So long as it doesn't rain!"
Person B: "You do realize we now actually are talking about the weather, right?"