Term for talking about a subject or idea you did not want to talk about?

Question

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?

An example:

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?"

Answer 1

Consider elephant in the room

a major problem or controversial issue which is obviously present but is avoided as a subject for discussion: they’ve steadfastly ignored the elephant in the room: the ever-growing debt burden on graduates

In a sense, it's a form of litotes

ironical understatement in which an affirmative is expressed by the negative of its contrary (e.g. I shan’t be sorry for I shall be glad)

The very process of avoidance results in the consideration of the avoided concept.