randomly decided after returning from their vet annual that they no longer knew each other, and would fight viciously.

How odd! I wonder if they smelled differently from being at the vet? Different enough to seem like a non-familiar animal?

I'm so lazy about litterboxes. I do my one cat's one box twice a week, which I worry might be pushing it a little. If she ever discovers that she can pee elsewhere, I'm going to kick myself.