There's always the one about the interviewer who said to the interviewee "Impress me"", then sat back in his chair with a paper. The interviewee then set fire to the paper, which supposedly got him/her a place. I think this is an urban myth, as it appears in all the Oxbridge interview books and newspapers around interview time.
My interviews were fairly straight forward, although in my general interview I had a conversation with one the tutors in which I spoke Swedish and he replied in Dutch! (We were learning our respective languages at the time, and I'd written about it in my Personal Statement.) I know someone who is an interviewer, and he likes to ask the candidate what they'd give him for dinner if he came round one evening - it's supposed to show that you're not just a robot force-fed on textbooks and syllabi (syllabuses?).
There must be loads of these stories that people can tell - every uni student I meet has some kind of wacky interview anecdote to tell when these conversations come up.