I've only ever answered one interview question in Scheme, the problem was perfect for it. :) I first had to check if the interviewer knew Scheme, and he did!

I fear it has been long enough that I could not repeat such a stunt now days, but last week a resume came by my desk that had LISP listed on it, so I prepared some problems that would be trivial in LISP just in case. Unfortunately it was as I had suspected, a stale language listing that should have been removed from the resume long ago.