Yes, depending on how you define "own volition"! I once settled into my seat feeling a bit under the weather..then had to get up against the flow of boarding pax to the lav where I began a terrible bout of the "trots". I was feeling so incredibly horrible I decided that I could not face the 4 hour flight..which was going to be a bumpy ride due to weather. I had the runs and was becoming nauseated and sweaty as well.

I got the FA, explained the situation, and they let me off..but they were reluctant to do it until I told them I did not check luggage. Apparently in this post 9/11 era the TSA is not keen on people getting off their flights before takeoff. They let me off but told me to stay in the term for a few minutes for "some paperwork" which I am guessing involved confirmation that I did not check luggage or leave anything on the plane. So I went to the terminal bathroom a few times and was on my way in about 5 minutes. Crawled back home, missed my trip, and was ill for 2 more days.