Funny, I had a student in a super cub turn the fuel off while trying to blindly trim the airplane just after lift off(there's no way we were above 500 feet). The thing went silent with me in the back seat(having access to nothing). The lady in the front said "you've got it" and gave up. I said, "Okay, there are a few things that I need you to do." and walked her through the standard suspects: mags, fuel, etc, all the while setting up to land in a field just next to our airport. She verified for me that the fuel was on a good tank as I turned base for said field, but something just wasn't sitting right with me, so I leaned as far forward as I could and barely got a hold of the fuel selector. Grabbed it, gave it a whirl and away we went. The thing lit right back up and we were on our way.
I briefed the trim and fuel selectors a little more thoroughly after that.