I had a student turn the fuel to off during the restart drills while doing a PFL in our Cub. Fortunately I realised what had happened and managed to get him to turn it back on (I can't reach the fuel valve from the back seat) and the engine restarted at 200ft. Good job too, because we would have hit the far hedge in the field he had chosen if we had to land. Doh!
The lesson I learned from this is to clearly brief "touch drills only" during Emergency drills.