I haven't instructed for a while now, but i remember one student who scared the !!!! out of me.
This guy was huge - he could bench press a mini van - which made him dangerous because he would never relax on the controls. He seemed to get tunnel vision, to the point where he couldn't even turn his neck - or relinquish control. I had to give up with him eventually because I realised if anything had gone wrong he would have fought me on the controls - and won.