When I was in my late teens (a long, long time ago), I owned a share in an SZD Pirat glider. One Sunday morning I was going to attempt my Silver distance award and prepped my glider and my retrieve crew etc. The moment looked good for a launch using a winch with some pretty active looking thermals directly ahead from the launch point. Off we went, and pulling back on stick to nicely bend the wings aiming to get 1500' or so on the launch. Just at the point of maximum concentration I felt an intense pain on my shoulder followed shortly after with a searing pain in my right ear. My 'buddies' had put the gliding club cat 'Roper' in the curtained off shelf immediately behind the pilots head. We both immediately freaked out and there's not much room in the cockpit the Pirat being a single seat glider. I detached Roper from my ear and his claws out of my should and shoved him into nose of the canopy on top of the instrument panel. I could see all the claws extended as he realised just how high we were. I managed to regain composure and finish the launch, I could see on the ground my mates who clearly saw what happened when Roper made his appearance, my glider apparently wobbled on the winch launch momentarily.
Roper stayed in position for the remainder of the flight, his response to loops was interesting. When we landed I have never seen a cat move so fast! I never did get even with my mates, apparently all of the members were in on it and there was great hilarity as I took off.