While riding around in the back of a friend's Piper Aztec, I would entertain myself by quietly unzipping the headliner aft ot the back seat, and finding the elevator trim cable up there. With a distraction of "hey what's that down there?" I could put the elevator a good bit out of trim, and amuse myself watching him re trim it, and figuring out why it was necessary.
Now on the other hand, the first time I ever flew a Cessna 340, my five fellow occupants, all of whom were pilots, left me flying left seat (to my profound delight) and went in the back for lemonade. Some time later, the plane pitched violently nose down, and I struggled to maintain control, as one of them entered the cockpit backward through the curtain. I had failed to be alert for the fact that they had been all moving farther and farther back in the cabin, and I had been retrimming, without wondering why I needed to. We all learned a startling lesson about the affect of changing the arm of the load in flight!