I glued up the top for the chest I am working on. A panel of two boards. Got the top nicely aligned and clamped and then noticed a slight variation at one end. One board was about 1/32" higher than the other. I put glue blocks top and bottom and put a heavy duty C-clamp on them to press the join flat. Tightened the clamp really tight. As I stood there admiring my work, the clamp backed off, unscrewing itself about a 1/4 turn. Not completely sure what I had seen I tightened the clamp again and it backed off again. I did it again and held it for a few seconds. It seemed to take.
I had recently cleaned and lubed my clamps with some sort of dry lube. Mostly thought this was curious and had never seen one back off before.