We’ve covered loose jam nuts before, but I’m showing them again because they are extremely prevalent. I have yet to inspect an airplane where I haven’t found at least one. In the top photo, the jam nut on the mixture cable was never tightened after an engine mount replacement.
The jam nuts on rod end bearings located on flight control surfaces help the bearing carry the load. Cracks can develop in the spars if they are not tightened. This is an easy preflight item, so no excuses! By the way, once tightened properly, and with the addition of some Torque Seal, I’ve never seen one come loose.