Nothing serious, but, on two occasions now, and with different types of adapter, I've had the flange that mates with the lens become loose from the body of the adapter. As might be expected, this results in out of focus shots, and that's the way that I have been able to identify the problem. Unfortunately this also means some spoiled images.
I now carry a small precision Phillips screwdriver that I can use to tighten the fixing screws. I guess a drop of loctite would provide a more permanent solution.