Sometimes when I shoot on manual the shutter speed will change by itself. I was shooting some volleyball last night. Poor lighting so ISO 12800. 16-80mm @ f4. Shutter speed set at 1/500 sec. Every now and then it would be at 1/400 sec even 1/320 sec., all the while the shutter speed dial would be still set to 1/500 sec. I rotated the shutter speed dial back and forth and it would go back to 1/500 sec. Has anyone had this happen to them?