Be sure to know exactly where your work is on the grinder at all times. I once was fine tuning the point of a finished knife when the point of the blade caught the contact wheel, shredding the belt as it briefly stuck into the rubber tire of the wheel. This resulted in an explosion; bang the knife was gone, out of my hands and stuck ¾ inches into the wall behind me. The 320 grit belt was shredded, and luckily the damage to my contact wheel was minimal, and I got away without injury. The knife had a bent tip as a result of the accident.
Wear eye protection, and a leather apron. Have a good grip on your work, and pay attention to what you are doing at all times. Let the machine do the work, and dont use excessive force, as this will lead to serious injury and premature wear of belts.