This little event has happened to me many times over the last 40+ years of reloading, and usually happens mostly when using my progressive press, but a primer has flipped on me unnoticed in my Rock Crusher with the same results.
One has only to decap the round in the normal manner, with gentle pressure, rather than quickly ramming the casing into the decapping die. In the event that the primer should go off, there is little chance for injury as the gas is not captured. I would recommend wearing safety glasses during the procedure however.