One time to be careful is if you are seating primers in brass that has had the primer pockets crimped. When you ream the crimps out sometimes little bits of brass can fall into the priming arm cup and when you seat the primer the piece of brass indents the primer like a firing pin.
I've made it a habit to make sure there is a die in the press so that if the primer blows it won't come up through empty hole In the press into my face.
