Never happened to me before but now you've got me worried. I don't make a habbit of test firing each batch since I only buy 2-3 magazines worth at a time. Oh well, guess I'm going to the range this PM.
Did you try to re-strike any of the primers/rounds? Did any go bang the 2nd time around?
Anything unusual about the ammo? Appearance? Was the primer dented?
What brand/cal/grain ammo???