In my personal experience, the bulk of problems with good quality scopes shifting come from mounting issues. Most optics companies do not like to tell their customers that they are [bleep] idiots and can't mount a scope, so they simply swap the scope out and take a loss.
ILya
This I very much agree with. Further I think in cases where the scope may be mounted ok but the zero shift is from shifting in the rings from the impact. Any slight shift in the rings causes a pretty big poi difference in most cases. Heck even adding a bit of torque on the rings causes a shift.
