The 788 ejects it's casings up and out at about a 60 to near 70* angle depending on which caliber it is.
With some scopes mounted as usual (windage adj knob on the right side of the tube) the ejected case can occasionally strike that adj knob on the way out.
Again it depends on which scope you happen to be using, how it's mounted (fore and aft position) and which 788.

The easy fix is to rotate the scope 90* CCW in the rings.
The windage adj is now on top and becomes your elevation.
What used to be the elevation is now on the left side out of the way and is now the windage adj.