When deleting shaders in Hypershade, its shading group is not deleted. This may result in orphaned shading groups, or worse, whatever was assigned to that shading group now only shows up as dense wireframe in hardware shading.
To get around the latter problem, reassign those objects to another shader.
When flipping back and forth between showing top/bottom/both tabs, the view is not reset, and may mislead the user into thinking that whatever was visible in one of the tabs is now invisible.
To resolve this, set View > Frame All for the problem tab.