What is zero thickness geometry?

Zero thickness geometry is when a feature is created that can not be solved due to the fact that there is precisely zero material at only a point or an edge. An example would be if you created an extruded circle that is tangent to an extruded square. There is a good explanation here: http://help.solidworks.com/2012/English/SolidWorks/sldworks/HIDE_ZeroThickness.htm
I've reposted the general guidelines from that site below as reference.

  1. Step 1:

    Every edge of a solid body must have exactly two adjacent faces.

  2. Step 2:

    Edge where zero thickness geometry is located.

  3. Step 3:

    Vertices where zero thickness is located.

  4. Step 4:

    Tangent line where zero thickness is located.

  5. Step 5:

    Zero thickness occurs when you attempt to extrude a cut tangent to a hole.
    This is frequently the cause of failed section views in drawings.