The surface is not watertight.  To identify the regions that are not watertight, perform the following steps:
	1) Define the expression: "external_node(meshname)"
	2) If some set of operations have been performed on the mesh, then you will need to turn on the DeferExpression 
	operator under Options->Plugin Manager and then apply this operator with the external node expression to your plot.
	3) Change the variable you are plotting by to the external nodes expression.  This will show where the 
	non-watertight nodes are located.
