There are three basic types of nonmanifold geometry -

1. three or more faces share a single edge
2. two faces share a single edge but have their normals in opposition
3. two or more faces are connected by a single vertex

Don't do any of them or you will have problems.

NOTE: In case 2 I model with highlight border edges turned "on" this thickens border edges which are by definition unattached. However, note example 2 - the edge connects the two faces (that is both faces share the center edge) however maya shows that center edge as a border edge. When you are modeling and you see faces that share an edge but the edge is displayed as a border edge this is a sure indication of flipped normals.

Hope this helps,
Rick