Simply Maya User Community - Tips ... on non-manifold

Well ..after reading a bit through Maya´s docs I came cross this little gem of info when modeling polygons .. and it should help you save trouble also. (Like i did when modeling my sub)

2-manifold topology basically means you can unfold the geometry so that it lies flat on a plane without overlapping pieces.

In the first example (the "T" shape), more than two faces share an edge.

In the second example (the "bowtie" shape), two faces share a single vertex without also sharing an edge. This shape is also possible where two three-dimensional shapes share a vertex (such as two cubes meeting at a single point).

In the third example, a single shape has non-contiguous normals (without border edges). This is a less obvious example of nonmanifold geometry.

The following operations can produce nonmanifold geometry:

Extrude Edge
Normals > Reverse (reverse normals without extracting geometry)
Merge Vertices
Delete Face (select the face and press the backspace key)
Collapse (Face or Edge)
To keep the polygon count down and make tasks such as mapping textures to your objects much easier and quicker, make the pieces of polygonal geometry making up a model fit properly. Avoid creating a polygonal edge that has no face. Also, try to create your polygons so that their normals point in the same direction. Although it is technically valid for the normals to point in opposite directions, textures may not behave as expected.