Disable 'Double Sided' rendering in the Mesh Buttons and check the normals (inverted normals show up as dark patches).
Enter edit mode and deselect all vertices. Use the Non-Manifold check (Ctrl-Alt-Shift-M) to detect any open or invalid edges. Non-manifold edges are edges that don't have *exactly* two faces attached to them. Fix these areas before proceeding to the next step.