After exporting a B-spline surface to STEP and importing it back to OCC, the surface is not displayed correctly in shaded mode (see image in the appendix). Wireframe mode is displayed correctly, also iso-lines can be shown, so the geometry is basically there, but shading fails. As far as I know such problems often point to some geometrical problems.
The problem occurs only after the surface was sewed together with other surfaces to form a solid.
However, according to DRAW's "checkshape" this solid is valid.
Furthermore, the problem does not occur if the surface spans less than 360°.
The problem occurs in OCC 6.5.1 as well as in OCC 6.3.1. Other versions, I dind't test.
With other export formats (BREP, IGES, STL) there is no problem.