The model (bugboard.3ds) is a 5x5 mesh with a protrusion on one side.
The surface texture (grd.jpg) is a jpeg image, thus with no transparency.
1) Mesh components are not properly depth sorted.
When rendered as a Nebula (bugboard.dsc), the far edge of the base is seen "through" the protrusion. It should not: the protrusion should block the view. (right-hand image)
When rendered as a Satellite (bugboard.ssc), the far edge of the base is correctly not seen. (left-hand image)

2) Typing a Control-W does not display the mesh of a Nebula.
It does display the mesh of a satellite.

A 25KB zip archive containing bugboard.dsc, bugboard.ssc, bugboard.3ds and grd.jpg plus a web page with Cel://URLs for the two viewpoints can be fetched from
http://www.lns.cornell.edu/~seb/celestia/bugboard.zip
added later: oops.
System configuration:
256MB 500MHz Pentium 3; WinXP Pro sp1
128MB Nvidia GeForce4 Ti 4200; Detonator 43.45, OpenGL 1.4
Celestia v1.3.0final and v1.3.0pre6
added yet later:
Although the original dsc file specifies galactic distances, the same problem isseen at mearly stellar distances. (Distance 100 Radius 1)
I had hoped that moving it within the 15KLY limit would help, but it doesn't.