The orbit of a selected moon is not fully drawn. At certain distances, the (red) orbit is drawn only partway across the screen. As the distance to the selected moon is increased, more of the orbit is drawn. As the distance is reduced, less is drawn.
Here's a picture showing the orbit of Mimas being drawn only halfway across the screen.
(as usual, this links to a larger image)
And here's a Cel://URL for this viewpoint:
cel://Follow/Sol:Saturn:Mimas/2003-09-18T01:36:50.33726?x=lj/66TRzDEKvDA&y=JA+jFTSmMRj+/////////w&z=mBzh7/jWVgFy/////////w&ow=0.964193&ox=0.002158&oy=-0.265167&oz=0.003776&select=Sol:Saturn:Mimas&fov=43.293781&ts=1.000000<d=0&rf=66359&lm=0
System:
256MB 500MHz P3, WinXP Pro SP1
128MB GF4 Ti4200, Detonator 44.03, OpenGL 1.4
Celestia v1.3.1pre10