Instructions to reproduce the problem:
Using the "OpenGL v2" render mode, when viewing "Planet Castor", Celestia crashes with the standard Windows popup when "Moon Castor" occults Canopus. The other render modes do not crash. This crash is also seen in V1.6.0 and prereleases of v1.6.1.
Canopus is adjacent to the moon's orbit at about the "7 o'clock" position. You probably should increase the speed of simulated time so that it happens in a reasonable amount of time. Canopus is actually irrelevant to the problem. It just happens to align with the orbital position of the moon which triggers the crash.
See shaders.log which is included.
Using "OpenGL v2" render mode, the planet is drawn Red (too many shsders?) when the "Planet Castor" url has been selected. This only happens for some positions of "Moon Castor". It looks OK in other the render modes. This problem is not seen in V1.6.0 or prereleases of v1.6.1.
The same problems (red planet and/or moon; Celestia crashes) are seen with Planet YY, although it crashes when the moon is at about the "8 o'clock" position in its orbit, not near Canopus.