I don't know if it il a bug, or the failure on my system depend upon video card drivers but Celestia 1.2.5 is not able to run full screen on my PC.
The program always crashes if I press Alt-Enter or if I use the menu option.
My system:
Athlon XP 1800
512 MB RAM
ATI Radeon 8500
Windows XP
I've noticed that the program dos not perform any check about he minimum required window size. So it is possible to resize in any way.
I think that the window should not be smaller than 640x480.