After upgrading to Windows XP Pro from Windows 98, and installing the Gimp
1.2.4 after previously using 1.2.3, I found that my layer masks were
getting lost when I saved my image.  I was saving as a .xcf file and
telling Gimp to determine the file type from the extension, and even if I
weren't I would expect the layer masks to be applied before the file was
saved, rather than just get thrown out.  I wasn't sure whether this problem
was due to some weird compatibility issue with Windows XP, or if it was
from upgrading to version 1.2.4 of the Gimp, so I found and installed 1.2.3
and the problem went away.  I've been using Jernej Simoncic's GimpWin
installers, but I wouldn't expect this problem to have anything to do with
the method of installation.  This would be a really obvious bug to anyone
who was affected by it, so the fact that I haven't seen any mention of it
makes me think that this isn't happening to most people.