Installing the guest addons in a guest machine fails when "distro" is Linux From Scratch. There are checks for "/usr/X11R6/lib/modules" while in LFS there is "/usr/X11R7.2/lib/X11/modules" and some other mismatches.
To see whether it is LFS, you may want to look for "/etc/lfs-release". Than you will know that the
- boot-scripts are in "/etc/rc.d/init.d/"
- X configuration should be "/etc/X11/xorg.conf"
- X11 modules dir can be assumed as "/usr/X11R6/lib/X11/modules" (since there is a compatibility symlink from /usr/X11R7.x to /usr/X11R6)
Do you any chance to add this to the VBoxLinuxAdditions.run script?
Btw, I tested this on VB-1.5.6, I'll test it again on 1.6.x when x becomes greater than zero ;-)