It's not an Installer-bug itself, but more a bug with Gentoo's mirroring policy.
Let me explain with an example, I recently hit: When you choose 'gentoo-sources' in the installer of the 2006.0 LiveCD, the installation goes fine until the point, where it wants to download the sources. The snapshot on the CD has gentoo-sources-2.6.15-r5 marked stable. Unfortunately not all patches are available on the mirrors any more. The Installer stops working and you are left behind with a half installed system. If you are an expert, you can chroot into the installation and finish manually (sync then install a newer kernel) or as a beginner you will restart and choose another kernel (like copiing the running CD-kernel).
I think, that we have to find a way, to mark sources on the mirrors unremovable, especially those which are needed for the packages in the current installation-snapshot. That means, we have to keep them for about half a year, when we do two releases per year.
The same problem exists for anyone doing a manual install and using the snapshot provided on the CD.
*** Bug 141214 has been marked as a duplicate of this bug. ***