boot record written to wrong partition
Binary package hint: ubiquity
I installed the 9.04 beta AMD64 on my Thinkpad x61t.
Using "advanced" on one of the screens (I forget which one), I specified that grub's boot record should be installed on /dev/sda9 (the only choice besides /dev/sda) (I admit that I don't remember if the terminology was actually (hd0) vs (hd0,8)).
I then tried to boot the machine. It should have booted Ubuntu 8.10 since the master boot record was still set to booting /dev/sda4. Instead, it hung.
To fix this, I booted the 9.04 beta live CD, used fdisk to move the boot flag to /dev/sda3. I then booted the grub on that partition (8.04) and got it to use the 8.10 grub menu and thus booted 8.10. I then fixed the boot record using 8.10's grub-install.
Summary: the installer somehow broke the bootloader on a partition it should not have touched.