I was just reading reports by people installing the latest Ubuntu release: Karmic Koala. Some reported no problems with the beta version, but massive problems with the release candidate. This is almost certainly caused by installing from a corrupted CD image. These errors can be silent, but will cause seemingly inexplicable failures.
When downloading CD images, always, always download the MD5SUMS file as well. Run "md5sum -c MD5SUMS' once you've downloaded the image and the MD5SUMS file. (Note: make sure the ISO and the MD5SUMS files are in the same directory.) This will ensure the CD image was downloaded correctly.
The second thing you must do is check that the CD image was burned without errors. You can do this at install time by choosing the "Check CD image" option from the install menu. However, I prefer to do this ahead of time because it is faster. Once you've burned the image to a CD-RW (or whatever you are using), you can use the "dd" command to be sure that no errors were introduced during writing. I find that errors crop up in burning CD's about 50% of the time, so this is the most common cause of installation problems. To check the newly created CD:
1) put it in the tray
2) you may need to unmount the CD if it gets mounted automatically. In a terminal, type "umount /mnt/cdrom". Your CD may be mounted on "/mnt/cdrom0" or other location. Type "mount" to find out where.
3) Now figure out what device the CD is attached to. Typing "dmesg | tail" after inserting the CD will usually show the device. Most often it will be "/dev/cdrom" or something similar. Now type "dd if=/dev/cdrom of=/dev/null bs=10M" and see if there are any IO errors. If not, you have a good CD. Install away.
I cannot tell you how many times I have seen bad installs because of corrupt CD's.