This is a human factors / ease of installation bug. Some modern CD-ROM drives require a few seconds after inserting a CD before you should attempt a mount command. During installation, if a user doesn't wait for about 5 seconds before clicking the OK button, signifying that she's inserted CD#2, the initial mount of CD#2 may fail. In the installer, if the very first mount of CD#2 fails, you should delay for about 3 seconds and retry the mount once. If this attempt fails, then you should display the error message.
Added for RC1