I see the following messages from both primary and alternate Ubuntu and Kubuntu Live Cds.
error: "prefix" is not set.
booting a command list you need to load a kernel first
Both these messages are displayed very early in the boot process, the first just after the boot begins and the second immediately after I select from the "Try or install" menu.
Selecting any option from "Try or install" leaves the system locked that needs a hard reboot.
I always see this on Live Cd's of Ubuntu/Kubuntu 12.04. But never on a Live USB.