This is also reported in ubuntu bug 564853 (https://bugs.launchpad.net/bugs/564853). There is a recreate script attached to the bug there. As diagnosed there, the prompt will occur if the user has made *both*: a. changes to grub-pc.postinst fields: GRUB_CMDLINE_LINUX, GRUB_CMDLINE_LINUX_DEFAULT, GRUB_TIMEOUT, GRUB_HIDDEN_TIMEOUT b. any other changes