Comment 6 for bug 1017678

I strongly believe that the root cause is in the linux-boot-prober as it returns a strange result when launched (from my Xubuntu Precise) against a Xubuntu Quantal partition:

(verbatim copy)
$ sudo linux-boot-prober /dev/sda7 2> /dev/null
/dev/sda7:/dev/sda7:Ubuntu' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-simple-7205122a-da09-4f1f-97ef-67d3f70df317:/boot/vmlinuz-3.5.0-15-generic:/boot/initrd.img-3.5.0-15-generic:root=UUID=7205122a-da09-4f1f-97ef-67d3f70df317 ro quiet splash $vt_handoff
/dev/sda7:/dev/sda7:Ubuntu, with Linux 3.5.0-15-generic' --class ubuntu --class gnu-linux --class gnu --class os $menuentry_id_option 'gnulinux-3.5.0-15-generic-advanced-7205122a-da09-4f1f-97ef-67d3f70df317:/boot/vmlinuz-3.5.0-15-generic:/boot/initrd.img-3.5.0-15-generic:root=UUID=7205122a-da09-4f1f-97ef-67d3f70df317 ro quiet splash $vt_handoff
(/verbatim copy)

More, this strange result also likely causes the following code in 30_os-prober to misbehave (returning two entries with strange names)

      LINUXPROBED="`linux-boot-prober ${DEVICE} 2> /dev/null | tr ' ' '^' | paste -s -d ' '`"
      prepare_boot_cache=

      for LINUX in ${LINUXPROBED} ; do

      (etc)

      done

I hope this helps