GRUB fallback boot
A very usefull feature of grub is booting a fallback system. Typically you will need it when booting into a newly built kernel on a remote server. In short, this feature makes grub to try to boot all entries one by one until the system starts or until every entry fails. So, assuming you have a current working kernel the last in your menu.lst - a remote copmputer will always boot 
Something i haven’t noticed reading article through, though smth. very important is
However, you need to run grub-set-default (see Invoking grub-set-default) when `A' starts correctly or you fix `A' after it crashes, since GRUB always sets next boot entry to a fallback entry. You should run this command in a startup script such as rc.local to boot `A' by default:
# grub-set-default 0
where `0' is the number of the boot entry for the system `A'.
Have fun and keep your nervs healthy!