[o11c]

I had a different GRUB2 upgrade breakage a couple years ago. Mine related to a hybrid ISO/MBR disk (extremely common for distro install disks); previously grub had silently just used the obvious partition table, but the new grub insisted on erroring out due to ambiguity. I had to do some really hairy "dd to back up the MBR; wipefs; dd to restore the MBR" to get a working system again; luckily I didn't lose power.

(note that many distros normally don't upgrade grub after the first install to avoid exactly this kind of breakage, but there was some "important security crap" at the time)