Comment 175 for bug 1289977

It doesn't matter whether or not this is caused by having done anything manually with grub before.
If you have a system that works 100% fine, you run dist-upgrade and you get a broken system that won't even boot, there's a bug, be it in grub or in dist-upgrade.
At the very least, dist-upgrade MUST check if there's something that will prevent the upgrade to work, refuse to perform the upgrade (or warn you) and tell you exactly what needs to be fixed before upgrading.

In my case, I never touched anything grub-related manually, or if I did, it was to fix something that the previous upgrade or clean install screwed up because of a bug.