Bug#766799:

[Dmitry Borisyuk]

I dare reopen this, since the bug actually exists.
Lowering severity, because this only occurs for a special cases.

In summary, if the root filesystem was created with no resize inode,
and is then resized, resize2fs adds meta_bg feature to it
(the same can happen in some other cases, see ext4(5) manpage).
After that, grub cannot recognize the filesystem, and thus cannot boot.