Comment 5 for bug 1787460

It does appear, however, that despite the above comment (which I think highlights ways we can make this foot a smaller target), this definitely began with human error:

Start-Date: 2018-08-14 13:37:10
Commandline: apt dist-upgrade
Install: linux-headers-4.15.0-32-generic:amd64 (4.15.0-32.35, automatic), linux-headers-4.15.0-32:amd64 (4.15.0-32.35, automatic)
Upgrade: linux-headers-generic:amd64 (4.15.0.31.33, 4.15.0.32.34), linux-libc-dev:amd64 (4.15.0-31.33, 4.15.0-32.35)
Remove: linux-signed-generic:amd64 (4.15.0.31.33), linux-generic:amd64 (4.15.0.31.33)
End-Date: 2018-08-14 13:37:16

That's a manual apt dist-upgrade which would have asked for confirmation on that removal. unattended-upgrades alone would have never gotten into this situation, as it won't remove to force an upgrade.