Recently I've got a bug about
#965164 - policykit-1: fails to install - Debian Bug report logs
It seems that it freezes after logged in. When I've noticed this bug by apt upgrade, so I could recover by manually.
I recalled the following bug about a year ago. It was also critical bug that I had got.
#932855 - e2fsprogs 1.45.3-1 breaks initramfs-tools-core <=0.133 - Debian Bug report logs
I have to admit that Debian unstable is truly "unstable" even though it seems that it works like a "stable" in mostly.
We need to be aware to execute piuparts before uploading a package. (Maybe should we forced?)