Comment 6 for bug 1185547

It's not entirely clear that the apt-cacher-ng user *should* be removed on package removal. See http://wiki.debian.org/AccountHandlingInMaintainerScripts for unresolved discussion of such ☺.

It's also worth noting that the patch as written will fail if adduser is installed *and* the apt-cacher-ng user is not a system user. You've traded an error on reinstall for an error on removal.

I've fixed a similar problem in colord recently by just checking whether the user exists before trying to create it in the postinst.

Finally, this does indeed seem like something that would also apply to debian - have you filed a Debian bug or contacted the maintainer about this?