While installing some other package:

Installing dependencies:
  libcurl4-gnutls  perl-modules-5.42

REMOVING:
  came  gnash-common  gosmore  libcupt3-0-downloadmethod-curl  libre-engine-re2-perl  sheepdog

Summary:
  Upgrading: 122, Installing: 2, Removing: 6, Not Upgrading: 552
  13 not fully installed or removed.
  Download size: 0 B / 16.5 MB
  Space needed: 3,286 kB / 2,719 MB available

Continue? [Y/n]
Retrieving bug reports... Done
Parsing Found/Fixed information... Done
Reading changelogs... Done
Extracting templates from packages: 100%
(Reading database… 815453 files and directories currently installed.)
Removing sheepdog (0.8.3-5+b1)…
Failed to stop sheepdog.service: Unit sheepdog.service not loaded.
invoke-rc.d: initscript sheepdog, action "stop" failed.
dpkg: error processing package sheepdog (--remove):
 old sheepdog package prerm maintainer script subprocess failed with exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 sheepdog
Processing was halted because there were too many errors.
needrestart is being skipped since dpkg has failed
Error: Sub-process /usr/bin/dpkg returned an error code (1)


So let's add some "set -x" to prerm and init scripts:

(Reading database… 815453 files and directories currently installed.)
Removing sheepdog (0.8.3-5+b1)…
+ [ -x /etc/init.d/sheepdog ]
+ invoke-rc.d sheepdog stop
Failed to stop sheepdog.service: Unit sheepdog.service not loaded.
invoke-rc.d: initscript sheepdog, action "stop" failed.
+ exit 1
dpkg: error processing package sheepdog (--remove):
 old sheepdog package prerm maintainer script subprocess failed with exit status 1
dpkg: too many errors, stopping
Errors were encountered while processing:
 sheepdog
Processing was halted because there were too many errors.
needrestart is being skipped since dpkg has failed
Error: Sub-process /usr/bin/dpkg returned an error code (1)