Comment 5 for bug 61371

The problem seems to be line 35 of the postinst script (which has been automatically inserted by dh_installinit):

        update-rc.d flashplugin-nonfree multiuser >/dev/null

which isn't valid use of update-rc.d. Instead it should be something like:

        update-rc.d flashplugin-nonfree defaults >/dev/null

I'm not 100% if that puts it at the right place in the boot sequence, but it chose S20 here. If you want to you can edit /var/lib/dpkg/info/flashplugin-nonfree.postinst and change that line and it will allow apt to continue without failing. HOWEVER, as I said, I'm not sure that that is the "correct" fix.