We override the behavior of some Debian packages, such that we need to re-make some changes each time they update. The /usr/local/sbin/fixperms script is responsible for doing this, so you should run it after an apt-get upgrade. Currently, it does this:
{{{chown list.list /usr/lib/cgi-bin/mailman/* chmod g-s /usr/lib/cgi-bin/mailman/* chmod u+s /usr/lib/cgi-bin/mailman/*
chown -R list /var/lib/mailman/lists/*}}}