I couldn't login after the last update to tcsh. After investigation, it ended up that /etc/shells had been modified in a way that removed my login shell from it which prevented users from loging in.

The post_upgrade() function from tcsh.install [1]
contains a sed call that changes all "/usr/bin/tcsh" to "/bin/tcsh" (same for csh). The problem is that it can match any substrings. On this specific machine, the login shell _has_ to be /opt/usr/bin/tcsh so I have such an entry in /etc/shells. Thus the sed call changed that line to "/opt/bin/tcsh" which prevented me from login in.

Wouldn't something like:
sed -i 's|^/usr/bin/tcsh$|/bin/tcsh|g' etc/shells
instead of
sed -i 's|/usr/bin/tcsh|/bin/tcsh|' etc/shells
be better (and same for all other sed calls)?
Note the "^", "$" and also the "g". While using both "^" and "$" would prevent substring substitution, the "g" would make sure any duplication get changed too.

https://projects.archlinux.org/svntogit/community.git/tree/trunk/tcsh.install?h=packages/tcsh