While upgrading from 11.2-RELEASE to 12.0-RELEASE ("Components src world kernel"), freebsd-update forgets several essential files, e.g. libypclient.so or libgssapi_krb5.so
The reason for this may be that the 11.2 system was built from source without GSSAPI or YPCLIENT. But it is obvious that freebsd-update should install all needed libraries and not only those that exist on the system.
Upgrading from sources ("Components src") does not work, see bug #234203
Sorry, I forgot to mention this: because of a missing libypclient.so, pam_unix.so is not loaded so no login is possible.