On http://fedoraproject.org/wiki/PackagingGuidelines under "Running scriptlets only in certain situations" it gives an example:

This means that for example a package that installs an init script with the chkconfig command should uninstall it only on erase and not upgrade with the following snippet:

%postun if [ $1 -eq 0 ]; then /sbin/chkconfig --del %{name} fi

However, that example is incorrect. By the time %postun is run, the /etc/init.d/%{name} file is already gone. This needs to be run instead in %preun.