Description of problem:
The iptables.spec file contains the following line:

  sed -i -e 's;^\(After=.*\)$;Before=ip6tables.service\n\1;' %{SOURCE3}

This has the evil side effect of changing a sources file of the RPM. When building with mock this may not be an issue, but it is an issue in the following situation:

  rpmbuild -bb iptables.spec (this has a side effect)
  rpmbuild -bs iptables.spec (now the side effect is in the resulting src.rpm)

A better way might be this:

  # install systemd service files
  install -d -m 755 %{buildroot}/%{_unitdir}
  sed -e 's;iptables;ip6tables;g' \
      -e 's;IPv4;IPv6;g' \
      -e 's;/usr/libexec/ip6tables;/usr/libexec/iptables;g' \
      -e 's;^\(After=.*\)$;\1,iptables.service;' \
      < %{SOURCE3} > ip6tables.service
  sed -e 's;^\(After=.*\)$;Before=ip6tables.service\n\1;' %{SOURCE3} > iptables.service
  install -c -m 644 iptables.service %{buildroot}/%{_unitdir}
  install -c -m 644 ip6tables.service %{buildroot}/%{_unitdir}

So instead of changing the source file, just create an in between file and move on from there

Version-Release number of selected component (if applicable):
iptables-1.4.21-18.2.el7_4

How reproducible:
100%

Steps to Reproduce:
1. rpm -i iptables-1.4.21-18.2.el7_4.src.rpm
2. rpmbuild -bb iptables.spec
3. rpmbuild -bs iptables.spec
4. Enjoy corrupted src.rpm

Actual results:
Accident waiting to happen

Expected results:
Happy RPM building

Additional info:
I would assume there are quality guidelines against this, but I'm not sure