OK, this just pisses me off. It's plain dumb -
rpmbuild expands macros before stripping comments. So what happened to me today (and I wasted an hour on) was that a multi-line macro was being inserted into a comment!
So now you know. If you are making a spec file for an RPM:
%files # %build
will screw you over like it did me. You need to:
%files # %%build