Firmant.Application.CrossReference.py

        for post in self.objs.get('posts',[]):
        
            def makeRef(name,value):
                for obj in self.objs.get(name,[]):
                    if obj.slug == value:
                        tag.posts.append(post)
                        post.tags[i] = tag
                        return True
                 warning = "%s '%s' referenced but not defined."
                 self.log.warning(warning % (name[:-1].capitalize(),value))
                 return False

            post.tags = [tag for tag in post.tags if makeRef('tags',tag)]
            post.feeds = [feed for feed in post.feeds if makeRef('feeds',feed)]

Line 26: Warning should warn instead of undefined feed referenced.
Lines 9 and 24: can be followed by continue since match has already been found

Since both if blocks need early escape and are virtually identical, we could factor out the code.

Oops, I'm missing a 'return False'

So this hasn't been tested because I don't have the dependencies and haven't checked out the Firmant testing framework recently.