Original: Note that under the official Wikipedia interpretation of what constitutes an aggregate work, it is already possible to add paragraphs or images which are not CC-BY-SA licensed and thus create an incompatible fork.
Simplified: Note that under the official Wikipedia interpretation of what constitutes an aggregate work (see Wikipedia: Verbatim copying), it is already possible to add paragraphs or images which are not GFDLed and thus create an incompatible fork.